Homework #8 Problem 1 -5 Solution

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Description

Consider a network of 6 routers shown in the gure below. The network is running OSPF routing protocol and the cost of each link is 10. Each router is announcing a single unique pre x, in total 6 pre xes are announced (pre x A, pre x B, … pre x F). Propagation delay for each link is 10msec. When a router has to choose between two or more equal cost paths to the same destination, it breaks the tie by picking the path whose next hop has smallest name (A < B < C < D < E < F). The network has been up and running for a long time. However, at time T=100 min, link A-F fails.

  1. How do node A and node F discover this link failure?

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  1. Will node C learn about this link failure? If so, does knowing this failure a ect C’s forwarding table?

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Problem 2

Consider the network shown below. Suppose all ASes (AS 1 { AS 4) are running OSPF for their intra-AS routing protocol. Suppose BGP is used for the inter-AS routing protocol (and iBGP is used inside each AS).

At some time T, the pre x x appears in AS4, adjacent to the router 4a. From which routing protocol (OSPF, eBGP, or iBGP):

  1. Router 1c learns about pre x x?

  1. Router 3b learns about pre x x?

  1. Router 4a learns about pre x x?

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Problem 3

  1. How does BGP detect loops in paths?

  1. Why does a BGP router not always choose routes with the shortest AS-path length?

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Problem 4

Suppose four active nodesnodes A, B, C and Dare competing for access to a channel using slotted ALOHA. Assume each node has an in nite number of packets to send. Each node attempts to transmit in each slot with probability p. The rst slot is numbered slot 1, the second slot is numbered slot 2, and so on.

  1. What is the probability that node A succeeds for the rst time in slot 5?

  1. What is the probability that any node (either A, B,C or D) succeeds in slot 4?

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Problem 5

Consider the following network topology with speci ed MAC addresses for network interfaces and the con-gured IP addresses:

00-00-00-00-00-05

192.168.1.253

Node C

00-00-00-00-00-02

192.168.0.2

00-00-00-00-00-01

Node A

2

192.168.0.1

192.168.1.254

Switch 1

Router 00-00-00-00-00-04

3

00-00-00-00-00-03

192.168.0.3

Node B

Assume the network mask for both subnetworks is 255.255.255.0.

  1. Assume that routing tables are properly con gured and the network just started (i.e., all caches are empty), ll the following table to enumerate Ethernet frames (in chronological order) needed for node B to send an IP packet to 192.168.0.2 and receive a response back.

frame #

dst MAC addr

src MAC addr

device(s) that can get the frame,

new entries added into

excluding the sender

the switch’s table (if any)

Problem 5 continued on next page. . . Page 5 of 6

CS 118 Winter 2019 : Homework 8

  1. Assume that the previous operation is done, ll the following table to enumerate Ethernet frames (in chronological order) for node B to send a packet to 192.168.1.253 and receive a reply.

frame #

dst MAC addr

src MAC addr

device(s) that can get the frame,

new entries added into

excluding the sender

the switch’s table (if any)

Page 6 of 6