Homework Assignment #3b Due 11/22 at 3:50pm EE122 Fall 2010

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1 Homework Assignment #3b Due 11/22 at 3:50pm EE122 Fall 2010 Please submit your solutions using BSpace ( Log in to BSpace with your CalNet ID, click on the EL ENG 122 Fa10 tab, and click on Assignments under Course Tools. Assignments should be submitted in one of the following formats:.txt,.pdf, or.ps. The various figures referred to in the problems are on the sheets at the end of the assignment. Hint: if you can t find the answer in the lecture notes, the textbook provides the needed information for several of these problems. Good luck! 1) General Routing (from K&R) [5 points] Consider the network in Figure 1, with ASes A, B, C, D. Each AS has some number of routers (labeled as A1, A2, etc.) and the domains are connected internally and with each other by the links depicted in the figure. Assume that ebgp and ibgp are used for interdomain routing, and that ASes A and D are using RIP for intradomain routing while ASes B and C are using OSPF for intradomain routing. Prefix x hangs off an interface on router C3. i) Router D3 learns about prefix x from which routing protocol: OSPF, RIP, ebgp, or ibgp? ii) Router D1 learns about prefix x from which routing protocol? iii) Router A3 learns about prefix x from which routing protocol? iv) Router A1 learns how to reach router A3 from which routing protocol? v) Will router A1 use interface 1 or interface 2 to reach prefix x? 2) BGP Policies I [4 points] Consider an interdomain network with domains A through F. For simplicity, assume that destinations in this problem are domains, not prefixes. Recall that routes are expressed in terms of the series of domains: e.g., [A- B- C] denotes a route that started with domain A and went to domain B and then to domain C (which is the destination). Domains always advertise the route to

2 themselves (i.e., domain X advertises the path [X] to all peers, customers, and providers). The following connectivity/business relationships exist: B is a customer of A C is a customer of A D is a customer of A B and C are peers C and D are peers E is a customer of B F is a customer of B F is a customer of C G is a customer of D Assuming that each domain s routing policies follow normal business practice, and that BGP has converged, i) What routes does A advertise to B? ii) What routes does C advertise to B? iii) What routes does E advertise to B? iv) What routes does F advertise to B? 3) BGP Policies II (3 points) We say an interdomain routing system is in a stable state if none of the domains wants to change their routes given the current set of routes they are receiving from their neighbors. Consider the network in Figure 2, and focus only on routes to AS D. Assume all domains advertise their current route to D to all the domains they are connected to (i.e., there are no restrictive route export policies). Hint: In answering the questions below, don t expect to analyze this theoretically. Just work through, by hand, how BGP would update route advertisements. In doing so, it is easiest to have one AS update at a time. Consider different advertisement and updating orders (for instance, you might have A update, then B, then C, then B, then A, then ). If you play with this for a while, the answers for the questions below should be obvious. Also, recall that at any given time an AS only offers one route to a particular destination (i.e., it doesn t advertise more than one route to a particular destination at once; announcing a new path amounts to withdrawing the previous path). i) In the beginning, the routing preferences of the three ASes for routes to AS D are as follows (with highest preference first): AS A: [A- B- D], [A- D]

3 AS B: [B- C- D], [B- D] AS C: [C- A- D], [C- D] Does the system have: (a) no stable states, (b) one stable state, (c) more than one stable state. ii) AS A decided that it actually likes the path [A- B- C- D] best, so the preferences are: AS A: [A- B- C- D], [A- B- D], [A- D] AS B: [B- C- D], [B- D] AS C: [C- A- D], [C- D] Does the system have : (a) no stable states, (b) one stable state, (c) more than one stable state. iii) Because ASes are dumb as bricks and just imitate each other (just kidding AT&T, please don t cut off my DSL!), AS B decides that it also likes longer path best, so the preferences become: AS A: [A- B- C- D], [A- B- D], [A- D] AS B: [B- C- A- D], [B- C- D], [B- D] AS C: [C- A- D], [C- D] Does the system have: (a) no stable states, (b) one stable state, (c) more than one stable state. 4) Distance- Vector [3 points] Consider a network depicted in Figure 3 which is running (in theory) a synchronous routing process: each node exchanges distance information with its peers at the same time, so all the routing tables are updated simultaneously. Assume the networking is running a distance vector routing protocol with poisoned reverse. a) When the network is first coming up, each router s routing table (containing the distances to each destination) only has entries for its directly connected neighbors. Fill in the router A s routing table for the first five steps of the updating process (note that the process might reach a stable state before step 5). Step 0: (initialized)

4 Step 1: 3 10 Step 2: Step 3: Step 4: Step 5: b) After calculating the distances in step 2, what distances does router A then advertise to router B (that router B will then use when computing the distances in step 3)? c) In general, for distance vector routing starting from the beginning (with routers only knowing about their direct connections), how long can it take for the routing process to converge? Letting d denote the longest shortest path in the network, pick from one of the following answers: d+1, 2d, d- 5, d- 1, 2d- 1, ln[d]. In other words, if all you know about the graph is d, what is the tight upper bound on the convergence process?

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