You have issued the following commands on RouterA: pseudowireclass boson ip pmtu RouterA receives a packet that is larger than the path MTU and that has a DF bit set to 1. Which of the following will RouterA do? (Select 2 choices.)

Last Updated on August 6, 2021 by Admin 3

You have issued the following commands on RouterA:

pseudowireclass boson
ip pmtu

RouterA receives a packet that is larger than the path MTU and that has a DF bit set to 1.

Which of the following will RouterA do? (Select 2 choices.)

  • RouterA will forward the packet.
  • RouterA will drop the packet.
  • RouterA will return an ICMP unreachable message to the sender.
  • RouterA will fragment the packet before L2TP/IP encapsulation occurs.
  • RouterA will fragment the packet after L2TP/IP encapsulation has occurred.
Explanation:
When RouterA receives a packet that is larger than the path maximum transmission unit (MTU) and that has a Don’t Fragment (DF) bit set to 1, RouterA will drop the packet and will return an Internet Control Message Protocol (ICMP) unreachable message to the sender. The ip pmtu command enables path MTU discovery (PMTUD) so that fragmentation issues can be avoided on the service provider backbone.
With PMTUD, the DF bit is copied from the IP header to the Layer 2 encapsulation header. If an IP packet is larger than the MTU of any interface on the path, the packet is dropped or fragmented based on the DF bit. If the DF bit is set to 0, the packet is fragmented before Layer 2 Tunneling Protocol (L2TP)/IP encapsulation occurs and is then forwarded. If the DF bit is set to 1, the packet is dropped and the router will return an ICMP unreachable message to the sender.

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