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Beginner

exact mtu to enable mpls over gre tunnel

hi,

I've a scenario like below

pc ------ rtr1 <--- p2p link ---> rtr2 ------ pc

rtr1 and rtr2 doesn't support interface mtu size more than 1500 in their wan interface.

I've configured mpls over gre but can't pass through clear 1500bytes packet. i can only pass through 1464bytes packet size live on wire.

how should i adjust mtu to pass through clear 1500bytes packet with or without fragmentation.

dipak

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Cisco Employee

Re: exact mtu to enable mpls over gre tunnel


Hello,

to allow 1500 byte packets to be sent across the tunnel, you will either have to:

1) increase the MTU of the WAN interfaces (apparently not possible here)

2) You can also try to configure "tunnel path-mtu-discovery" under the tunnel interfaces and let them find the best MTU based on the path they cross or decrease the mpls mtu. Though, if you the tunnel MTU is decreased, that means fragmentation will occur. If applications are using the df-bit, you will then need to clear the df-bit for each packets entering the router.

Which router/module/interface are you using here ?

Beginner

Re: exact mtu to enable mpls over gre tunnel

both routers are 7206 vxr (npe300) with all fe interfaces using c7200-p-mz.122-31.SB16.bin

router didn't allow me to configure mpls mtu on tunnel interface. "tunnel path-mtu-discovery" is enabled on tunnel.

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