Eliminating High Transmit Discards on Internet Router

Unanswered Question
Jun 29th, 2007

We just deployed a new Internet router that is running a multi-homed BGP setup. Our company provides ICMP and SNMP performance monitoring, mostly via VPN connections.

What we are seeing is a very high rate of transmit discards. I believe these are the result of packet bursts from our monitoring servers. We are using QoS to tag our VPN and monitoring traffic and give it 1 Mbps priority. I have also tuned the buffers to eliminate buffer misses. However, the router is still discarding packets at a very high rate (12,000 per hour). This is causing quite a lot of timeouts for our monitoring systems and affects our reporting accuracy.

Does anyone have any suggestions on how to eliminate these discards. So far I haven't come up with a solution and Cisco TAC hasn't found one for me yet either.


Attached is a copy of the router config (with most of the Ip info cleared out). I'm also attaching a graph showing the discard rates. Again everything we've tried so far has been ineffective. I did force a failover to the AT&T T1 by shutting down the IMA group - that cut the discards in half, but we obviously can't run on our failover link full time.

I have this problem too.
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Pavel Bykov Fri, 06/29/2007 - 08:43

Are discards on input or output? Because QoS seems to be one way.

Also, can this be a burst issue? Try shaping and bandwidth commands, instead of priority.

e.g. (in class)

bandwidth 1024

shape average 1024

chrislisser Fri, 06/29/2007 - 08:48

I think it is a burst issue, and the discards are on output (transmit on the WAN circuits). I'll try the shaping and see what it does. Thanks!

chrislisser Fri, 06/29/2007 - 10:02

Unfortunately this did not work. I cannot use shaping on an IMA group. Any other suggestions???


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