WAAS -- Packets Fragmented

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Feb 26th, 2010
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We are seeing excessive hitcounts against packets fragmented:

Packets fragmented for egress:  776
Packet pullups needed:   778

GRE fragments redirected:  779

We tried lowing the MTU to 1400 on both WAAS interfaces and are still getting this. Is this normal?

WCCP Branch Setup:


WCCP Negotiated Return

Connected to ISR

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Zach Seils Fri, 02/26/2010 - 10:26
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How long has WAAS been active at this site?  It's possible that the fragmentation is associated with connections that were already established when WAAS was brought into service.


johng231 Fri, 02/26/2010 - 10:35
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The one I mentioned before was just configured for 1400 MTU size on gig 1/0 and gig 2/0. I'll wait a little bit and see if it is still incrementing. I just noticed it increment right after I brought WCCP back online. Now for our other locations, we have it incrementing all the time, and they're using the default MTU size of 1500. Would you want to modify it for 1400 or modify the MSS value from 1432 to say 1300?

Other location where the packets fragmented are high:

GRE fragments redirected:                     114041

Packets fragmented for egress:            114041
Packet pullups needed:                         114082

Zach Seils Fri, 02/26/2010 - 10:38
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I wouldn't recommend modifying the MTU.  It should be left at 1500 on the WAAS devices.  For TCP traffic where WAAS sees the connection setup (SYN, SYN-ACK), the TCP MSS should be adjusted to account for the WCCP GRE overhead.

Can you share a copy of the configuration where WCCP is enabled?


Zach Seils Mon, 03/01/2010 - 06:51
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Thanks for the information.  On one of the WAEs with incrementing fragments, can you please capture the fragments with the following commands:

tcpdump -i any -s0 -w frags.cap ((ip[6:2] > 0) and (not ip[6] = 64))

While this capture is running, you can check the fragments statistics through another terminal session.  Once they have incremented, please download the frags.cap file from the WAE and send it to me.




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