Question : Cisco router path MTU discovery RFC 1191

How can I check if a cisco router supports path MTU discovery (RFC 1191)?

Answer : Cisco router path MTU discovery RFC 1191

If the packet is bigger than the router can receive, it will be discarded.  DF and PMTU work in the context where the router receives a packet that is too big to *forward*.  For instance, going from an Ethernet LAN (~1500 MTU) to an ISDN WAN link (576? MTU).  If DF isn't set, the router will fragment the packet to fit and it's up to the recipient to reassemble it.  If DF is set, the router isn't supposed to do that, but instead generate the ICMP back to the originating host.

So to do PMTU, the originator sends out a packet (with DF set) that fits its local MTU, and if it gets back a a "would fragment" ICMP, reduces the size and tries again.