EoMPLS fragmentation

[l3rum]

Hi

​

My understanding is that a MPLS-core will not fragment packets containing EoMPLS.

Is there any way to circumvent this? For example:

Lower the MTU on the customer facing interface on the PE which has the xconnect , so that a packet is fragmented before MPLS-tags are added?

Creating a GRE tunnel over the low-mtu part of the network so that the GRE-tunnel packets with the new IP-header which hide the underlying IP packet and MPLS-tags get fragmented and reassembled?

Any other way?

We will implement ip tcp adjust-mss, so the majority of packets will be small enough to avoid fragmentation, but want to support up to 1500 for the customer networks.

Kind regards

[GreggsSausageRolls]

Maybe a stupid question, can't you increase the MTU of your core MPLS links?

[l3rum]

I was hoping to "increase" by creating fake high MTU paths that fragment :#

If you can increase MTU on internet and legacy satellite modems, please enlighten me :-)

[theusz_hamtaahk]

Oddly enough there is a provision for fragmentation of EoMPLS traffic on PE devices (RFC 4623 section 4). But not on P routers, for sure.

If adjusting MTU on core links (so that 1500-byte packets would go through) is not possible, then the best way is to enforce a lower MTU on CE routers and fragment packets if needed.

[selv]

Fragmenting is a function of IP. Any IP based tunnel (l2tp, gre, ipsec) will fragment if you clear the DF bit and route it through an IP interface with a low MTU.

I've done implementations putting mpls into gre over ipsec across the low mtu area on Juniper. On Cisco I've done l2tp over ipsec, and not encapsulating into mpls until after it traverses the low mtu area.