MTU Problem

Unanswered Question
Dec 18th, 2007
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I have a 3750G switchA connected to a 3750G switchB acting as a router.

Servers on switchB are set to 9000 byte mtu.

Both switchA and switchB are set to:

system mtu jumbo 9000

Speed between servers on same vlan is good but intervlan traffic is horrible and drops.

My guess is its an mtu problem since the routing MTU is 1500 and jumbo is 9000.

How can I begin to troubleshoot? is there a debug command I can run on the 3750G router?

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cpubob Tue, 12/18/2007 - 13:20
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MTU may not necessarily be the problem. What type of transfer are you doing? Does it use tcp or udp? Remember TCP connection negotiate the MTU of a connection via the MSS field in the TCP 3-way handshake. Get a sniffer like wireshark and setup a monitor port for your server and capture the session. Look for duplicated packets, late or missing acks, or anything else that would be considered an error. Are you interfaces to the servers both full-duplex?


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