Hello Renuka
Yes, that does look strange, doesn’t it? Well, it has to do with the fact that each router will advertise only its best route to the destination. Let’s take a look at each case:
For R3
- R3 receives an update from R4 about the destination of the 4.4.4.4 network, sharing its best route to the destination. The network is directly connected to R4, so of course this is its best route.
- R3 also receives an update from R1 telling it that its best route to 4.4.4.4 is via R2.
- Therefore, R3 has two different routes in the BGP table to reach 4.4.4.4.
For R1
- R1 receives an update from R3 telling it about its best route to 4.4.4.4 via R4
- R1 receives an update from R2 telling it about its best route to 4.4.4.4 via R2
- Thus R1 has two routes in the BGP table to reach 4.4.4.4
- Note that R1 chooses the route via R2 as the best route due to the lower BGP router ID of that neighbor.
For R2
- R2 receives an update from R4 telling it about its best route to 4.4.4.4 via R4 (connected route)
- R2 receives an update from R1 telling it about its best route to 4.4.4.4 via R2 (itself)
- Therefore, R2 learns about a route to R4 via itself, therefore it only installs one route in the BGP table via R4 directly.
Does that make sense?
I hope this has been helpful!
Laz