How to solve this

Question

Consider the network shown below. Suppose AS3 and AS2 are running OSPF for their intra-AS routing protocol. Suppose AS1 and AS4 are running RIP for their intra-AS routing protocol. Suppose eBGP and iBGP are used for the inter-AS routing protocol. Initially suppose there is no physical link between AS2 and AS4.

a)  Which routers in AS1 run eBGP?

b)  Which routers in AS1 run iBGP?

c)  Which routers in AS1 run OSPF?

e) Router 4b learns about x from which routing protocol?

Answer 1

Without a diagram this is extremely difficult to answer:

However, as a rule of thumb:

eBGP is external BGP and is used at the edge routers for communications between Autonomous Systems (AS's)

iBGP is internal BGP and is used for communications within an AS.

An igp (OSPF, RIP, etc) can be used by internal routers as a transport between BGP routers.

So (I know this is crude):

----------------AS1------------|----------------AS2------------|----------------AS3------------|----------------AS4------------

1R1-1R2-1R3---------------2R1-2R2-2R3---------------3R1-3R2-3R3---------------4R1-4R2-4R3

eBGP would be running between 1R3 and 2R1, 2R3 and 3R1, 3R3 and 4R1

iBGP would be running between 2R1 and 2R3, 3R1 and 3R3

RIP would be running on all routers in AS1 and AS4 - need to be on the same subnet

OSPF would be running on all routers in AS2 and AS3 - need to be on the same subnet

BGP neighbors do not need to be on the same subnet. But they do need an igp (RIP, OSPF, etc) to provide connectivity between them. Therefore, router 2R2 does not need to be running bgp for a neighbor relationship for form between 2R1 and 2R3, provided there is a valid route in OSPF between the IP's on 2R1 and 2R3 being used for BGP (usually a Loopback or system address).

Clear as mud?