[c-nsp] Troubleshooting Lag between GigE interfaces

[Paul Stewart]

We have a 7513 and a 6509 connected via GigE SX Fiber.  Frequently we
see "lag" on the connection lasting 5-10 seconds causing 60-80ms delay.

When I look at the interfaces I see the following:

7513

GigabitEthernet8/1/0 is up, line protocol is up
  Hardware is cyBus GigabitEthernet Interface, address is 0001.64ef.a108
(bia 0001.64ef.a108)
  Description: Gig Fiber to 6509
  Internet address is XXX.XXX.XXX.XXX/24
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 8/255, rxload 6/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full-duplex, 1000Mb/s, link type is autonegotiation, media type is SX
  output flow-control is XOFF, input flow-control is unsupported
  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 never
  Input queue: 3/75/913188/2076260 (size/max/drops/flushes); Total
output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  30 second input rate 26395000 bits/sec, 9394 packets/sec
  30 second output rate 35145000 bits/sec, 9900 packets/sec
     1790737826 packets input, 2276828279 bytes, 0 no buffer
     Received 1895463 broadcasts, 0 runts, 0 giants, 34543 throttles
     0 input errors, 0 CRC, 0 frame, 36 overrun, 0 ignored
     0 watchdog, 520626 multicast, 0 pause input
     0 input packets with dribble condition detected
     1655202857 packets output, 359511296 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 babbles, 0 late collision, 0 deferred
     2 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

6509

GigabitEthernet1/2 is up, line protocol is up (connected)
  Hardware is C6k 1000Mb 802.3, address is 0006.d65b.853d (bia
0006.d65b.853d)
  Description: Connection to 7513
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
     reliability 255/255, txload 8/255, rxload 11/255
  Encapsulation ARPA, loopback not set
  Full-duplex, 1000Mb/s, media type is SX
  input flow-control is off, output flow-control is on
  Clock mode is auto
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never
  Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops:
0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 43500000 bits/sec, 12035 packets/sec
  5 minute output rate 33545000 bits/sec, 11573 packets/sec
     5952975396 packets input, 2640933868846 bytes, 0 no buffer
     Received 1696859 broadcasts (68504 multicast)
     0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     0 watchdog, 0 multicast, 0 pause input
     0 input packets with dribble condition detected
     6088478711 packets output, 2200836239462 bytes, 0 underruns
     0 output errors, 0 collisions, 1 interface resets
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 PAUSE output
     0 output buffer failures, 0 output buffers swapped out

The 6509 looks nice and clean but the 7513 shows a tonne of buffer
issues it seems.  Is this a buffer issue that I should start trying to
tune or would something else be the actual cause do you think?

Thanks in advance,

Paul