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MTU tuning on L2TPv3

Unanswered Question
Jun 1st, 2006
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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....


Peter

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vkapoor5 Wed, 06/07/2006 - 05:23
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Here is a Cisco document that explains the MTU issues while using L2TP VPN.


http://www.cisco.com/en/US/tech/tk801/tk703/technologies_tech_note09186a0080094c4f.shtml


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.


http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

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