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Internetworking with TCP/IP: Pearson New International Edition

Douglas E. Comer

Chapter 13

Routing Among Autonomous Systems (BGP) - all with Video Answers

Educators

Chapter Questions

Problem 1

If your site runs an Exterior Gateway Protocol such as BGP, how many routes do you advertise? How many routes do you import from an ISP?

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Problem 2

Some implementations of BGP use a hold down mechanism that causes the protocol to delay accepting an $O P E N$ from a peer for a fixed time following the receipt of a cease request message from that neighbor. Find out what problem a hold down helps solve.

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Problem 3

The formal specification of BGP includes a finite state machine that explains how BGP operates. Draw a diagram of the state machine and label transitions.

Adriano Chikande
Adriano Chikande
Numerade Educator

Problem 4

What happens if a router in an autonomous system sends BGP routing update messages to a router in another autonomous system, claiming to have reachability for every possible Internet destination?

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Problem 5

Can two autonomous systems establish a forwarding loop by sending BGP update messages to one another? Why or why not?

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Problem 6

Should a router that uses BGP to advertise routes treat the set of routes advertised differently than the set of routes in the local forwarding table? For example, should a router ever advertise reachability if it has not installed a route to that network in its forwarding table? Why or why not? (Hint: read RFC 1771.)

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Problem 7

With regard to the previous question, examine the BGP-4 specification carefully. Is it legal to advertise reachability to a destination that is not listed in the local forwarding table?

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Problem 8

If you work for a large corporation, find out whether it includes more than one autonomous system. If so, how do they exchange routing information?

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Problem 9

What is the chief advantage of dividing a large, multi-national corporation into multiple autonomous systems? What is the chief disadvantage?

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Problem 10

Corporations $A$ and $B$ use BGP to exchange routing information. The network administrator at Corporation $A$ configures BGP to omit network $N$ from advertisements sent to $B$, which is intended to prevent computers in $B$ from reaching machines on network $N$. Is network $N$ secure? Why or why not?

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Problem 11

Because BGP uses a reliable transport protocol, KEEPALIVE messages cannot be lost. Does it make sense to specify a keepalive interval as one-third of the hold timer value? Why or why not?

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Problem 12

Consult the RFCs for details of the Path Attributes field. What is the minimum size of a BGP UPDATE message?

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