communication from one vlan via router to jumbo vlan

vanoverschelde · ‎12-11-2008

If I have communication that is initiated from a vlan that has normal 1500 mtu to a vlan on which all devices have mtu 6144, will the router (in our case a 3750 or 6509) chop up the reply packet from the destinatino (in the jumbo frame vlan)?

Giuseppe Larosa · ‎12-11-2008

Hello Dominique,

the router should fragment the packets originated in the jumbo frame vlan.

This could cause performance issues (on the router if traffic is high).

This happens if DF bit in IPv4 header is not set.

TCP handshake should provide the MSS parameter so may be TCP based communications are able to detect the max MSS can be used on the path

Hope to help

Giuseppe

vanoverschelde · ‎12-11-2008

This happens by default in the router?

No need to configure some option?

Giuseppe Larosa · ‎12-11-2008

Hello Dominique,

IP fragmentation is part of IPv4 specifications so a router is supposed to be able to support fragmentation.

To be noted fragmentation reassembly is performed by the final endpoint and not by any other router on the path.

If the traffic volume is very high and the device is a low-end multilayer switch its cpu can rise usage to 100% (think of a C3560 or the likes)

Hope to help

Giuseppe

Hope to help

Giuseppe

vanoverschelde · ‎12-16-2008

Thank you.

Does the 6500 with sup720 support ip fragmentation in hardware?

I suppose the 3750-E supports it only in software?