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New Member

3750E and mtu size

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?

7 REPLIES
Hall of Fame Super Blue

Re: 3750E and mtu size

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

New Member

Re: 3750E and mtu size

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?

Hall of Fame Super Blue

Re: 3750E and mtu size

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

Re: 3750E and mtu size

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

Re: 3750E and mtu size

if your server is windows, you can adjust the mtu size to improve tcp/ip performance

http://news.zdnet.co.uk/communications/0,1000000085,2123537,00.htm

Franco

New Member

Re: 3750E and mtu size

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?

Silver

Re: 3750E and mtu size

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

-----------------------------------------

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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