I have a question regarding inter-AS MPLS interconnect configuration.
We are currently experiencing an MTU problem on an interconnect using EBGP redistribution of labelled vpnv4 routes (type B in RFC 4364). It appears that labelled packets of size 1500 bytes or more are not able to traverse the inter-AS link when the "Do not fragment" flag is set in the IP packet header even though the interface MTU is set to 4470 bytes.
An ICMP "undeliverable, cannot fragment" message is returned to source from our ASBR. This message even shows that the MTU of the next hop link is 4466 bytes and that the undeliverable packet size is 1500 bytes. IP (i.e. not labelled) pings > 1500 bytes can be sent across the link without problems with the DF bit set. This problem does not occur on LDP or RSVP enabled MPLS interfaces in our network.
The interface configuration for the outgoing interface on our ASBR isâ¦.
description >>> XX to router eer01 port 3/0 Suite 00/149 position 08 <<<<
ip address X.X.X.X 255.255.255.252
no ip directed-broadcast
carrier-delay msec 0
no negotiation auto
mpls bgp forwarding
Does MPLS MTU need to be explicitly configured on type B interconnect interfaces?