The penultimate para in Section 3, Page 6 of 6PE RFC 4798 states the following:
"Otherwise, routers in the IPv4 MPLS network have the option to generate an ICMP "Packet Too Big" message using mechanisms as described in Section 2.3.2 .... of [RFC3032]"
As per RFC3032, the routers in the IPv4 MPLS network can generate ICMP "Time Exceeded" message or "Destination Unreachable because fragmentation needed and DF set" message.
Can someone please explain, how a IPv4 MPLS router will generate an ICMP "Packet Too Big"? This requires that the router in IPv4 MPLS network be a dual stack router to understand the IPv6 header under the label stack. Is my understanding correct?
If the router is an intermediate LSR, how will it know the path to the IPv6 destination even if it is dual stack router?
Thanks in advance.
The "Packet too big" ICMPv6 message will never exceed 1280 bytes (including the IPv6 header), which is the minimum MTU that all interfaces should support as per RFC2460, Section 5. So as long as all your MPLS core interfaces support an MTU of at least 1280+label overhead, you should be fine. This should be a non issue in todays MPLS cores.