Такое чувство, что в 4948 забыли добавить возможность повесить на l3 порт или vlan ip accounting (access-violation) и прочее. При всём при этом:

#sh ip accounting ?
  access-violations  show access violations in accounting database
  checkpoint         The checkpointed IP accounting database
  output-packets     show output packets in accounting database
  |                  Output modifiers
  <cr>

При этом:
(config-if)#ip ac?
access-group

Порт сам:
#sh int g1/1
GigabitEthernet1/1 is up, line protocol is up (connected)
  Hardware is Gigabit Ethernet Port, address is 001c.5875.79bf (bia 001c.5875.79bf)
  Description: --- L3 ---
  Internet address is 10.xx.xx.xx/24
  MTU 1500 bytes, BW 100000 Kbit, DLY 10 usec,
     reliability 255/255, txload 4/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, link type is auto, media type is 10/100/1000-TX
  input flow-control is off, output flow-control is off
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 5000 bits/sec, 7 packets/sec
  5 minute output rate 1916000 bits/sec, 573 packets/sec
  L3 in Switched: ucast: 10250 pkt, 1406812 bytes - mcast: 0 pkt, 0 bytes
  L3 out Switched: ucast: 15389538 pkt, 7349054958 bytes - mcast: 0 pkt, 0 bytes
     59758 packets input, 5581950 bytes, 0 no buffer
     Received 49133 broadcasts (41867 IP multicasts)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 input packets with dribble condition detected
     15396764 packets output, 7635599732 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier
     0 output buffer failures, 0 output buffers swapped out

Может я какую команду забыл дать, но блин на 3550 это работает!
sh ver:
Cisco IOS Software, Catalyst 4500 L3 Switch Software (cat4500-ENTSERVICESK9-M), Version 12.2(37)SG1, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Mon 30-Jul-07 13:47 by prod_rel_team
Image text-base: 0x10000000, data-base: 0x11863D38

#sh bootflash:
-#- ED ----type---- --crc--- -seek-- nlen -length- ---------date/time--------- name
1   .. image        EB4C1715  DC82CC   39 14189132 Jul 31 2007 18:43:12 +03:00 cat4500-entservicesk9-mz.122-37.SG1.bin
Может конечно спасёт релиз 12.2(40), но его я не нашёл :(