When you configure the ip pmtu command under an L2TPv3 pseudowire class, which two things can
happen when a packet exceeds the L2TP path MTU? (Choose two.)
A.
The router drops the packet.
B.
The router always fragments the packet after L2TP/IP encapsulation.
C.
The router drops the packet and sends an ICMP unreachable message back to the sender only if
the DF bit is set to 1.
D.
The router always fragments the packet before L2TP/IP encapsulation.
E.
The router fragments the packet after L2TP/IP encapsulation only if the DF bit is set to 0.
F.
The router fragments the packet before L2TP/IP encapsulation only if the DF bit is set to 0.
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