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CSC41200: Computer Networks
Written Homework 2
Due Dec 3, 2003
Problem 1: Distance vector routing (10 points).
Consider the network shown in the figure below with the indicated static link costs.
Considering the distance vector routing algorithm, show the final distance table in node C and
the next hop that should be taken to each destination. Use a table similar to Table 4.6 in the
textbook (slide 19 of Chapter 4, part 1, in the notes). Note: for this problem, you do not need to
show how you computed the distance table entries; indeed, I recommend that you do it by
“inspection”; just give the result of the algorithm.
Problem 2: Distance vector routing (20 points).
Consider the network shown in the figure below with the indicated static link costs.
Considering the distance vector routing algorithm, and its current state (right after
initialization phase, - indicates infinity). Assume that all nodes exchange messages with
their neighbors within one time step.
a) What are the new distance tables for nodes A, B, C and D?
b) Which nodes have changed their route after this step is complete?
c) Which nodes will send which updated values to which neighbors for the next step
of the computation (after the current step is complete)?
Problem 3: Hierarchical routing (20 points)
Consider a two-level routing hierarchy (e.g., similar to Figure 4.11 in textbook, slide 32,
Chapter 4, part 1 in the notes). Suppose that shortest hop routing is used in both inter-AS
and intra-AS routing. For inter-AS routing, this means that a packet that is to be routed at
the inter-AS level to a destination in network X will be routed to the closest (shortest
number of hops) inter-AS router in network X.
Draw a figure similar to Figure 4.11 in textbook (slide 32, Chapter 4, part 1 in the notes)
showing hosts and routers in different domains, that illustrates that when two-level
shortest hop routing is used, the overall length of the path taken may be longer than if flat
(non-hierarchical) routing had been used. Indicate the path and the path length between
two hosts that would be taken if hierarchical routing or flat routing was being used.
e
a
C
b
b
a
a
A
b
d
c
B
c
Problem 4: BGP (20 points)
Consider the network shown in Figure 4.37 of the textbook (slide 44, Chapter 4, part 2 in
the notes). Recall that BGP sends path information in its routing advertisements (as
opposed to some metric), and based on this path information, a receiver of BGP
advertisements can learn about the overall network topology. Note that each destination
advertisement in BGP has an AS-path which indicates the series of ASs that will be
traversed to reach that destination. Based on the BGP information that Y receives from C,
Y will have the following "view" of the network topology:
a) Why is it that network Y does not know about the existence of network B?
b) Starting with the network shown in Figure 4.37 of the textbook (slide 44, Chapter
4, part 2 in the notes), what are the paths that networks B and C will advertise to
network X?
c) Draw a diagram similar to the one above, showing the view of the topology at
network X, based on the BGP advertisements it receives.
Problem 5: IP Addressing (30 points)
Consider the simple network shown below with the IP addresses and network prefix
indicated, and suppose that A is sending datagrams to E.
a) Write down an IP address for each interfaces at all hosts and routers in the
diagram. Note that the IP addresses and network prefix for A and E are as given.
You should assign IP addresses so that interfaces on the same network have the
same network prefix in their IP address. Remember to consider the fact that the
addresses containing all ones and all zeros for the host part of the address are
reserved and cannot be assigned to interfaces.
b) Given your IP addresses from above, what are the contents of the routing table in
the router. Consult Figure 4.22 in the text to see and example of the general
format of a routing table (slide 13, Chapter 4, part 2 in the notes).
c) What are the IP source and destination addresses in the IP datagram as it passes
from A to the router?
d) What are the IP source and destination addresses in the IP datagram as it passes
from the router to E?
e) Suppose the router is removed from the network, and all five hosts are connected
together through one shared link. Would the IP addresses need to be changed?
Explain your answer.