[c-nsp] Overruns on 2621 FE interface

[Chris Roberts]

I have a 2621 with 2 x VWIC-1MFT-G703 cards connected to two unframed E1
circuits, load-balancing via ip load-sharing per-packet with CEF enabled.

This particular router is showing overruns on its FastEthernet interface -
I'm trying to understand how this could happen. Cisco.com has:
overrun
 Shows the number of times that the receiver hardware was incapable of
handing received data to a hardware buffer because the input rate exceeded
the receiver's capability to handle the data. 
 
However, this router is barely even stretched:
CPU utilization for five seconds: 9%/9%; one minute: 12%; five minutes: 12%

And the interface rxload/txload and line rates are very low, so I can't see
how the receiver can be stretched, the overruns are happening even at low
line rates:
FastEthernet0/0 is up, line protocol is up
  Hardware is PQUICC_FEC, address is 0011.216c.977c (bia 0011.216c.977c)
  Description: *** Customer LAN interface ***
  Internet address is 10.1.8.1/22
  MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec,
     reliability 255/255, txload 3/255, rxload 3/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 100Mb/s, 100BaseTX/FX
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:02:55
  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 1329000 bits/sec, 639 packets/sec
  5 minute output rate 1234000 bits/sec, 510 packets/sec
     122772 packets input, 33312820 bytes
     Received 2406 broadcasts, 0 runts, 0 giants, 0 throttles
     10 input errors, 0 CRC, 0 frame, 10 overrun, 0 ignored
     0 watchdog
     0 input packets with dribble condition detected
     96532 packets output, 29677703 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

Any ideas? Dodgy NIC on the LAN/connecting switch, or could it be that the
E1 cards are stretched meaning that CEF can't switch the packet into a
transmit buffer on the E1 cards?

Cheers,
Chris.

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