I have the following AS-level topology with 3 ASes, i.e., A, B and C.
I am asked to
illustrate the idea of hierarchical routing in the particular case of the Internet by describing the process by which an IP packet would be routed from a source router
Ato a destination router
Yin . Moreover, for each step, I should briefly describe which routing information is used, which protocol determines that routing information, and how routing information is transmitted to the router.
I know that within an AS there's a internal routing protocol. The most common ones are link-state or distance-vector-based protocols. OSPF (open-shortest path protocol) is a link-state based routing protocol that is quite used nowadays. Another one apparently is IS-IS. A common, if I am not wrong, old distance-vector based protocol is RIP (routing information protocol).
There are then external routing protocols, which are protocols that allow ASes to communicate. The most common one is BGP (border gateway protocol).
Of course from one router
X to send something to another router
Y in a different AS these two kind of protocols (internal and external) should be used.
I think that the
GWs are border routers that allow the ASes to communicate. My understanding is that
X should first send its packet to
A's internal routing protocol. Then
GW1 would contact
GW2 using for example
BGP, and the packet would then be transferred to
GW3 (using e.g. RIP), and then from
GW4 using again maybe BGP. Once arrived at
GW4, again, another internal routing protocol, such as OSPF, would be used to send
A's original packet to
Apart from that, I don't think I would be able to give an exhaustive answer to the question above.
Any help is appreciated!