[c-nsp] overruns on a lightly loaded ethernet port

[Burton Windle]

I have a 3640, running 12.2(28), connected directly to a 6509 via 100bT. 
Every now and then, I see a large burst of 'overruns' reported on the
3640's FastEth int; however, this connection is very lightly used 
(~2megabit/sec average). I am under the impression that overruns on an 
Ethernet interface happen when one side tries to send too fast, but this
interface never pushes more than 5 megabit/sec.

5-3640-WAN>sh proc cpu
CPU utilization for five seconds: 17%/16%; one minute: 12%; five minutes: 17%

!
interface FastEthernet0/0
  description ***6509_PORT2/1***
  ip address 192.168.1.4 255.255.255.0
  ip route-cache flow
  speed 100
  full-duplex

5-3640-WAN>sh int fastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
   Hardware is AmdFE, address is 0006.2889.e301 (bia 0006.2889.e301)
   Description: ***5-6509-1_PORT2/1***
   Internet address is 192.168.1.4/24
   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 2d01h
   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 1304000 bits/sec, 592 packets/sec
   5 minute output rate 1476000 bits/sec, 598 packets/sec
      62322869 packets input, 4032355232 bytes
      Received 157135 broadcasts, 0 runts, 0 giants, 0 throttles
      750 input errors, 0 CRC, 0 frame, 750 overrun, 0 ignored
      0 watchdog
      0 input packets with dribble condition detected
      60861002 packets output, 108203385 bytes, 0 underruns
      0 output errors, 0 collisions, 0 interface resets
      0 babbles, 0 late collision, 0 deferred
      0 lost carrier, 0 no carrier



5-6509-1> sh port 2/1
Port  Name                 Status     Vlan       Duplex Speed Type
----- -------------------- ---------- ---------- ------ ----- ------------
  2/1  ***5-3640-WAN***     connected  1          full   100   10/100BaseTX

Port  AuxiliaryVlan AuxVlan-Status     InlinePowered     PowerAllocated
                                    Admin Oper   Detected mWatt mA @42V
----- ------------- -------------- ----- ------ -------- ----- --------
  2/1  none          none           off   off    no       0     0


Port  Security Violation Shutdown-Time Age-Time Max-Addr Trap     IfIndex
----- -------- --------- ------------- -------- -------- -------- -------
  2/1  disabled  shutdown             0        0        1 disabled      59

Port  Num-Addr Secure-Src-Addr   Age-Left Last-Src-Addr     Shutdown/Time-Left
----- -------- ----------------- -------- ----------------- ------------------
  2/1         0                 -        -                 -        - 
-

Port     Broadcast-Limit Multicast Unicast Total-Drop
-------- --------------- --------- ------- --------------------
  2/1                   -         -       -                    0

Port  Send FlowControl  Receive FlowControl   RxPause    TxPause
       admin    oper     admin     oper
----- -------- -------- --------- ---------   ---------- ----------
  2/1  off      off      off       off         0          0

Port  Status     Channel              Admin Ch
                  Mode                 Group Id
----- ---------- -------------------- ----- -----
  2/1  connected  off                      4     0

Port  Align-Err  FCS-Err    Xmit-Err   Rcv-Err    UnderSize
----- ---------- ---------- ---------- ---------- ---------
  2/1           0          0          0          0         0

Port  Single-Col Multi-Coll Late-Coll  Excess-Col Carri-Sen Runts     Giants
----- ---------- ---------- ---------- ---------- --------- --------- ---------
  2/1           0          0          0          0         0         0         0

Port  Last-Time-Cleared
----- --------------------------
  2/1  Fri Apr 1 2005, 22:16:01



Any idea what would be causing these?

-- 
Burton Windle                           bwindle at fint.org