Which three statements are correct regarding ping mpls command operations? (Choose three.)
A. MPLS OAM has to be enabled on the router using the mpls oam command.
B. They use a 127/8 address as the destination address in the MPLS echo request packet.
C. They use ICMP echo request and ICMP echo reply packets.
D. They are used to test for a broken LSP.
E. If there is a broken LSP, instead of using label switching, the packet can still be forwarded based on the destination IP address in the mpls ping echo request packet.
Correct Answer: ABD
Explanation/Reference:
Explanation:
How MPLS Ping works?
MPLS Ping doesn’t rely on ICMP echo messages. Instead it uses UDP protocol with both source and destination port as 3503 and relies on MPLS Echo request and MPLS Echo reply. When MPLS ping is triggered from any MPLS router, it will generate UDP segment with source/destination port as 3503. The source address will be selected as usual while the destination address will be 127.0.0.1. The IP TTL will be set to 1.
Below is a sample IP format when MPLS Ping is originated from R5 to 150.1.6.6/32,
Now the originating LSR will look into the LFIB and populate the label header with respective labels to reach the FEC, in our case 150.1.6.6/32.
R4 on receiving the MPLS packet will be able to send to the actual destination only if the LSP is end to end. If the LSP is broken between R4 and R6, R4 will look into the destination IP address which will be 127.0.0.1 and won’t be able to perform IP forwarding. As per RFC 1812, a router should not forward any packet that has destination address of 127.0.0.0/8
