While studying for my CCIE in the section of MPLS L3 VPN I was reading all the time that we should configure the MP-BGP on the /32 loopback interfaces, But it wasn’t so clear WHY?. So I decided to practice it in a lab and know what will happen if we configured the MP-BGP to be on the connected interfaces.

Once I did that – look at the topology figure – I faced a reachability problem when I tried to reach CE2 from CE1 or in the opposite direction.

Before continuing with the problem we should know about PHP operation that most of us ignore or think it’s trivial in the MPLS…

PHP is penultimate hop popping which means remove the label one hop before its destination. The Edge LSR sends label #3 “implicit-null” to his neighbors for all the directly connected interfaces and summary addresses, so that when the neighbor receive a packet with this destination the PHP will be done.

Lets go back to our main problem, I know that most of  you figured out what is the reason of the reachability problem.Unfortunately the control plan is OK and the real problem is in the forwarding itself. look at the below topology:

During establishing LSPs, ELSR2 will send a VPN label for CE2 to ELSR1 using MP-BGP so that when we do show ip route CE2 network we will find that the route is known via BGP and the next hop is ELSR2 P1/0 interface which is known via IGP “IS-IS or OSPF”.

For the IGP control plan LSR see that ELSR2 P1/0 interface is directly connected to it so it sends label# 3 “implicit-null” to ELSR1 due to PHP which is ON by default and can not be disabled.

Lets assume that CE1 wants to reach CE2, that IP packet will arrive to ELSR1 and it starts to build the label stack by imposing the VPN label and it will find that it received “implicit-null” from LSR as we mentioned before, so that the packet will arrive to LSR with only the VPN label which it doesn’t know anything about it so the packet will be dropped.

In the other hand what will happen if we configured the MP-BGP to be on loopbacks?

The ELSR1 will know the route via BGP with next hop loopback address which is not directly connected to LSR so that ELSR2 will send a LDP label# 3 “implicit-null” to LSR and LSR will send a LDP label “random number” to ELSR1, so that LSR will receive a labeled packet that it knows.