The World-Wide Internet
Practice Problem 17.1
Consider the picture below, showing the interconnection of four AS's. Note that traffic can route from New
York through Paris and Cannes and eventually get to Bonn, but traffic that goes from New York to Berlin to
Bonn and on to Stuttgart cannot proceed onward to Cannes. What are the categories of each of the AS's?
AS2
AS1
Solution AS1 is a stub, AS2 is transit, AS3 is multihomed and AS4 is transit.
AS3
Practice Problem 17.2
Problems with routing protocols arise from issues of scalability or arise from issues of administration. Classify each of the problems below as a problem of scalability or of administration.
(a) Verizon wants Netflix to pay for routing data through its network.
(b) Routers can only hold a limited amount of table entries.
(c) Extremely large routing tables cause delays in packet forwarding.
(d) Brazil and Europe decided not to route their traffic through the United States to avoid NSA spying.
Solution: (a) Administration (b) Scalability (c) Scalability (d) Administration
Practice Problem 17.3
Consider the network shown below.
(a) What type of Autonomous System is AS3?
(b) What would happen if AS1 declared itself to be a multi-homed AS?
Solution: (a) Stub
(b) No communication between AS2 and AS3 would be possible.
2

Practice Problem 17.4
You are the BGP router for AS1 in the network shown below, and you would like to send data to AS3.
(a) What path is used if the administrator for AS1 has set a policy that no data from AS1 may go to AS2?
(b) What path is used if the administrator for AS1 has set no specific local preferences?
Solution:
(a) Since we cannot traverse AS2, we select the path with the fewest AS's: AS1-AS4-AS5-AS3.
(b) Since there is no local preference, we select the path with the fewest AS's: AS1-AS2-AS3.
Practice Problem
Suppose that in the network below, the administrator for AS1 has set a policy that no data from AS1 may go to
AS2. Additionally, AS1 has set a local preference value of 500 on the AS4-AS5-AS3 path and a value of 100 on the AS6-AS7-AS3 path. Which path does data traverse from AS1 to AS3?
Solution: AS1-AS4-AS5-AS3
3