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

[Robert Larsen]

We've seen similar problems on FE interfaces on 2621 routers, and this is
most likely because the burst of traffic is too much for the input buffer on
the interface to deal with, so the router has to effectively tail drop
packets which causes the overruns in the interface stats.

Although these "low-end" routers (2600 and 3600 series) have FE interfaces,
they have fairly limited buffers and processing power, and as such could
never deal with traffic arriving at anything approaching line rate for any
length of time.

We've done some testing on a 2621XM a few years ago, and up to approx. 2Mbps
with a packet size of 192 bytes (or greater) it just about coped (CPU was
running at 80-100%), but any increase in traffic rate *or* decrease in
packet size just maxed the box out completely.

Cheers,

Rob.

-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of Burton Windle
Sent: 11 May 2005 22:38
To: cisco-nsp at puck.nether.net
Subject: [c-nsp] overruns on a lightly loaded ethernet port

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

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