Hello Walid.
Thanks for the clarification, I will attempt to answer below.
In the previous post I mentioned that the only prerequisite for two BGP neighbours to form a neighbourship was that the IP addresses used as the BGP sources should be reachable via an IGP (or static routing). However, this is one more requirement and that is that they be only one hop away from each other. If they are more than one hop away from each other as is the case if you use the L0 addresses, then you must have the ebgp-multihop X command where X is the number of hops between the BGP source IP addresses. Now because the two loopback addresses are not directly connected but are two hops away from each other, X must be 2, and only then will a neighbour relationship form.
Unlike IGPs, when BGP forms a neighbour relationship, it only forms in one direction. In order to understand what I mean, take a very simple topology such as the following:
Imagine you enable BGP on both routers and you put in the following command on the R1 router:
neighbor 192.168.12.2 remote-as 2
This will create a one way BGP neighbour relationship between R1 and R2. This means that if you enter the show ip bgp neighbors command on R1 you will see the neighbour relationship. If you go to R2 and enter the same command you will see no neighbours. Neighbours on the R2 router will appear once the following command has been entered on R2:
neighbor 192.168.12.1 remote-as 1
Once this is complete, then there is a mutual neighbour relationship.
The same thing occurs when you implement the topology in the lesson. When you put in the update-source lo0 command on one router, the neighbour relationship comes up in only one direction. If you go to the other router, you won’t see any neighbours until you put in the corresponding command in that router as well.
I hope this has been helpful!
Laz