Suppose P, Q, and R, are network service providers, with respective CIDR address

Question

Suppose P, Q, and R, are network service providers, with respective CIDR address allocations d1.0.0.0/8, d2.0.0.0/8, and d3.0.0.0/8 (using hexadecimal dotted notation with mask). Each provider’s customers initially receive address allocations that are a subset of the provider’s address space.

P has the following customers:

• PA, with allocation d1.a0.0.0/16, and

• PB, with allocation d1.b3.0.0/12.

Q has the following customers:

• QA, with allocation d2.0a.10.0/20, and

• QB, with allocation d2.0b.0.0/16.

(a) Assume there are no other providers or customers, and that each provider connects to both of the others. Give the routing table for a router in provider Q and indicate, for each destination entry, the next hop using the name of the domain (provider or customer). Also assume that we want to be able to send a datagram to any destination address, i.e. we have routing entries for the address range/subrange that contains that destination. Also, you may assume that the path selection is based on the shortest AS path criterion. (Please make any other assumptions clear in your answer.)

(b) Now suppose customer QB switches to provider P, and customer PA switches to provider R. Use the CIDR longest prefix match rule to give the routing table for a router in Q that allows QB and PA to switch without renumbering (i.e., keeping their initial address allocations).

Explanation / Answer

(a) Using and analyzing the next hop:

P’s routing table                                  Q’s routing table                     R’s routing table

Address Next Hop                         Address       Next Hop             Address       Next Hop

d2.0.0.0/8        Q d3.0.0.0/8         R d2.0.0.0/8        Q

d3.0.0.0/8         R d1.0.0.0/8       P d1.0.0.0/8         P

d1.a0.0.0/16    PA                             d2.0a.10.0/20   QA

d1.b3.0.0/12.    PB                             d2.0b.0.0/16      QB

So Q's routing table can be used to know the next hop based on the next hops from routing table of P and R.

(b) To switch without renumbering, we have to define routing table not depending on R i.e. ignoring R.

P’s routing table                                  Q’s routing table                     R’s routing table

Address       Next Hop                         Address       Next Hop             Address       Next Hop

d2.0.0.0/8        Q d3.0.0.0/8         R d2.0.0.0/8        Q

d3.0.0.0/8         R d1.0.0.0/8       P d1.0.0.0/8         P

d1.a0.0.0/16    PA d2.0a.10.0/20   QA

d1.b3.0.0/12.    PB d2.0b.0.0/16      QB

C2.0A.10.0/20   QA                            C1.A3.0.0/16    PA    

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