I have the following AS-level topology with 3 ASes, i.e., A, B and C.

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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 X in A to a destination router Y in . 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 GW1 using 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 GW3 to 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 Y.

Apart from that, I don't think I would be able to give an exhaustive answer to the question above.

Any help is appreciated!

closed as off-topic by Ron Maupin, Teun Vink Jan 20 '16 at 17:43

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  • I think the subject is too broad. Please try to formulate specific questions. – Everton Jan 20 '16 at 15:28
  • @Everton I understand that the answer could be too broad, but honestly I would not know how to shorten it. I just wanted to know your opinion regarding the question and my answer. If am wrong or right, and what could I say more. For example, I don't know which internal routing protocol is used by AS A, so I cannot say anything about it, I can just assume or suppose. – nbro Jan 20 '16 at 15:31
  • 1
    Unfortunately, all "education, certification, or homework" questions are explicitly off-topic here. – Ron Maupin Jan 20 '16 at 15:34

If AS'es A,B,C are all you have:

  1. Both AS A and AS C could use static default routing towards AS B.
  2. AS B could point specific static route X to A and specific static route Y to C.
  3. No dynamic routing protocol between AS'es.
  4. AS B could run OSPF between GW2 and GW3.

This could be thought as if AS B would lie in a routing level hierarchically "above" both AS A and AS C.

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