E-Commerce: Doing Business on the Internet
CPSC 155b, Spring 2003

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Notes on Homework 1

Question 3 (OSPF Routing)

• If there are two or more equally shortest (lowest-weight) paths, just pick one to use in the forwarding tables in your answers. The next hop of any shortest path will be given credit.
• LSPs are only sent when there is a change in the network. In part B, node B has nothing new to report, so it does not send an updated LSP. Assume that it is still connected as in part A.

Question 4

• If you weren't able to get a copy of the textbook from the bookstore, the preliminary online version should be enough for the homework assignment.

Question 6 (BGP Routing)

• Please remember the order in which ASs obtain information about routes. This process is outlined in the homework question and in the example. The order is always followed strictly: (1) advertise routes / destinations to your neighbors, (2) decide on a best route to a destination based on your policies, (3) advertise that best route to your neighbors, (4) repeat when updates are received. Information about link connectivity, especially about links that don't connect you to a neighbor, is not transmitted in BGP. Only routes are transmitted, and those only after decisions by your neighbors.
• ASs cannot tell their neighbors what routes to choose; that's the point of BGP -- choose best routes based on one's own routing policies and preferences. In particular, in part II, AS14 can never tell AS13 to choose a specific route; it can only tell AS13 as a part of a local business agreement what preference it would like assigned to the link purchased between them.