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Help with Underrun errors

I am experiencing a high amout of underrun packet drops for egress traffic out my inside interface on a ASA 5520 (avg 10 packet drops p/sec).  I am not completely clear on the understanding of underruns, but my understanding is that the harware is working faster the the software processing.  I am hoping someone can give me a few suggestions as to where I should look for my problems, or what typically leads to this type of performance.  I am sure traffic patterns matter in this type of situation, and I have about 50Mbps sustained traffic ingress on my Untrusted port, and 60-70 MBps ingress on my DMZ port.  Traffic peaks at higher rates, those are just daily averages.  Do I need to move up models?  Thanks so much for your help.


Cisco Employee

Re: Help with Underrun errors

An Overrun is when an incoming (ingress) packet hits the PIX's NIC, and the rx ring is full.  This is generally caused by elevated CPU, or cpu hogs or infected hosts.

An Underrun is when part of the packet is in the tx ring, and the driver starts transmitting it on the wire, but is unable to get the remaining part of the packet by the time it has finished transmitting the first part.

Pls. take a look at CSCso66911    ASA55x0 GE output stuck and underrun errors


Resolved in 008.000(005)          008.000(004.005)          007.002(004.009)

Also this one CSCsz58391    Burst Traffic causes underrun when QoS shaping is enabled on ASA

Cisco Employee

Re: Help with Underrun errors

It seems you might be oversuscribing your box.

If you see overrung/underruns and have 40-70Mbps with higher bursts it is likely to be overloaded when there are traffic spikes.

Keep an eye on your cpu too and try to alleviate it if possible. If your cpu is constantly high due to traffic you might need more hardware.

I hope it helps.


Community Member

Re: Help with Underrun errors

PK -

Thanks for the suggestion.  I was leaning the same way.  About 85% of the traffic is web, and I don't have any sort of proxy solution in place.  Probably try to mitigate it with a couple of Squid Servers to see if I can get the load down.

Thanks again.

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