L4 - Routing

Question 1

Internet routing consists of 2 very distinct parts relating to Autonomous Systems. What are they?

Answer 1

1 - Intra-domain routing (inside an AS)

2 - Inter-domain routing (between AS’s)

Question 2

What are the 2 types on intra-domain routing?

Answer 2

1 - Distance Vector

2 - Link State

Question 3

What is distance vector routing?

Answer 3

This is when a router sends its neighbors copies of its own routing table. Generally it uses the Bellman-Ford algorithm.

When costs decrease, the network converges quickly. But when failures occur, bad news can travel slowly (count to infinity problem, solved by poison reverse).

Routing Information Protocol (RIP) is an example of a distance vector protocol.

Question 4

What is link state routing?

Answer 4

This is when routers distribute a network map to every other node in the network. Each node then performs the shortest path computation itself.

Question 5

What are two common link state routing protocols?

Answer 5

1 - Open Shortest Path First (OSPF)

2 - Intermediate System-Intermediate System (IS-IS) - this is the most common one.

Question 6

What is a problem with Link State Routing?

Answer 6

Scaling - complexity grows as n^3. We can deal with this using hierarchies (areas in OSPF, or levels in IS-IS)

Question 7

What is the protocol used in Interdomain routing (i.e. between AS’s)?

Answer 7

Border Gateway Protocol (BGP). Specifically, external BGP (eBGP). There are 3 attributes in a BGP route advertisement - destination, nexthop, and ASpath.

Question 8

What is the protocol used for transmitting routing info inside an AS about external destinations?

Answer 8

internal BGP (iBGP).

Question 9

How does BGP pick best route?

Answer 9

In order of importance:
1 - Higher local preference value (only known to AS, determines exiting traffic)
2 - Shorter AS paths
3 - Lower MED (multi-exit discriminator) values for AS’s with multiple exit nodes, determines incoming traffic.
4 - Shortest internal path to AS exit (‘hot potato routing’)
5 - Any consistent tie-breaker

Question 10

What are the basic rules of preference in interdomain routing regarding customer/peer/provider routes?

Answer 10

customer > peer > provider routes

i.e. customer routes most preferred

Question 11

Is BGP guaranteed to be stable?

Answer 11

No, in fact interdomain routing can oscillate indefinitely