[c-nsp] MPLS and links with limited MTU size

Alexandre Snarskii snar at snar.spb.ru
Sun Jan 24 09:35:06 EST 2016

On Sat, Jan 23, 2016 at 01:06:12PM +0600, Victor Sudakov wrote:
> Dear Colleagues,
> There is an IP MPLS network with MTU=1600 on all routers' interfaces.
> There are plans to add some backup Ethernet links whose maximum frame
> size cannot exceed 1500 (hardware limit). 
> Is there any way those links can be put to any good use in the MPLS
> network?

Dirty trick is to run MPLS over GRE. You shall never do this, but sometimes
there are no other options :( 
MTU issues can be avoided by the means of fragmenting GRE packets itself: 
GRE is actually "GRE over IP" and IP packets can be fragmented and reassembled.
Of course this means that tunnel ingress router must be able to fragment
MPLSoGREoIP if it exceeds link MTU, and that tunnel egress router must be 
able to reassemble GRE thus restoring original MPLS frame. 

Not sure if it can be done with Cisco, with Juniper MX it's possible 
since JunOS 14.2.

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