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Authors:
James F. Kurose, Keith W. Ross
Chapter:
The Network Layer
Exercise:
Problems
Question:38 | ISBN:9780132856201 | Edition: 6

Question

Referring to the previous problem, once router 1d learns about x it will put an entry (x, I) in its forwarding table.

a. Will I be equal to I 1 or I 2 for this entry? Explain why in one sentence.

b. Now suppose that there is a physical link between AS2 and AS4, shown by the dotted line. Suppose router 1d learns that x is accessible via AS2 as well as via AS3. Will I be set to I 1 or I 2 ? Explain why in one sentence.

c. Now suppose there is another AS, called AS5, which lies on the path

between AS2 and AS4 (not shown in diagram). Suppose router 1d learns that x is accessible via AS2 AS5 AS4 as well as via AS3 AS4. Will I be set to I 1 or

I2?Explain why in one sentence.

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Answer

a)

 

Consider the data: The router "1d" learns about "x" it will put an entry (x, I) in its forwarding table.

  •  The interface starts the least-cost path "1d" towards the router  "1c". 
  • The path "I1" is the least-cost path with 2 hops from the Router "1d"  through the router "1c". 
  • The path "I2" is the least-cost path with 3 hops from the Router "1d"  through the router "1c". 

So, the value of "I"  will be equal to  "I1" or  "I2".

 

b)

 

Assume there is a physical link between AS2 and AS4, shown by the dotted line.

Assume the router "1d" learns that "x" is accessible through AS2 as well as via AS3. 

Then, both the routers have equal AS-PATH length is 2 hops. So, the value of "I"  will be set to   "I2".

 

c)

 

Assume there is another AS, called AS5, which lies on the path between AS2 and AS4.

Assume the router"1d" learns that "x"  is accessible via AS2 AS5 AS4 as well as through AS3 AS4.

Then, the path  "I1" handle the shortest AS-PATH than it takes place through the path   "I2". So, the value of "I"  will be set to   "I1".

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