Computer Networking - BGP

Question

Consider the network shown above. Suppose all ASs are running OSPF for their intra-AS routing protocol, with the cost of each link equal to 1. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Initially suppose there is no physical link between AS2 and AS4.

Routers(1a,1b,1c,1d,2a,2b,2c,3a,3b,3c,4a,4b,4c) learns about prefix x from which routing protocol: OSPF, RIP, eBGP or iBGP?

I'm so confused on BGP, I think the answers should be either eBGP or iBGP. Can anyone help me with this and explain why? (I think 1c and 3c should be eBGP, and 3a and 1d should be iBGP. Not sure about others)

Answer 1

First of all, since all the ASs run OSPF why you believe it's either eBGP or iBGP and not OSFP? What point exactly confuses you?

The border routers 3c, 1c, 2a will learn it through eBGP, because they will learn the prefix from a router outside their AS.

The other routers will learn it through the intra-domain routing protocol, which is OSPF, because they will learn the prefix from a router that belongs to the same AS.

Think how the prefix is propagated to by each router to their next-hop neighbours to understand from which protocol the prefix will be learned at each router.

The prefix will be propagated as follows: 4a -> 4b -> 4c --> 3c -> 3b -> 3a --> 1c -> 1a -> 1d and 1b --> 2a ->2b and 2c

I think the solution is simple and because the question is 9 days old I hope you took the time to study routing if it was a homework. Generally, it would be better to explain and discuss which points you don't understand rather than go directly for the solution, it will help you more in the long term.

Answer 2

I could not comment on the Vasilis's answer since i have not enough reputation points... There is nothing said that the BGP routes are redistributed into OSPF, thus route to X can't be learned via OSPF in ASs 1,2,3. OSPF simply does not have this route in its routing table. We can only assume that X is present int OSPF in AS4 and is redistributed into BGP, thus, all routers within AS will learn it via OSPF (and iBGP is we assume OSPF is redistributed into BGP in AS4). Next, the route propagates to AS3, 3c will learn the route via eBGP, other routers will learn it via iBGP. Same with other ASs because there isn't link between AS2 and AS4 only "upstream" routers (closer to AS4) in each AS will learn it via eBGP, it is a very important point in this task. If there was link between AS2 and AS4, each AS border router (the one that is connected to another AS) whould learn it via BOTH eBGP and iBGP but would prefer eBGP. There are actually many missing points in this task such as redistribution of route X in AS4.