4 - Routing

Question 1

Examples of an Autonomous System

Answer 1

Internet service provider, content provider, campus network

Question 2

intra-domain routing

Answer 2

process by which traffic is routed inside any single autonomous system

Question 3

inter-domain routing

Answer 3

process of routing traffic between autonomous systems

Question 4

A topology inside of an autonomous system consist of:

Answer 4

nodes and edges.

Question 5

Points of Presence (PoP)

Answer 5

nodes inside of an autonomous system

Question 6

PoP is typically located in a dense population center so that:

Answer 6

It can be close to the PoPs of other providers for easier interconnection and also close to other customers for cheaper back haul to customers that may be purchasing connectivity from this particular AS

Question 7

PoPs are constrained by:

Answer 7

fiber paths

Question 8

Routing is the process by which:

Answer 8

nodes discover where to forward traffic so that it reaches a certain node.

Question 9

2 types of intra-domain routing

Answer 9

distance vector and link state

Question 10

Distance Vector routing

Answer 10

Each node sends multiple distance vectors to each of its neighbors, which amounts to copies of its own routing table.

routers then compute costs to each destination in the topo, based on shortest available path.

based on bellman-ford algorithm.

Question 11

Routing Information Protocol (RIP)

Answer 11

a distance vector routing protocol.

edges have unit cost

infinite = 16

table refresh: 30 seconds or when updates occur.

Tell all neighbors except for one that caused update (“split horizon”)

Question 12

Downsides of Routing Information Protocol (RIP)

Answer 12

1. slow convergence

2. count to infinity

Question 13

Link State Routing

Answer 13

used in most networks

distribute network map

each node performs shortest path (SPF) computation

Iniitially adds costs of immediate neighbors, flood costs to neighbors

Dijkstras algorithm

Question 14

2 common link state routing protocols and which one is more commonly used?

Answer 14

1. Open Shortest Paths First (OSPF)

2. Intermediate System - Intermediate System (IS-IS) (used more)

Question 15

Problem with link state routing

Answer 15

scale

The complexity of a link state routing protocol grows as n cubed where n is the # of nodes in the network

Question 16

How to cope with scale (related to link state routing)

Answer 16

introduce hierarchy

OSPF: Areas

IS-IS: Levels

Shortest path computed within area or stitching together the shortest path to the area zero backbone router, and the shortest path across area zero, followed by another intra area shortest path

Question 17

Route Advertisements

Answer 17

Each AS advertises reachability to some destination by sending these (in interdomain routing).

used Border Gateway Protocol (BGP)

Question 18

Route advertisement contains these 3 things (BGP route attributes)

Answer 18

Destination IP, Next Hop IP, AS path

Last number on AS path is known as the “origin AS”, which originated the announcement

Question 19

eBGP (external Border Gateway Protocol)

Answer 19

Consisting of route advertisements solely between border routers of adjacent autonomous systems

Responsible for transmitting routing information between border routers of adjacent ASes about external destinations

Question 20

iBGP (internal border gateway protocol)

Answer 20

The protocol that is used to transmit routes inside an autonomous system for external destinations

Responsible for disseminating BGP route advertisements about external destinations to routers inside any particular AS

Question 21

IGP (intra-domain routing protocol)

Answer 21

routes inside an AS to internal destinations

Question 22

iBGP (internal-gateway border protocol)

Answer 22

routes inside an AS to external destinations

Question 23

BGP Route Selection Process

Answer 23

1. Prefer higher “local preference”
2. Shortest AS path length
3. Multi-exit discriminator (MED) (lower med values preferred and only comparable among routes from the same AS)
4. Shortest IGP path (“hot potato” routing)
5. Tiebreak -> “most stable”, lowest router ID