Explanation/Reference:
Explanation:
If you enable the ip pmtu command in the pseudowire class, the L2TPv3 control channel participates in the path MTU discovery. When you enable this feature, the following processing is performed:
ICMP unreachable messages sent back to the L2TPv3 router are deciphered and the tunnel MTU is

updated accordingly. In order to receive ICMP unreachable messages for fragmentation errors, the DF bit in the tunnel header is set according to the DF bit value received from the CE, or statically if the ip dfbit set option is enabled. The tunnel MTU is periodically reset to the default value based on a periodic timer.
ICMP unreachable messages are sent back to the clients on the CE side. ICMP unreachable messages

are sent to the CE whenever IP packets arrive on the CE-PE interface and have a packet size greater than the tunnel MTU. A Layer 2 header calculation is performed before the ICMP unreachable message is sent to the CE.
ReferencE. http://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/l2tpv325.html