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This FAQ consists of frequently asked questions about routers manufactured by cisco Systems, Inc. Additionally, it provides additional information on IP routing that may be useful in non-Cisco environments.
Archive-name: cisco-networking-faq
Last-modified: $Date: 1996/04/28 05:55:19 $
Version: $Revision: 1.10 $

This FAQ is edited by John Hawkinson, <>.


Please contribute answers to the questions in the Todo section! If
your answer is somewhat complicated, posting would probably be best
(to Otherwise, e-mail it to
Please note that a LOT of these questions have been hanging around for
some time, and if knowledgable people could take the time to answer a
few of them, that'd help.

This draft FAQ is in RFC1153 digest format, so you can follow each
question with your newsreader. I suppose that question-numbers should
be moved to the From: field. Note that Date: fields represent
last-modification times for the questions.

Since this FAQ was first developed, cisco has written up a lot of
useful information on their web site,  If you
can't find what you're looking for here, please check there, too.

Table of Contents

1.      How can I contact cisco?
2.      What is this newsgroup?
3.      What does ``cisco'' stand for?
4.      How do I save the configuration of a cisco?
5.      Where can I get ancillary software for my cisco?
6.      Is there a World-Wide-Web (www) information source?
7.      How can I get my cisco to talk to a third party router over 
8.      How can I get my cisco to talk to a 3rd-party router over Frame Relay?
9.      How can I use debugging?
10.     How can I use NTP (Network Time Protocol) with my cisco?
11.     Sample cisco NTP Configurations
12.     How do I avoid the annoying DNS lookup if I have misspelled a command?
13.     Tracing bad routing information
14.     How to use access lists
15.     The cisco boot process
16.     Where can I get cisco hardware?
17.     Where can I get IETF documents (RFCs, STDs, etc.)?
18.     Future features in cisco software
19.     How do cisco routers rate performance-wise?
20.     How are packets switched?
21.     How does one interpret buffer statistics?
22.     How should I restrict access to my router?
23.     What can I do about source routing?
24.     Is there a block of private IP addresses I can use?
25.     Is DHCP supported?
26.     Where can I get cisco documentation?
27.     What's the latest software for the CSC/3?
28.     What IP routing protocol should I use?
29.     How do I interpret the output of ``show version''?
30.     What is the maximum number of Frame Relay PVCs?
31.     How much memory is necessary to telnet to a cisco router?
32.     Where can I purchase flash RAM?
33.     When are static routes redistributed?
34.     When is the next hop of a route considered ``reachable''?
35.     How do name and phone number of ``dialer map'' interfere?
36.     What's the purpose of the network command?
37.     What is VLSM? 
38.     What are some methods for conserving IP addresses for serial lines?
39.     Why do some ip addresses get rejected?
40.     How do 4xxx serial numbers correspond to models?
41.     Where can I find more info on TACACS+
99.     Acknowledgements.


*  What is SNMP and how can I use it? What software is available and how do
   I use cisco enterprise MIBs? MIBs on and
*  Pointers to other net resources, like comp.protocols.tcp-ip, RFCs,
   the firewalls mailing list, etc (bgpd?[or is it cidrd now? :-)]).
*  Hints about confusing and not-well documented things like xtacacs...
*  Comments on interoperability issues WRT other vendors.
*  What's SMARTnet, why should I subscribe, how much does it cost,
   and what do I get?
*  What should I name my router, my interfaces, etc.?
*  Should we adjust the buffer parameters on the routers?  What should
   be the indicator before tunning the buffer parameters?  How should
   one fine tune the buffer parapeters?
*  What is CIDR and why do I care (or a more general acronym decoder) ?
*  How do I configure my cisco to use variable-length subnetting ?
*  Is there a block of private network numbers I can use
   within my organization only?  When should I use them?
   How do I access them from outside?
*  What do I do if I have to partition a network number?
*  Questions and answers about access lists
   access-list reference list (lots of questions on that)
*  I forgot to mention that routing DECnet over X.25 is a problem.
*  Where PD network applications for SLIP/PPP are.
*  What is HSRP and how does it work?  When is it available (10.0)
   (Hot Standby Routing Protocol)
*  Should I run 10.0, 10.2, 10.3, 11.1, or what?
*  What's the difference between IBGP and EBGP? Why should I run BGP?

Actual content.


From: Question 1
Date: 31 October 1994
Subject: How can I contact cisco?

Corporate address:

                cisco Systems
                170 West Tasman Drive
                San Jose, CA 95134

The following phone numbers are available:

  Technical Assistance Center (TAC)                     +1 800 553 2447
                                                              (553 24HR)
                                                        +1 800 553 6387
                                                        +1 408 526 8209
  Customer Service (Documentation, Warranty &           +1 800 553 6387
      Contract Services, Order Status
  Engineering                                           +1 800 553 2447
                                                              (553 24HR)
  On-site Services, Time & Materials Service            +1 800 829 2447
                                                              (829 24HR)
  Corporate number / general                            +1 408 526 4000
  Corporate FAX (NOT tech support)                      +1 408 526 4100

The above 800 numbers are US/Canada only.

cisco can also be contacted via e-mail:           Technical Assistance Center      European TAC        Literature and administrative (?) requests            *UNRELIABLE*, special-interest, ``non-support''

Please follow the directions available on CIO before doing this.
cisco provides an on-line service for information about their routers
and other products, called CIO (cisco Information Online).  telnet to for more details.

The collective experience of this FAQ indicates that it is far wiser to
open a case using e-mail than FAXes, which may be mislaid, shredded,

For those of you still in the paperfull office (unlike the rest of us),
cisco Systems' new corporate address is:

        170 West Tasman Drive
        San Jose, CA 95134

Mail to should include your service contract number, your name,
telephone number, a brief one line problem/question description, and a
case  priority in the  first 5 lines. For example:

        Cisco service contract number       92snt1234a 
        First and last name                 Jane Doe 
        Best number to contact you          415-555-1234 
        Problem/question description        Cannot see Appletalk zones 
        Case Priority                       3

CASE PRIORITIES are defined as one of the following:

Pri 1           Production network down, critical business impact 
Pri 2           Production net seriously degraded, serious impact 
Pri 3           Network degraded, noticeable impact to business 
Pri 4           General information, non production problems


From: Question 2
Date: 26 July 1994
Subject: What is this newsgroup?, which is gatewayed to the mailing list, is a newsgroup for discussion of cisco
hardware, software, and related issues. Remember that you can also
consult with cisco technical support.

This newsgroup is not an official cisco support channel, and should
not be relied upon for answers, particularly answers from cisco
Systems employees.

Until recently, the mailing list was gatewayed into the newsgroup,
one-way. It is possible that this arrangement may resume at somet time
in the future.


From: Question 3
Date: 31 October 1994
Subject: What does ``cisco'' stand for?

cisco folklore time:

At one point in time, the first letter in cisco Systems was a
lowercase ``c''. At present, various factions within the company have
adopted a capital ``C'', while fierce traditionalists (as well as some
others) continue to use the lowercase variant, as does the cisco
Systems logo. This FAQ has chosen to use the lowercase variant

cisco is not C.I.S.C.O. but is short for San Francisco, so the story
goes.  Back in the early days when the founders Len Bosack and Sandy
Lerner and appropriate legal entities were trying to come up with a
name they did many searches for non similar names, and always came up
with a name which was denied. Eventually someone suggested ``cisco''
and the name wasn't taken (although SYSCO may be confusingly similar
sounding). There was an East Coast company which later was using the
``CISCO'' name (I think they sold in the IBM marketplace) they ended
up having to not use the CISCO abberviation.  Today many people spell
cisco with a capital ``C'', citing problems in getting the lowercase
``c'' right in publications, etc. This lead to at least one amusing
article headlined ``Cisco grows up''. This winter we will celebrate
our 10th year.

[This text was written in July of 1994 -jhawk]


From: Question 4
Date: 31 October 1994
Subject: How do I save the configuration of a cisco?

If you have a tftp server available, you can create a file on the
server for your router to write to, and then use the write network
command. From a typical unix system:

        mytftpserver$ touch /var/spool/tftpboot/myconfig
        mytftpserver$ chmod a+w /var/spool/tftpboot/myconfig

        myrouter#write net
        Remote host []?
        Name of configuration file to write [myrouter-confg]? myconfig
        Write file foobar on host [confirm] y

Additionally, there's a Macintosh TFTP server available:

Additionally, you can also use expect, available from:

or, in shar form from

Expect allows you to write a script which telnets to the router and
performs a ``write terminal'' command, or any other arbitrary set of
command(s), using a structured scripting language (Tcl).


From: Question 5
Date: 5 July 1994
Subject: Where can I get ancillary software for my cisco?

Try ftping to

It's a hodgepodge collection of useful stuff, some maintained and some
not. Some is also available from

Vikas Aggarwal has a very customised tacacsd:

A new version of xtacacsd is available via anonymous FTP from:


From: Question 6
Date: 28 April 1996
Subject: Is there a World-Wide-Web (www) information source?

You can try the WWW page for this FAQ:

or the cisco Educational Archive (CEA) home page:

or the cisco Information Online (CIO) home page:


From: Question 7
Date: 5 July 1994
Subject: How can I get my cisco to talk to a third party router over 
a serial link?

You need to tell your cisco to use the same link-level protocol as the
other router; by default, ciscos use a rather bare variant of HDLC
(High-level Data Link Control) all link-level protocols use at some
level/layer or another. To make your cisco operate with most other
routers, you need to change the encapsulation from HDLC to PPP on the
relevant interfaces. For instance:

        sewer-cgs#conf t
        Enter configuration commands, one per line.
        Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
        interface serial 1
        encapsulation ppp

        sewer-cgs#sh int s 1
        Serial 1 is administratively down, line protocol is down 
          Hardware is MCI Serial
          MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, rely 255/255, load 1/255
          Encapsulation PPP, loopback not set, keepalive set (10 sec)

If you're still having trouble, you might wish to turn on serial interface

        sewer-cgs#ter mon
        sewer-cgs#debug serial-interface


From: Question 8
Date: 27 July 1994
Subject: How can I get my cisco to talk to a 3rd-party router over Frame Relay?

You should tell your cisco to use ``encapsulation frame-relay ietf''
(instead of ``encapsulation frame-relay'') on your serial interface
that's running frame relay if your frame relay network contains a
diverse set of manufacturers' routers. The keyword ``ietf'' specifies
that your cisco will use RFC1294-compliant encapsulation, rather than
the default, RFC1490-compliant encapsulation (other products, notably
Novell MPR 2.11, use a practice sanctioned by 1294 but deemed verbotten
by 1490, namely padding of the nlpid).  If only a few routers in your
frame relay cloud require this, then you can use the default
encapsulation on everything and specify the exceptions with the
frame-relay map command:

        frame-relay map ip 56 broadcast ietf

(ietf stands for Internet Engineering Task Force, the body which
evaluates Standards-track RFCs; this keyword is a misnomer as both
RFC1294 and RFC1490 are ietf-approved, however 1490 is most recent and
is a Draft Standard (DS), whereas 1294 is a Proposed Standard (one step
beneath a DS), and is effectively obsolete).


From: Question 9
Date: 26 July 1994
Subject: How can I use debugging?

The ``terminal monitor'' command directs your cisco to send debugging
output to the current session. It's necessary to turn this on each time
you telnet to your router to view debugging information. After that,
you must specify the specific types of debugging you wish to turn on;
please note that these stay on or off until changed, or until the
router reboots, so remember to turn them off when you're done.

Debugging messages are also logged to a host if you have trap logging
enabled on your cisco. You can check this like so:

        sl-panix-1>sh logging
        Syslog logging: enabled (0 messages dropped, 0 flushes, 0 overruns)
            Console logging: level debugging, 66 messages logged
            Monitor logging: level debugging, 0 messages logged
            Trap logging: level debugging, 69 message lines logged
                Logging to, 69 message lines logged

If you have syslog going to a host somewhere and you then set about a
nice long debug session from a term your box is doing double work and
sending every debug message to your syslog server. Additionally, if you
turn on something that provides copious debugging output, be careful
that you don't overflow your disk (``debug ip-rip'' is notorious for

One solution to this is to only log severity ``info'' and higher:

        sl-panix-1#conf t
        Enter configuration commands, one per line.  End with CNTL/Z.
        logging trap info

The other solution is to just be careful and remember to turn off
debugging. This is easy enough with:

        sl-panix-1#undebug all

If you have a heavily loaded box, you should be aware that debugging
can load your router.  The console has a higher priority than a vty so
don't debug from the console; instead, disable console logging: t
        Enter configuration commands, one per line.  End with CNTL/Z.
        no logging console

Then always debug from a vty.  If the box is busy and you are a little
too vigorous with debugging and the box is starting to sink, quickly
run, don't walk to your console and kill the session on the vty.  If
you are on the console your debugging has top prioority and then the
only way out is the power switch.  This of course makes remote
debugging a real sweaty palms adventure especially on a crowded box.
Caveat debugger!

Also, if you for some reason forget what the available debug commands
are and don't have a manual handy, remember that's what on-line help
is for. Under pre 9.21 versions, ``debug ?'' lists all commands. Under
9.21 and above, that gives you general categories, and you can check
for more specific options by specifying the category: ``debug ip ?''.

As a warning, the ``logging buffered'' feature causes all debug
streams to be redirected to an in-memory buffer, so be careful using

Lastly, if you're not sure what debugging criteria you need, you can
try ``debug all''. BE CAREFUL!  It is way useful, but only in a very
controlled environment, where you can turn off absolutely everything
you're not interested in.  Saves a lot of thinking.  Turning it on on
a busy box can quickly cause meltdown.


From: Question 10
Date: 5 July 1994
Subject: How can I use NTP (Network Time Protocol) with my cisco?

>What level of software is required for NTP support in
>a cisco router?

9.21 or above.

>Which cisco routers support NTP?

It is a software feature exclusively. Anything that supports
9.21 or 10 will run NTP (when running that s/w).

>How do I set it up?

The basic hook is:
        ntp server <host> [version n]
        ntp peer <host> [version n]

depending on whether you want a client/server or peer relationship.
There's a bunch of other stuff available for MD5 authentication,
broadcast, access control, etc.  You can also use the
context-sensitive help feature to puzzle it out; try ``ntp ?'' in
config mode.

You'll also want to play with the SHOW NTP * router commands.  Here
are two examples.


router# show ntp assoc

      address         ref clock     st  when  poll reach  delay  offset    disp
+~      .WWVB.            1   109   512  377    97.8   -2.69    26.7
*~     .GOES.            1   309   512  357    55.4   -1.34    27.5
 * master (synced), # master (unsynced), + selected, - candidate, ~ configured


router#show ntp stat
Clock is synchronized, stratum 2, reference is
nominal freq is 250.0000 Hz, actual freq is 249.9981 Hz, precision is 2**19
reference time is B1A8852D.B69201EE (12:36:13.713 PDT Tue Jun 14 1994)
clock offset is -1.34 msec, root delay is 55.40 msec
root dispersion is 41.29 msec, peer dispersion is 28.96 msec

For particular cisco NTP questions, feel free to ask in

For broader NTP info, see  The file
clock.txt in that directory has info about various public NTP servers.
There is also information on radio time receivers that can be
connected to an NTP server (this is handy on private networks, if you
have an entire campus to get chiming, or if you become a hard core

The ``ntp clock-period'' command is added automagically to jump-start
the NTP frequency compensation when the box is rebooted.  This is
essentially a representation of the frequency of the crystal used as
the local timebase, and may take several days to calculate otherwise.
(Do a ``write mem'' after a week or so to save a good value.)

Caveat of obsolecence: Note that the CS-500 will not be able to
achieve quite the same level of accuracy as other platforms, since its
hardware clock resolution is roughly 242Hz instead of the 1MHz
available on other platforms.  In practice this shouldn't matter for
anyone other than true time geeks.


From: Question 11
Date: 5 July 1994
Subject: Sample cisco NTP Configurations

You will need to substitute your own NTP peers, timezones, and GMT
offsets into the examples below, of course.  Example 1 is in US Central
Time Zone, while example 3 is in US Pacific Time Zone.  Both account
for normal US Daylight Savings Time practices.

EXAMPLE 1 (Charley Kline):
clock timezone CST -6
clock summer-time CDT recurring
ntp source eth 0
ntp peer <host1>
ntp peer <host2>
ntp peer <host3>

EXAMPLE 2 (Tony Li):
ntp source Ethernet0/0
ntp update-calendar
ntp peer <host1> 
ntp peer <host2> prefer

EXAMPLE 3 (Dave Katz):
service timestamps debug datetime localtime
service timestamps log datetime localtime
clock timezone PST -8
clock summer-time PDT recurring
interface Ethernet0
ip address <mumble>
ntp broadcast
ntp clock-period 17180319
ntp source Ethernet0
ntp server <host1>
ntp server <host2>
ntp server <host3>

        The config file is commented with date and time (and user id,
if TACACS is enabled) when the system thinks the clock is accurate.
I've enabled timestamping of debug and syslog messages.  I send NTP
broadcast packets out onto the local ethernet.  I'm in Pacific
Standard Time, with U.S. standard daylight saving time rules.  I use
the IP address of the ethernet as the source for all NTP packets.


From: Question 12
Date: 5 July 1994
Subject: How do I avoid the annoying DNS lookup if I have misspelled a command?
By default, all lines are configured to automatically try a telnet
connection if the first word in a input line is not recognized as a
valid command.  You can disable this by setting ``transport preferred
none'' on every line (con, aux and vty). For instance:

        sl-panix-1#conf t
        Enter configuration commands, one per line.  End with CNTL/Z.
        line vty 0 10
        transport preferred none

You can see the number of vty's currently configuered with ``show lines''

Also, you can suspend connect attempts with ^^ followed by ``x'', ie
shift-cntrl-6 x.

[It has been suggested that ``no ip ipname-lookup'' to turn off IEN116
helps. I think this is the default -jhawk ]

From: Question 13
Date: 31 Oct 1994
Subject: Tracing bad routing information

or: How do I find out which non-cisco systems on my networks generate IP-RIP
   information without letting them mess up my routing tables.
Here you could work with a default administrative distance.
Administrative distance is the basis upon which the cisco prefers
routing information of one protocol over another. In this example:

        router rip
        distance 255
        distance 120     ! list all valid RIP suppliers

the value 255 has the implicit meaning of not putting this information
into the routing table. Therefore, setting an administrative distance
of 255 means that all RIP suppliers are by default accepted but their
information is not put into the routing table. The administrative
distance for the router has been reset to the default
(for RIP) of 120, causing its routes to be accepted into the routing table.

Then you can look them up with ``show ip protocols'' and restore the
original administrative distance for the ones you want to fill in the
routing table.

The same results can be acheived with an ip access-list, but with
that, ``show ip protocols'' will only show the valid ones. But often
it is more useful to see which systems were generating routing
information at all.

This trick works for other routing protocols as well, but please select
the proper adminstrative distance (rather than 120) for the protocol
you're using.


From: Question 14
Date: 5 July 1994
Subject: How to use access lists

[The following is wholesale included; at some point it'll
probably be editted a bit and reformatted... -jhawk ]

                    Frequently Asked Questions
                    contributed by Howard C. Berkowitz
                    PSC International
                [probably will be my permanent 
                                     personal account]
                    PSC's domain is in mid-setup

Where in the router are access lists applied?

    In general, Basic access lists are executed as filters on
outgoing interfaces.  Newer releases of the cisco code, such as
9.21 and 10, do have increased ability to filter on incoming ports.
Certain special cases, such as broadcasts and bridged traffic,
can be filtered on incoming interfaces in earlier releases.
There are also special cases involving console access.

Rules, written as ACCESS-LIST statements, are global for the entire
cisco box; they are activated on individual outgoing interfaces by
ACCESS-GROUP subcommands of the INTERFACE major command.
    Filters are applied after traffic has entered on an incoming
interface and gone through a routing process; traffic that originates in
a router (e.g., telnets from the console port) is not subject to

             |     GLOBAL        |
             |                   |
             | Routing           |
             | ^  v       Access |
             | ^  v       Lists  |
             | ^  v        ^   v |
             | ^  v        ^   v |
A----------->|-|  |>>>>Access  >>----------->B
             |1        Group   2 |
<------------|                   |<-----------
             |                   |
             |                   |

    Some types of ``filter,'' using ``filter'' as a broader class than
ACCESS-LIST, can operate on incoming traffic.  For example, the INPUT-
SAP-FILTER used for Novell networks is applied to Service Advertisement
Packets (SAP) seen at incoming interfaces.  In general, incoming
filtering can only be done for ``system'' rather than user traffic.

Rules of thumb in defining access lists.

    First, define what you want to do and in which directions.  An
informal drawing is a good first step.  As opposed to the usual
connectivity drawings among routers, it's often convenient to draw
unidirectional links between routers.
    Second, informally write out your filtering rules.  In general, it
is best to go from most specific to least specific. Modify the order of
writing things to minimize the number of rules needed.
    Third, determine which rules need to be on which routers.
Explicitly consider the direction of flow, and the possible existence of
additional paths that could inadvertently bypass a filter.

Can a cisco router be a ``true'' firewall?

    This depends on the definition of firewall.  Some writers (e.g.,
Gene Spafford in _Practical UNIX Security_) define a firewall as a
host on which an ``inside'' and/or an ``outside'' application process run,
with application-level code linking the two.  For example, a firewall
might provide FTP access to the outside world, but it would not also
provide direct FTP service to the inside world.  To place a file on
the FTP external server, a designated user would explicitly log onto
the FTP server, transfer a file to the server, and log off.  The
firewall prevents direct FTP connectivity between the inside and
outside networks; only indirect, application-level connectivity is
   Firewalls of this sort are complemented by chokes, which filter on
network addresses and/or port numbers.  Cisco routers cannot do
application-level control with access control lists.
   Other authors do not distinguish between chokes and filters.  Using
the loose definition that a firewall is anything that selectively blocks
access from the inside to the outside, routers can be firewalls.

IP Specific

Can the ``operand'' field be used with a protocol keyword of IP to filter
on protocol ID?

    No.  Operand filtering only works for TCP and UDP port numbers.

How can I prevent traffic for a certain Internet application to flow in
one direction but not the other?

    Remember that Internet applications flow from client port to server
port.  Denying traffic from port 23, for example, blocks flow from the
client to the server.

             |                   |
A----------->|                   |----------->B
             |1                 2|
<------------|                   |<-----------
             |                   |

    If we deny traffic to Port 23 of address B by placing a filter at
interface 2, we have blocked A's ability to telnet to B, but not B's
ability to telnet to A.  A second filter at interface A would be needed
to block telnet in both directions.
    Assume that we only have the filter at interface 2.  Telnets to A
from B will not be affected because the filter at 2 does not check
incoming traffic.

With the arrival of in-bound access lists in 9.21, it should be noted
that both inbound and access lists are about equally efficient, in
case any of you were wondering.

It's worth remembering that there are some kinds of problems
that packet-filtering firewalls are not best suited for. There's
reasonably good information in:

	"Network (in)security through packet filtering"


From: Question 15
Date: 26 July 1994
Subject: The cisco boot process

What really happens when a cisco router boots, from boot start to live
First it boots the ROM os version.  It reads the config.  Now, it
realizes that you want to netboot.  It loads the netbooted copy in on
top of itself.  It then re-initializes the box and re-reads the
config.  Manly, yes, but we like it too....

[[ Ummm... in particular it loads the netbooted copy in as WELL as
itself, decompresses it, if necessary, and THEN loads on top of
itself.  Note that this is important because it tells you what the
memory requirements are for netbooting: RAM for ROM image (if it's a
run from RAM image), plus dynamic data structures, plus RAM for
netbooted image. ]]
The four ways to boot and what happens (sort of):
           I (from bootstrap mode)
The ROM monitor is running.  The I command causes the ROM monitor to
walk all of the hardware in the bus and reset it with a brute force
hammer.  If the bits in the config register say to auto-boot, then
goto B
           B (from bootstrap mode)
Load the OS from ROM.  If a name is given, tell that image to start
silently and then load a new image.  If the boot system command is
given, then start silently and load a new image.
Does some delay stuff to let the power settle.  Goto I.
           reload (from the EXEC)
Goto I.


From: Question 16
Date: 18 July 1994
Subject: Where can I get cisco hardware?

Try calling 800-553-NETS and asking for your local sales office.
That's probably the best plan.


From: Question 17
Date: 18 April 1995
Subject: Where can I get IETF documents (RFCs, STDs, etc.)?

                   Where and how to get new RFCs

RFCs may be obtained via EMAIL or FTP from many RFC Repositories.  The
Primary Repositories will have the RFC available when it is first
announced, as will many Secondary Repositories.  Some Secondary
Repositories may take a few days to make available the most recent

Primary Repositories:

RFCs can be obtained via FTP from DS.INTERNIC.NET, NIS.NSF.NET,


Secondary Repositories:

        Directory:      rfc

        Directory:      rfc

        Site:           EUnet Germany
        Directory:      pub/documents/rfc

        Site:           Institut National de la Recherche en Informatique
                        et Automatique (INRIA)
        Notes:          RFCs are available via email to the above
                        address.  Info Server manager is Mireille 
                        Yamajako (

        Site:           EUnet
        Directory:      rfc
        Notes:          RFCs in compressed format.

        Site:           Centre d'Informatique Scientifique et Medicale
        Directories:    pub/rfc/*       Classified by hundreds
                        pub/mirrors/rfc Mirror of Internic
        Notes:          Files compressed with gzip. Online
                        decompression done by the FTP server.

        Site:           FUNET
        Directory:      rfc
        Notes:          RFCs in compressed format.  Also provides 
                        email access by sending mail to

        Directory:      pub/rfc

        Site:           University of Copenhagen
        Directory:      rfc

Australia and Pacific Rim

        Site:           munnari
        Contact:        Robert Elz <>
        Directory:      rfc
                        rfc's in compressed format rfcNNNN.Z
                        postscript rfc's

United States

        Site:           cerfnet
        Directory:      netinfo/rfc

        Site:           NASA NAIC
        Directory:      files/rfc

        Site:           NIC.DDN.MIL (DOD users only)
        Host:           NIC.DDN.MIL
        Directory:      rfc/rfcnnnn.txt
        Note:           DOD users only may obtain RFC's via FTP 
                        from NIC.DDN.MIL.  Internet users should NOT
                        use this source due to inadequate connectivity.
        Site:           uunet
        Contact:        James Revell <>
        Directory:      inet/rfc

UUNET Archive

     UUNET archive, which includes the RFC's, various IETF documents,
     and other information regarding the internet, is available to the
     public via anonymous ftp (to and anonymous uucp, and
     will be available via an anonymous kermit server soon.  Get the
     file /archive/inet/ls-lR.Z for a listing of these documents.

     Any site in the US running UUCP may call +1 900 GOT SRCS and use
     the login "uucp".  There is no password.  The phone company will
     bill you at $0.50 per minute for the call.  The 900 number only
     works from within the US.


From: Question 18
Date: 22 April 1996
Subject: Future features in cisco software

[This could be more fleshed out (still!)]

Kerberos and RADIUS in 11.1
RIP version 2 in 11.1 (allows VSM, etc.)
Policy-based routing (routing based on source address or interface, or just
about anything else you want) in 11.0 *released*
PPP Multilink in 11.0(3) *released*
Frame Relay payload compression in 11.0(4) *released*
IPX Per-Host load balancing in 11.1


From: Question 19
Date: 27 July 1994
Subject: How do cisco routers rate performance-wise?

People often ask about performance of the cisco routers and are shyed
away from answering their questions because we don't know where to send

Scott Bradner keeps the results of his performance tests on the
Internet.  You can find them for ftp on the system
in the /pub/ndtl.  There is a README file in that directory that
explains what is available.  In addition, cisco has just started
publishing a piece of literature called ``The Harvard Benchmark Test
Results: Summary of cisco Systems Performance''.  The only number I
can find on the doc is Lit. #700901.  Don't know if you can order it
by this number, but at least there's a title to go on.


From: Question 20
Date: 22 April 1996
Subject: How are packets switched?

There are 3 basic types of switching (in order of increasing performance).

        process switching
        fast switching
        autonomous switching

Process and fast switching support inbound and outbound, simple and
extended, access lists. Of course, for fast switching, such lists only
restrict traffic on the particular fast-switched interface.

Autonomous switching is done in the switch processor, a microcoded device that
is capable of switching IP, IPX, and bridging packets in the 100kpps range.
This is known as the "SP" card on the 7000 and the CBUS controller on the AGS+.
Encapsulation support is rather limited (Ethernet, HDLC, HSSI...).

The cisco 7000 also supports:

        silicon switching

Silicon switching is done in the silicon switching engine (creative, eh? ;-).

The silicon switch processor (SSP) is the board which combines both the
switch processor and a silicon switching engine.

The SSP supports simple and extended outbound access lists in 10.3 and later.
The SSP supports simple and extended inbound access lists in 11.1 and later.

The cisco 75xx series supports:

	"optimal" switching (cruddy name, eh?)
	"flow" switching
	"distributed" switching

* "optimal" switching (cruddy name, eh?)

The 7500 platform does not have a separate SP or SSP card, rather the RISC
processor on the "integrated route/switch processor card (IRSP)" handles
switching directly, similar to the 4000 series routers.  There are several
hardware and software enhancements made though to increase the throughput to
a level that is several times above what you would normally get from "fast"
switching.  Everything that "fast" switching supports is supported in
"optimal" switching.

* "flow" switching

Basicly the "optimal" switching method, however things have been front-ended
with an additional small "flow" cache.  This flow cache contains information
about source/destination addresses & ports which allow the router to make more
informed queueing decisions and process access lists faster.  This is a win in
routers that would tend to carry a reasonably small number of flows at any one
time, such as what you would expect in a corporate network or in a smaller
internet service provider network.  It's unclear if there are any advantages
in a large internet backbone.

* "distributed" switching

cisco has announced a new type of interface-processor card, called a "VIP"
available in the 7500 platform that is intelligent enough to switch packets
with no intervention on the part of the IRSP card.  This once again separates
switching from routing, as in the earlier CBUS/SP/SSP design.

The first packet of every session or connection is always Process Switched.
The route table is consulted (this resides in DRAM on the CPU) and the
"result" is cached in the system memory cache. If the protocol can only be
process switched, then it will continue this way and interrupt the CPU for a
route table lookup each time. [comment: Process Switching is brutally slow
compared to other switching methods. Some features (usually new features do
this for the first few software releases) force every packet to be process
switched. If you can't avoid process-switching every packet, at least get a
router with a fast CPU, such as the 75xx, 4500, and 4700. The 4700 is
currently the fastest at process-switching packets, with the 4500 and 75xx
tied for second. The 75xx can optimum-switch, however, so it's a lot faster
than either of the 4x00s, if you can use it).

The second and subsequent packets of each session are capable of being Fast
Switched (more session types are becoming fast-switchable), and will consult
only the route-cache. This still involves a memory lookup on the board, but
the packet can be transferred from the source card directly to the
destination card without requiring full storage on the CSC [the CSC refers
to the CPU card, basically].

There are some undocumented commands that are useful for obtaining
per-interface statistics on what sort of switching was performed.

For instance:

frobozz-magic-robot>sh int atm4/0 switch
         Throttle count:          0
     Protocol       Path    Pkts In   Chars In   Pkts Out  Chars Out
           IP    Process     104851    7669968     116378   11180988
            Cache misses      35826
                    Fast          0          0          0          0
               Auton/SSE          0          0          0          0
frobozz-magic-robot>sh int atm4/0 stat
          Switching path    Pkts In   Chars In   Pkts Out  Chars Out
               Processor     105024    7679155     116422   11184108
         Route cache/FIB          0          0          0          0
       Distributed cache          0          0          0          0
                   Total     105024    7679155     116422   11184108


From: Question 21
Date: 31 October 1994
Subject: How does one interpret buffer statistics?

Buffer statistics may be obtained with:>sh buffers
        Buffer elements:
             433 in free list (500 max allowed)
             82320311 hits, 0 misses, 0 created
        Small buffers, 104 bytes (total 202, permanent 120):
             185 in free list (20 min, 250 max allowed)
             34289219 hits, 4297 misses, 1307 trims, 1389 created
        Middle buffers, 600 bytes (total 104, permanent 90):
             102 in free list (10 min, 200 max allowed)
             6829533 hits, 1432 misses, 483 trims, 497 created
        Big buffers, 1524 bytes (total 90, permanent 90):
             90 in free list (5 min, 300 max allowed)
             3403884 hits, 56 misses, 1 trims, 1 created
        Large buffers, 5024 bytes (total 5, permanent 5):
             5 in free list (0 min, 30 max allowed)
             49984 hits, 13 misses, 20 trims, 20 created
        Huge buffers, 18024 bytes (total 0, permanent 0):
             0 in free list (0 min, 4 max allowed)
             0 hits, 0 misses, 0 trims, 0 created
        5683 failures (0 no memory)

You can interpret them:

Total   Number of buffers of that size that exist. 

Free    Number of free buffers.

Max     Maximum size that the free list can grow to before we start
        throwing them away.

Hit     Buffer got used.

Miss    Someone requested a buffer and we had to go carve it up out of
        free memory.  If we couldn't because we were at interrupt
        level, it's also an allocation failure.  If we couldn't
        because we were out of memory, then it's also a ``no memory''

Trim    There are more free buffers on the free list than there need
        to be and we threw some away.

Create  Number of buffers we created after a miss.


From: Question 22
Date: 22 April 1996
Subject: How should I restrict access to my router?

Many admins are concerned about unauthorized access to their routers
from malicious people on the Internet; one way to prevent this
is to restrict access to your router on the basis of IP address.

Many people do this, however it should be noted that a significant number
of network service providers allow unrestricted access to their routers
to allow others to debug, examine routes, etc. If you're comfortable doing
this, so much the better, and we thank you!

If you wish to restrict access to your router, select a free IP access
list (numbered from 1-100) -- enter ``sh access-list'' to see those
numbers in use.

        yourrouter#sh access-list
        Standard IP access list 5
            permit, wildcard bits

Next, enter the IP addresses you wish to allow access to your router
from; remember that access lists contain an implicit "deny everything"
at the end, so there is no need to include that. In this case, 30
is free:

        yourrouter#conf t
        Enter configuration commands, one per line.  End with CNTL/Z.
        yourrouter(config)#access-list 30 permit

(This permits all IP addreses in the network, i.e. 172.30.*.*).
Enter multiple lines for multiple addresses; be sure that you don't
restrict the address you may be telnetting to the router from.

Next, examine the output of ``sh line'' for all the vty's (Virtual ttys)
that you wish to apply the access list to. In this example, I want
lines 2 through 12:

        yourrouter#sh line
         Tty Typ    Tx/Rx    A Modem  Roty AccO AccI  Uses    Noise   Overruns
           0 CTY             -    -      -    -    -     0        0        0/0
           1 AUX  9600/9600  -    -      -    -    -     1  3287605        1/0
        *  2 VTY  9600/9600  -    -      -    -    7    55        0        0/0
           3 VTY  9600/9600  -    -      -    -    7     4        0        0/0
           4 VTY  9600/9600  -    -      -    -    7     0        0        0/0
           5 VTY  9600/9600  -    -      -    -    7     0        0        0/0
           6 VTY  9600/9600  -    -      -    -    7     0        0        0/0
           7 VTY  9600/9600  -    -      -    -    7     0        0        0/0
           8 VTY  9600/9600  -    -      -    -    7     0        0        0/0
           9 VTY  9600/9600  -    -      -    -    7     0        0        0/0
          10 VTY  9600/9600  -    -      -    -    7     0        0        0/0
          11 VTY  9600/9600  -    -      -    -    -     0        0        0/0
          12 VTY  9600/9600  -    -      -    -    -     0        0        0/0

Apply the access list to the relevant lines:

        yourrouter#conf t
        Enter configuration commands, one per line.  End with CNTL/Z.
        yourrouter(config)#line 2 12
        yourrouter(config-line)# access-class 30 in
        yourrouter(config-line)# ^Z

(This apply access list 30 to lines 2 through 12. It's important to
restrict access to the aux port (line 1) if you have a device (such
as a CSU/DSU) plugged into it.a) 

Be sure to save your configuration with ``write mem''.

Please note that access lists for incoming telnet connections do NOT
cause your router to perform significant CPU work, unlike access lists
on interfaces.


From: Question 23
Date: 1 November 1994
Subject: What can I do about source routing?

What *is* source routing?

Soure routing is an IP option which allows the originator of a packet
to specify what path that packet will take, and what path return packets
sent back to the originator will take. Source routing is useful when the
default route that a connection will take fails or is suboptimal for some
reason, or for network diagnostic purposes. For more information on
source routing, see RFC791.

Unfortunately, source routing is often abused by malicious users on
the Internet (and elsewhere), and used to make a machine (A), think
it is talking to a different machine (B), when it is really talking to
a third machine (C). This means that C has control over B's ip address
for some purposes.

The proper way to fix this is to configure machine A to ignore
source-routed packets where appropriate. This can be done for most
unix variants by installing a package such as Wietse Venema,
<>,'s tcp_wrapper:

For some operating systems, a kernel patch is required to make this
work correctly (notably SunOS 4.1.3). Also, there is an unofficial
kernel patch available for SunOS 4.1.3 which turns all source routing
off; I'm not sure where this is available, but I believe it was posted
to the firewalls list by Brad Powell soimetime in mid-1994.

If disabling source routing on all your clients is not posssible, a
last resort is to disable it at your router. This will make you unable
to use ``traceroute -g'' or ``telnet @hostname1:hostname2'', both
of which use LSRR (Loose Source Record Route, 2 IP options, the first
of which is a type of source routing), but may be necessary for some.
If so, you can do this with

        foo-e-0#conf t
        Enter configuration commands, one per line.  End with CNTL/Z.
        foo-e-0(config)#no ip source-route

It is somewhat unfortunate that you cannot be selective about this; it
disables all forwarding of source-routed packets through the router,
for all interfaces, as well as source-routed packets to the router
(the last is unfortunate for the purposes of ``traceroute -g'').


From: Question 24
Date: 22 April 1996
Subject: Is there a block of private IP addresses I can use?

Yes there is, however whether you wish to do so is an issue of
some debate.

You could consult:

1627 Network 10 Considered Harmful (Some Practices Shouldn't be
     Codified). E. Lear, E. Fair, D. Crocker & T. Kessler. June 1994.
     (Format: TXT=18823 bytes)

1918 Address Allocation for Private Internets. Y. Rekhter, B.
     Moskowitz, D. Karrenberg, G. J. de Groot & E. Lear. February 1996.
     (Format: TXT=22270 bytes) (Obsoletes RFC1627, RFC1597) (Also BCP0005)

In any event, RFC 1918 documents the allocation of the following
addresses for use by ``private internets'':        -      -     -

Most importantly, it is vital that nothing using these addresses
should ever connect to the global Internet, or have plans to do so.
Please read the above RFCs before considering implementing such
a policy.

As an additional note, some Internet providers provide network-management
services, statistics gathering, etc. It is unlikely (if at all possible)
that they would be willing to perform those services if you choose to
utilize private address space.

With the increasing popularity and reliability of address translation
gateways, this practice is becoming more widely accepted. Cisco has acquired
Network Translation, who manufacture such a product. It is now available as
the Cisco Private Internet Exchange. With it, you can use any addressing you
want on your private internet, and the gateway will insure that the invalid
addresses are converted before making out onto the global Internet. It also
makes a good firewall. Information on this product is available at


From: Question 25
Date: 18 April 1995
Subject: Is DHCP supported?

DHCP, the Dynamic Host Configuration Protocol (RFC1533), is essentially
a more extended and flexible version of BOOTP, which allows configuration
parameters and other control information to be carried to hosts.

Forwarding of DHCP packets (to a DHCP server elsewhere in the network) is
supported in 9.21(4) and 10.0(3), as well as later releases.


From: Question 26
Date: 18 April 1995
Subject: Where can I get cisco documentation?

Cisco no longer distributes printed documentation with their routers;
instead, they distribute a CDROM.

Paper documentation may be purchased, however if you purchase a
support contract, documentation is free.

Cisco documentation is also available on the web -- if you have
a fast Internet conneciton this may be more useful
than the CD. Try:


From: Question 27
Date: 18 April 1995
Subject: What's the latest software for the CSC/3?

The last supported release on the CSC/3 is 9.1(15). cisco
does not plan to release further software for the CSC/3.


From: Question 28
Date: 19 May 1995
Subject: What IP routing protocol should I use?

This is a really complicated question, and a full answer
is beyond the scope of this document. Here are the beginnings
of an answer.

Note that Hello is no longer shipped with cisco routers, and that EGP has been
declared Historical (and thus obsolete) by the IETF. Don't use them.

Protocol        RIP     HELLO  IGRP   OSPF    EIGRP  IS-IS  EGP     BGP4
Type            IGP     IGP    IGP    IGP     IGP    IGP    EGP     EGP
Algorithm       DV      DV     DV     SPF     DUAL   SPF    DV      PV
Metrics         Hopcnt  Delay  Speed  Arb.    Speed  Arb.   Policy  Policy
Convergence     Slow    Unstb  Mdt    Fast    Fast   Fast   Slow    Fast
Standard?       IETF    No     No     IETF    No     ISO    Hist.   IETF
Complexity      Simple  Simple Simple Complx  Complx Complx Simple  Complx
Multipath?      Yes     Yes    Yes    Yes     Yes    Yes    Yes     [*]
Var-netmask?    No      No     No     Yes     Yes    Yes    No      YES


IGP = interior gateway protocol, used to build routing tables within an AS.
EGP = exterior gateway protocol, used to communicate reachability
information between AS's.

DUAL = DV with diffusing update algorithm (Garcia-Luna-Aceves et al)
DV   = Distance Vector (Bellman-Ford)
PV   = "Path Vector"
SPF   = Shortest-path-first (Dijkstra)


A metric is how the protocol measures the network to determine the
"best" path.

"Speed" refers typically to link speed, not available bandwidth.
"Arb." indicates that the metrics are arbitrary and configurable.

HELLO tried to use available bandwidth by monitoring round-trip delay,
but was not generally successful at this.

Metrics are not directly exchangable when redistributing routing
information from one protocol to another. IGRP and EIGRP use
compatible and automatically convertable metrics.


Qualitatively, convergence measures how fast routers using this
protocol will adapt to changes in the topology of the network.

"Unstb" indicates a protocol which in general never decided on a
stable configuration but continually oscillated between alternatives.


An observation of how complex the protocol is to implement.


Multipath indicates whether the protocol support and transport
multiple equal- or different- cost pathways across between endpoints?

[*] indicates that BGP4 supports multipath for IBGP (Internal BGP, a
full mesh of all border routers within an AS), but not for EBGP
(External BGP).

Variable netmask (Var-netmask)

Indicates whether the protocol allows for and transports different
masks for the subnets of a routed network.


From: Question 29
Date: 18 April 1995
Subject: How do I interpret the output of ``show version''?

Typing ``show version'' or ``show hardware'' yields a response like:>sh version
        Cisco Internetwork Operating System Software 
        IOS (tm) GS Software (GS7), Experimental Version 10.2(11829) [pst 113]

System-type (imagename) Version major.minor(release.interim)[who] Desc

System-type:  type of system the software is designed to run on.
imagename:  The name of the image.  This is different (slightly) for
        run-from-rom, run-from-flash, and run-from-ram images, and also
        for subset images which both were and will be more common.
"Version": text changes slightly.  For example, if an engineer gives you
        a special version of software to try out a bug fix, this will say
        experimental version.
Major: Major version number.  Changes (in theory) when there have been
        major feature additions and  changes to the softare.
Minor: minor version number.  Smaller but still signficant feature added.
        (in reality, cisco is not very sure what the difference between
         "major" and "minor" is, and sometimes politics gets in the way,
        but either of these "incrementing" indicates feature additions.)
        EXCEPT: 9.14, 9.17, and 9.1 are all somewhat similar.  9.1 is
        the base, 9.14 adds specical feature for low end systems, 9.17
        added special features specific the high end (cisco-7000)  This
        was an experiment that we are trying not to repeat.
release: increments (1 2 3 4 ...) for each maintenance release of released
        software.  Increments for every compile in some other places.
interim: increments on every build of the "release tree", which happens
        weekly for each release, but is only made into a generically
        shipping maintenance release every 7 to 8 weeks or so.
[who]:  who built it.  Has "fc 1" or similar for released software.
        has something like [billw 101] for test software built Bill
        Westfield (
Desc:   additional description.

The idea is that the image name and version number UNIQUELY identify
a set of sources and debugging information somewhere back at cisco,
should anything go wrong.

        Copyright (c) 1986-1995 by cisco Systems, Inc.
        Compiled Thu 09-Mar-95 23:54 by tli
        Image text-base: 0x00001000, data-base: 0x00463EB0

Copyright, compilation date (and by whom), as well as the
starting address of the image.
        ROM: System Bootstrap, Version 5.0(7), RELEASE SOFTWARE
        ROM: GS Software (GS7), Version 10.0(7), RELEASE SOFTWARE (fc1)

The version of ROM bootstrap software, and the version of IOS
in ROM.
           uptime is 2 weeks, 4 days, 18 hours, 38 minutes
        System restarted by reload

How long the router has been up, and why it restarted.

        System image file is "sse-current", booted via flash

How the router was booted.
        RP (68040) processor with 16384K bytes of memory.

Type of processor.

        G.703/E1 software, Version 1.0.
        X.25 software, Version 2.0, NET2, BFE and GOSIP compliant.
        Bridging software.
        ISDN software, Version 1.0.

Various software options compiled in.

        1 Silicon Switch Processor.
        2 EIP controllers (8 Ethernet).
        2 FSIP controllers (16 Serial).
        1 MIP controller (1 T1).
        8 Ethernet/IEEE 802.3 interfaces.
        16 Serial network interfaces.
        128K bytes of non-volatile configuration memory.
        4096K bytes of flash memory sized on embedded flash.

Hardware configuration.
        Configuration register is 0x102

Lastly, the "configuration register", which may be set via
software in current releases...


From: Question 30
Date: 22 April 1996
Subject: What is the maximum number of Frame Relay PVCs?

This is covered fairly thoroughly in Product Info/Product
Bulletin/Frame Relay Broadcast Queue, Cisco Product Bulletin # 256,
available on CIO.

Via the web (requires CIO username and pasword)

An excerpt:

(Virtual Interfaces)

   It should be noted that in the IOS (Internetworking Operating System)
   10.0 software there is a limit of 256 Virtual and physical
   interfaces. Hence, if each DLCI is given its own virtual interface,
   the router is limited to 256 DLCIs. This restriction is expected to be
   removed in a future release.
   In most scenarios, it is not necessary that each DLCI have its own
   Virtual Interface. In particular, IP has the facility which allows
   disabling of split-horizon routing and hence does not require Virtual
   Interfaces to support partial mesh topologies.
(Appendix 1: How many DLCIs Can Cisco Support on an Interface?)

   This question is similar to the question of how many PCs can you put
   on an Ethernet. In general, you can put a lot more than you should
   given performance and availability constraints.
   When dimensioning a router in a large network, the following issues
   should be considered:
   DLCI Address Space: The only hard limits are the roughly 1000 DLCI
   limit due to the 10 bit DLCI address space in the Frame Relay frame
   LMI Status Update: The LMI protocol requires that all status reports
   fit into a single packet and generally limits the number of DLCIs to
   less than 800.

  Max DLCIs (approx) = (MTU -20)/5,
        where MTU is the MTU size in bytes on the Frame Relay link.

   Broadcast Replication: When sending, the router must replicate the
   packet on each DLCI and this causes congestion on the access link. The
   Broadcast Queue reduces this problem. In general, the network should
   designed to keep the routing update load to below 20 percent of the
   access lines speed. It is also important that memory requirements for
   the Broadcast Queue be considered. A good technique to reduce this
   restriction is the use of default route or extending the update
   Broadcast Receipt: When receiving, the router must receive updates
   from the network. The issue here is that the upstream switch can be
   overloaded and drop packets. When routing updates are dropped, routing
   instability occurs. Again, the receiving routing update load should be
   kept to less than 20 percent of the access link speed and preferably
   lower. Where very high speed links are used, a limit of 128 Kbit/s
   worth of routing updates is recommended.
   Routing Stability: When using a link state protocol to reduce the
   update traffic, the dimensioning should be done assuming the periodic
   update process and the worst case for Link State Updates (i.e.,
   assuming link and power instability). Dimensioning should not be based
   on the Hello traffic. As a rule of thumb, dimension assuming a
   distance vector protocol, but assume that extra bandwidth is available
   for user data.
   User Data Traffic: Clearly, the number of DLCIs is dependent on the
   traffic on each DLCI and the performance requirements to be met. In
   general, Frame Relay accesses should be run at lower loads than
   router-to-router links since the prioritisation capabilities are not
   as strong in many cases and in general the marginal costs of
   increasing access link speed are lower than with dedicated lines.
   Many of the issues covered here are included in the Internet Design
   Guide manual that Cisco provides.


The limit of 256 PVCs goes away in IOS 11.1. I think the number is now
something like 1024 per router or some even more ludicrous number. There are
still lots of reasons you never want to do that. ;-)
The limit of 256 PVCs goes away in IOS 11.1. I think the number is now
something like 1024 per router or some even more ludicrous number. There are
still lots of reasons you never want to do that. ;-)


From: Question 31
Date: 18 April 1995
Subject: How much memory is necessary to telnet to a cisco router?

In order to login to a cisco router, it needs to have at least 64k
of contiguous free memory.


From: Question 32
Date: 18 April 1995
Subject: Where can I purchase flash RAM?

There are two varieties:

        MEM-1X8F                8meg
        MEM-2X8F                16meg

*******************************     2500       ********************************
*******************************   8M Flash     ********************************
--------        ---
MEM-1X8F         1
MEM-2X8F         2

Part Number: 16-0975-01      
Description: IC,FEPROM,  2Mx32,100ns,SIM80     SC: P  REV: A0 S/UM: EA P/UM: EA
       --- -------------------- ---------- ------------------------------
   1-    1 SM732C2000B-10       KITTING01  SMART MODULE

Smart Modular is located in Freemont, California.

For small orders, Smart Modular recommends you contact:

	PC Complete

They carry both	Flash RAM and DRAM.


From: Question 32
Date: 19 May 1995
Subject: When are static routes redistributed?

In the simple case, any static route *in the routing table* is
redistributed if the ``redistribute static'' command is used, and some
filter (set with either ``route-map'' or ``distribute-list out'')
doesn't filter it out.

Whether the static route gets into routing table depends on:

	Whether the next hop address is reachable (if you use
	static route pointing to a next hop)
	Whether the interface is up (if you use static route
	pointing to an interface). 

If one of these is true, an attempt is made to add the route to the
routing table; whether that succeeds depends on the administrative
distance of the route -- a lower administrative distance (the route
is "closer") than a preexisting route will cause the preexisting route
to be overwritten.


From: Question 33
Date: 19 May 1995
Subject: When is the next hop of a route considered ``reachable''?

When a static route is added, or during an important event (eg:
interface up/down transition), the next hop for a route is looked up
from the routing table (i.e. recursive routing).

As a consequence, if a route which is depended upon for evaluation
of the next hop of a static route goes away, a mechanism is required
to remove that (now-invalid) static route.

Scanning all static routes each time the routing table changes is
too expensive, so instead, a period timer is used. One a minute, static
routes are added and removed from the routing table based on the routes
they depend upon.

It should be noted that a particular static route will be reevaluated
when its interface transitions up or down.


From: Question 35
Date: 22 April 1996
Subject: How do name and phone number of ``dialer map'' interfere?

How do name and phone number of `dialer map' interfere?

We use the telephone number first actually.  If the
caller id matches the telephone number to call, then you don't need the
'name' parameter with a phone number.

I realized that the above is ambiguous, so let's do this.  You have:

  dialer map ip x.x.x.x name <param1> <phone-num>

<param1> is used for incoming authentication.  It can be either the hostname,
for PAP and CHAP, or it can be a number as returned by caller id.  If this
is not there, and it is an imcoming call, and there is caller id, we will
compare against <phone-num> to see if that matches.

Not sure I've been clear here.


From: Question 36
Date: 22 April 1996
Subject: What's the purpose of the network command?

>*  what is the real purpose of the network subcommand of
>   router commands?  When do I not want to include a network
>   I know about?

The real purpose of the 'network' sub-command of the router commands is to
indicate what networks that this router is connected to are to be
advertised in the indicated routing protocol or protocol domain. For
example, if OSPF and EIGRP are configured, some subnets may be advertised
in one and some in the other. The network command enables one to do this.

An example of such a case is a secure subnet. Imagine the case where a set
of subnets are permitted to communicate within a campus, but one of the
buildings is intended to be inaccessible from the outside. By placing the
secure subnet in its own network number and not advertising the number, the
subnet is enabled to communicate with other subnets on the same router, but
is unreachable from any other router, barring static routes. This can be
extended by using a different routing protocol or routing protocol domain
for the secure network; subnets on the various routers within the secure
domain are mutually reachable, and routes from the non-secure domain may be
leaked into the secure domain, but the secure domain is invisible to the
outside world.


From: Question 37
Date: 22 April 1996
Subject: What is VLSM? 

A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing
resources to subnets according to their individual need rather than some
general network-wide rule. Of the IP routing protocols supported by Cisco,
OSPF, Dual IS-IS, BGP-4, and EIGRP support "classless" or VLSM routes.

Historically, EGP depended on the IP address class definitions, and
actually exchanged network numbers (8, 16, or 24 bit fields) rather than IP
addresses (32 bit numbers); RIP and IGRP exchanged network and subnet
numbers in 32 bit fields, the distinction between network number, subnet
number, and host number being a matter of convention and not exchanged in
the routing protocols. More recent protocols (see VLSM) carry either a
prefix length (number of contiguous bits in the address) or subnet mask
with each address, indicating what portion of the 32 bit field is the
address being routed on.

A simple example of a network using variable length subnet masks is found
in Cisco engineering. There are several switches in the engineering
buildings, configured with FDDI and Ethernet interfaces and numbered in
order to support 62 hosts on each switched subnet; in actuality, perhaps
15-30 hosts (printers, workstations, disk servers) are physically attached
to each. However, many engineers also have ISDN or Frame Relay links to
home, and a small subnet there. These home offices typically have a router
or two and an X terminal or workstation; they may have a PC or Macintosh as
well. As such, they are usually configured to support 6 hosts, and a few
are configured for 14. The point to point links are generally unnumbered.

Using "one size fits all" addressing schemes, such as are found in RIP or
IGRP, the home offices would have to be configured to support 62 hosts
each; using numbers on the point to point links would further compound the
address bloat.

One configures the router for Variable Length Subnet Masking by configuring
the router to use a protocol (such as OSPF or EIGRP) that supports this,
and configuring the subnet masks of the various interfaces in the 'ip
address' interface sub-command. To use supernets, one must further
configure the use of 'ip classless' routes.


From: Question 38
Date: 22 April 1996
Subject: What are some methods for conserving IP addresses for serial lines?

VLSM and unnumbered point to point interfaces are the obvious ways.

The 'ip unnumbered' subcommand indicates another interface or sub-interface
whose address is used as the IP source address on messages that the router
originates on the unnumbered interface, such as telnet or routing messages.
By doing this, the router is reachable for management purposes (via the
address of the one numbered interface) but consumes no IP addresses at all
for its unnumbered links.

When a serial ip interface connects several sites, as an SMDS link might,
then the use of an appropriate subnet mask (and a routing protocol that can
make good use of the information) will minimize address consumption.


From: Question 39
Date: 23 April 1996
Subject: Why do some ip addresses get rejected?

How come my cisco router doesn't accept an address like:
        "ip address"
or      "ip address"

When "subnetting" of IP networks was first sanctioned by the IETF, the first
and last subnets (the all zeros subnet and all ones subnet) were reserved for
rather obscure uses and because of the confusion that would be caused with
routing protocols that don't carry net mask information.  It was technically
illegal to place hosts or routers on those two subnets.

Several hosts and most other vendor's router products have problems operating
with the reserved subnets,  so their use is discouraged.  However, in 1995,
the IETF removed the restrictions on the use of these reserved subnets as part
of the classless routing effort.

If you would like to use the reserved subnets, simply add the line
"ip subnet-zero" to your cisco configuration.

You might consider adding "ip subnet-zero" to all your configurations as a
metter of course, to avoid being bitten by this in the future.


From: Question 40
Date: 27 April 1996
Subject: How do 4xxx serial numbers correspond to models?

              show version      serial #        Label
4000            Rev A0          440xxxxx        C4000
4000M           Rev B0          445xxxxx        C4000
4500                            450xxxxx        C4500
4500M                           455xxxxx        C4500
4700                            470xxxxx        C4700


From: Question 41
Date: 28 April 1996
Subject: Where can I find more info on TACACS+

In addition to sundry cisco documentation and ftp-able
info, there exists a TACACS+ mailing list.

For more information, see


From: Question 99
Date: 19 May 1995
Subject: Acknowledgements.

The following people contributed to this FAQ, and their contributions
are greatly appreciated, both questions and answers (in alpha order):

        Arpakorn Boonkongchuen <>
        Robert Kiessling <>
        "Ronnie B. Kon" <>
        Alain Martineau <> (Barton F. Bruce / CCA)
        Bill Miskovetz <>
        Charley Kline <>
        Dave Katz <>
        Eriks Rugelis <eriks@YorkU.CA>
        Howard C. Berkowitz, PSC International, <>
        Jim Forster <>
        John Wright
        Pete Siemsen <>
        Phillip Remaker <>
        Ran Atkinson <>
        Robert Kiessling <>
        Sanjay Rungta~ <>
        Sean McGrath <SEAN@oak.his.ucsf.EDU>
        Srinivas Vegesna <>
        Steve Cunningham <>
        Warren Lavallee <>
        William "Chops" Westfield <> (Ran Atkinson) (Bruce Pinsky) ($ Burkhard Kohl) (Fred Baker) (Jerry Anderson) (John Hawkinson) (John Wright) (John Temples) (Paul Ferguson) (Peter Radig) (Tony Li) (Thomas R. Kimpton)
        vikas@Tudor.Com (Vikas Aggarwal) (Jim Warner)

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