3750E and mtu size

Unanswered Question
Mar 25th, 2008

If a 3750E switch that is only doing layer 2 and not layer 3 receives a packet from a server that's larger than the 1514 byte packet, say 4098, what does it do with the large packet?

I have this problem too.
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rmoneal Tue, 03/25/2008 - 13:16

The size hasn't been changed. Would the switch respond to the server that the packet is to large and fragment or just drop the packet?

Jon Marshall Tue, 03/25/2008 - 13:17

As far as i know the switch will simply discard the packet because it acting at layer 2 so it will not be sending icmp messages for example.

Jon

francisco_1 Tue, 03/25/2008 - 14:41

Yes switch cannot fragment frames when forwarding between LANs of differing MTU sizes because data-link connections rarely have a mechanism for fragment reassembly at the receiver. Packets can be dropped because the Layer 2 switch cannot fragment the data.

The network layer (Layer 3) protocol determines the MTU from the data link layer (Layer 2) protocol and fragments the messages into the appropriate frame size, making the frames available to the lower layer for transmission without further fragmentation.

Franco

rmoneal Wed, 03/26/2008 - 07:00

You said the switch cannot fragment frames between LANS of differing MTU size. This particular setup the traffic is between a Citrix and SQL server on the same 3750E switch and the MTU size on the switch is default (1500). It appears the SQL server is sending larger frames than 1500, up to 4096, and the switch is passing it onto the Citrix server and the Citrix server is rejecting it and requesting it to be fragmented. Is this possible?

AJAZ NAWAZ Thu, 04/30/2009 - 06:03

Jon Marshall wrote:

Hi

Assuming you have no modified the system mtu size it will discard the packet. You can use the "system mtu jumbo " command to change the size to greater then 1514.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750e_3560e/software/release/12.2_44_se/command/reference/cli3.html#wp1948947

HTH

Jon

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And which counter would you expect to see incrementing (if any), each time a frame is discarded by the switch because it is too large?.

I would assume 'mtu exceeded' from the show interface controllers command. It's only that it doesn't seem to be doing much, at least with native IOS anyway.

Ajaz

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