Change of MTU size from default to 1600

[peterledwidge]

Hi,

if you are looking to change the MTU size of a link, do you have to do this at the same time for all connections along a path between a source and destination?

In this case the source and destination are within the same AS.

Thanks

[sean]

Its best to have the same MTU between source and destination, but this is not always possible. If there are different MTU values, fragmentation will occur along the way unless the df bit (don't fragment) is set. If this is the case, then when an MTU limit is reached, the packet will be dropped. Just out of curiosity, for what reason are you trying to change to an MTU of 1600? Hope this helps.

[peterledwidge]

Hi Sean,

it's to do with a problem we have with

a checkpoint vpn client connecting back to it's gateway.

Routing path is ok and we've also tried to remove an ACL's between souce and destination.

[kamlesh.sharma]

hi peter,

if you are curious about changing an MTU across the AS is possible and will definately give you the network with more guranteed utilization bcos than there will not be any fragmentation in effect no DE bit no packet loss. but if the destination is somewhere in different AS number than you don't have the choice of changing MTU in your network only.

well if i were there in your place i will suggest you to keep this as it is in 1500 bcos it is universaly same for all router ethernet interface where all the host use to be connected and major work is done by only host in the network.

cosistant networ MTU is always prefered and standered is 1500 except the server form where you have server to server communication and are connected to gigabit ethernet which is having greater MTU size of upto 9000.

please rate if it helps.

kamlesh

[sean]

The checkpoint vpn client software should have modified the TCP/IP stack of the machine it was installed on and set the MTU for the PC to a lower value. The cisco VPN client does this. 1600 is very large for running in an ethernet environment. You normally don't see a need for an MTU larger than 1500 unless you are running MPLS VPNs, which requires an MTU of 1526 on some links, but this is adding a lot of extra to the packet including dot1q info. Is the client having difficulty connecting and establishing the tunnel or is it problems with applications after the tunnel is established? Hope this helps.