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MTU tuning on L2TPv3

Hi all,

I have a small problem with the MTU on L2TPv3.

I built in my lab small IP core with Cisco routers and to edge IP routers I connected a PCs (or any IP routers) and I simulated L2TPv3 L2 VPN over them. The scenario working very good, without any problems. But I have a problem with large frames on Ethernet (which is encapsulated with L2TPv3 header). According to documentation, I tried to setup ip pmtu max <n> in pseudowire-class, but it didn`t work correctly. So, I applied on Ethernet interface ip tcp-adjust mss <n>...but I`m not sure that this is the right solution for me.

Please, can you help me with some idea to do not fragmentation of tunneled IP packets to smaller?

I know, that L2TPv3 do not supported fragmentation of packets before the encapsulation, so only encapsulated IP packets with L2TPv3 header is possible, but I don`t want it.

There is a one blocking point, I think. My lab is built on Cisco 1751-V Series routers and according to documentation, L2TPv3 feature is supported from Cisco 7200 Series routers.....

Thanks for an answers....



Re: MTU tuning on L2TPv3

Here is a Cisco document that explains the MTU issues while using L2TP VPN.

This is another excellent Cisco document that explains PMTU and IP fragmentaion issues. though this document is specific to IPSec, but would be very useful.

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