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mtu&MPLS VPN

chenleish
Level 1
Level 1

PC-CE-PE-P-PE-CE-PC

I connect the network use FE port,and set the tag-swicthing mtu between P&PE to 1512,the other mtu use default size(1500)。PE-PE use MPBGP.

when i use 2547bits VPN,it work well.

but when i use draft-martini to set up PC-PC connection,use ftp ,it cann't list the file name.

change the tag-swithing mtu to 1520,it work well.

why?

3 Replies 3

Harold Ritter
Spotlight
Spotlight

This is because in the context of mpls vpn (2547bis), the core routers supports PMTUD for packets coming from vpn customer networks.

This is not the case for draft Martini (I assume you are referring to eompls).

Hope this helps,

Regards,
Harold Ritter, CCIE #4168 (EI, SP)

but even the customer network would send the 1500 length packet, plus the mpls tag for 4 bytes ,plus the vlan info for 4 bytes ,it's only 1508 bytes,still not exceed 1512.

why cann't it pass through ?

With EoMPLS, the ethernet frames is transported in its entirety through the cloud. a 4 bytes control word is also added (Martini encapulation).

This gives you

4 bytes for the pseudowire label +

4 bytes for the VC label +

4 bytes for the control word +

12 bytes for the source and destination addresses +

4 bytes for the vlan info +

2 bytes for the ethertype +

1500 bytes for the payload =

1530 bytes total

Hope this helps,

Regards,
Harold Ritter, CCIE #4168 (EI, SP)