I am investigating network performance issues on a DMVPN network. There is evidence that packet fragmentation is occurring on traffic traversing DMVPN tunnels indicating that Path MTU Discovery is failing.
I've reviewed some of the router configurations and noticed that "no ip unreachables" is set on every interface including the multilink and tunnel interfaces. This doesn't seem to be the correct usage.
Is there a good discussion of what this command is supposed to accomplish and where it is applied in the flow through each interface?
A side issue is what is the proper way to define the MTU to prevent packet fragmentation of data passing through a tunnel? Using "ip tcp mss" does not seem a complete solution. Shouldn't "ip mtu" be used?