BGP can be pretty daunting when you are new to it. First of all, I would recommend to start with this lesson:
That explains exactly how we use IBGP and why we need an IGP for it.
Let’s say you are sending a packet from one R1 to R9, and it’s not working. On each device in the path from source to destination, here’s what you should do:
- Check the routing table, is there an entry for the destination here? If not, do you see it in the BGP table? If it's in the BGP table but not in the routing table then the next hop is probably unreachable. Fix this.
- If this device knows how to reach the destination, move to the next router in the path and do the same checks.
- Do this for all devices until you are at the final router.
- All routers in the path should be able to route to the destination.
Now you are at the router where the destination is, you have to check if it knows how to get back to the source. Do the same checks all the way back to the source:
- Does the router have an entry in the routing table for the source? If not, check the BGP table. Is the next hop unreachable? Fix it.
- Move to the next router, check all routers in the path to the source to see if they know how to reach the source.
- All routers in the path should be able to reach the source.
If you troubleshoot like this, then you’ll find the problem.