Suppose a site A is multihomed, in that it has two Internet connections from two different providers, P and Q. Provider-based addressing as in Exercise 5 is used, and A takes its address assignment from P.

Q has a CIDR longest match routing entry for A.

(a) Describe what inbound traffic might flow on the A–Q connection. Consider cases where Q does and does not advertise A to the world using BGP.

(b) What is the minimum advertising of its route to A that Q must do in order for all inbound traffic to reach A via Q if the P–A link breaks?

(c) What problems must be overcome if A is to use both links for its outbound traffic?

Exercise 5

Suppose P, Q, and R are network service providers, with respective CIDR address allocations C1.0.0.0/8, C2.0.0.0/8, and C3.0.0.0/8. Each provider’s customers initially receive address allocations that are a subset of the provider’s. P has the following customers:
PA, with allocation C1.A3.0.0/16, and PB, with allocation C1.B0.0.0/12.
Q has the following customers:
QA, with allocation C2.0A.10.0/20, and QB, with allocation C2.0B.0.0/16.
Assume there are no other providers or customers.

(a) Give routing tables for P, Q, and R assuming each provider connects to both of the others.

(b) Now assume P is connected to Q and Q is connected to R, but P and R are not directly connected. Give tables for P and R.

(c) Suppose customer PA acquires a direct link to Q, and QA acquires a direct link to P, in addition to existing links. Give tables for P and Q, ignoring R.