[nsp] Strange multilink ppp issue with T1s

[Bill Wichers]

I've been having an odd problem with multilink ppp for a while now, and
google doesn't seem to hold the answer so I upgraded to here :-)

We have three T1 circuits on one end (a 7507) going into a PA-MC-4T1 in a
vip2-40, the other end consists of three channels on a DS3 in a PC-MC-2T3+
on another vip2-40 in one side of a 7576. Both routers have RSP4's running
IOS v12.2(6a).

What we are seeing is the the multilink bundle has large levels of packet
loss at times, and at other times none -- but all three T1 circuits are
clean at all times. The link seems to have unpredictable lag (and behaves
especially poorly in packet lag and loss when loaded) regardless of the
settings for fragmentation. The strangest part is that the 7507 end sees
the circuit frequently moving far more traffic than the bundles max
capacity of 4.6 Mb/s, while the other end sees more believeable traffic
numbers. The available bandwidth is reported correctly on both ends. I
included results of 'show ppp multilink' and 'show multilink1' for each
end. And we have tried the 7507 both with distributed CEF and without. No
difference... Fragmentation is disabled on both ends at the moment (and in
the 'show' info below).

Really hoping someone can provide a bit of insight since we can't seem to
find any info on this anywhere and have tried everything we can think of
to fix it.

     -Bill

[7507 begin]
troy1>show int multilink1
Multilink1 is up, line protocol is up
  Hardware is multilink group interface
  Internet address is x.x.x.x/30
  MTU 1500 bytes, BW 4608 Kbit, DLY 100000 usec,
     reliability 255/255, txload 27/255, rxload 135/255
  Encapsulation PPP, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 2 seconds on reset
  LCP Open, multilink Open
  Open: IPCP
  Last input 00:00:00, output never, output hang never
  Last clearing of "show interface" counters 4w3d
  Queueing strategy: fifo
  Output queue 0/40, 2069415 drops; input queue 0/75, 396 drops
  30 second input rate 2442000 bits/sec, 654 packets/sec
  30 second output rate 5698000 bits/sec, 1018 packets/sec
     692034835 packets input, 3061367520 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     1017262671 packets output, 1171349294 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 458140128 output buffers swapped out
     0 carrier transitions
troy1>
troy1>show ppp multilink

Multilink1, bundle name is sfld2core1
  Bundle up for 3w0d
  118828 lost fragments, 43010323 reordered, 5 unassigned
  120023 discarded, 120023 lost received, 1/255 load
  0x534FDF received sequence, 0x192D02 sent sequence
  Member links: 3 active, 0 inactive (max not set, min not set)
    Serial4/0/1:0, since 3w0d, last rcvd seq 534FE7
    Serial4/0/2:0, since 3w0d, last rcvd seq 534FDF
    Serial4/0/0:0, since 1w2d, last rcvd seq 534FE6
[7507 end]

[7576 begin]
sfld2core1>show int multilink1
Multilink1 is up, line protocol is up
  Hardware is multilink group interface
  Internet address is y.y.y.y/30
  MTU 1500 bytes, BW 4608 Kbit, DLY 100000 usec,
     reliability 255/255, txload 4/255, rxload 164/255
  Encapsulation PPP, loopback not set
  Keepalive set (10 sec)
  DTR is pulsed for 2 seconds on reset
  LCP Open, multilink Open
  Open: IPCP
  Last input 00:00:02, output never, output hang never
  Last clearing of "show interface" counters 3w0d
  Queueing strategy: fifo
  Output queue 0/40, 0 drops; input queue 0/75, 5 drops
  30 second input rate 2972000 bits/sec, 587 packets/sec
  30 second output rate 86000 bits/sec, 128 packets/sec
     335627760 packets input, 2579977254 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     124442011 packets output, 1807068880 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     0 output buffer failures, 0 output buffers swapped out
     0 carrier transitions
sfld2core1>
sfld2core1>show ppp multilink
Multilink1, bundle name is troy1
  Bundle up for 3w0d
  Bundle is Distributed
  5318 lost fragments, 3944230 reordered, 0 unassigned
  441 discarded, 441 lost received, 84/255 load
  0x193A1C received sequence, 0x555F07 sent sequence
  Member links: 3 active, 0 inactive (max not set, min not set)
    Serial1/0/0/25:0, since 3w0d, no frags rcvd
    Serial1/0/0/24:0, since 3w0d, no frags rcvd
    Serial1/0/0/26:0, since 1w0d, no frags rcvd
[7576 end]
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Waveform Technology
UNIX Systems Administrator