Cisco 3750 and 3550 losing fragments

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Apr 7th, 2009
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I'm working with a system that has a 1 G-bit network connected through a 3750 switch to a 100 M-bit network connected through a 3550.

When sending large (50k) UDP packets on the 1 G-bit network, I can see that the UDP message is fragmented into 35 IP packets, however, on the 100 M-bit network, I only see the first 17 (up to offset 37000) fragments being received. All of the rest are missing.

I'm only sending a 50k packet every 100 ms.

I'm not sure if the 3750 or the 3550 are dropping fragments.

What's the best approach to diagnose, and are there any configuration parameters with regards to IP fragments on these switches (didn't see anything in the configuration documentation) or buffer space?


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vkapoor5 Tue, 04/14/2009 - 14:06
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If you configured QOS then disable your Qos. And verify any Framentation loss or not. After qos-disabling, no packet lost in Catalyst 3750 then re-enabled qos entered your recommended settings for "mls qos queue-set".

rtipartner Tue, 04/21/2009 - 10:09
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Thanks for your reply!

I'm actually working with a customer who has the setup that I described. They report that if they disable the mls qos, that the problem goes away. But they are concerned about running in this configuration.

How do you mean by "recommended settings for mls qos queue-set"?

Thanks again!


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