1. COMP 3270 Computer Networks
Routing
COMP 3270 Computer Networks
Computing Science
Thompson Rivers University

2. Learning Objectives
Update the routing table after a RIP router receives the routing information from a neighbor RIP router.
Understand the motivations of Link State Routing and Path Vector Routing.

3. UNICAST ROUTING PROTOCOLS
A routing table can be either static or dynamic.
A static table is one with manual entries.
A dynamic table is one that is updated automatically when there is a change somewhere in the Internet. [Q] How?
A routing protocol is a combination of rules and procedures that lets routers in the Internet inform each other of changes.
Topics discussed in this section:
Autonomous Systems
Intra- and Inter-domain Routing
Distance Vector Routing and RIP
Link State Routing and OSPF
Path Vector Routing and BGP

4. Autonomous Systems
An Autonomous System (AS) is a collection of connected Internet Protocol (IP) routing prefixes (i.e., address blocks) under the control of one or more network operators that presents a common, clearly defined routing policy to the Internet.
[Q] What is the definition of the Internet?
Another definition of the Internet with ASes:
The internet is a network of ASes.
AS-1
AS-2
AS-N

5. Intra- and Interdomain Routing
The Internet of autonomous systems
Border gateways
At a router,
When the destination of a packet is in another AS, Packet forwarding using default routing entry, up to a border gateway.
When the destination of a packet is in the same AS, Packet forwarding using network specific routing entry to the destination host.
At a border gateway,
Forwarded to another border gateway, or
Forwarded to a router in the same AS
OSPF
RIP
BGP
OSPF
BGP
BGP
RIP

6. Popular routing protocols
i.e., within an AS
i.e., between ASes
(Routing Information Protocol)
(Open Shortest Path First)
(Border Gateway Protocol)

7.  How to update routing tables?
Distance Vector Routing Algorithm
Initialization of routing tables
 How to update routing tables?

8. In distance vector routing, each node shares its routing table with its immediate neighbors periodically and when there is a change, i.e., exchange routing tables between immediate neighbors.

9.  The cost between A and C ?
Updating
 The cost between A and C ?
1. Add the cost (cost 2 between A and C in this example) to the
cost for each advertised destination.
2. Repeat the following steps for each advertised destination:
If (destination is not in the routing table)
Add the advertised information to the table.
Else
If (next-hop field is the same)
Replace entry in the table with the advertised one.
If (advertised hop count smaller than one in the table)
Replace entry in the routing table.
3. Return.

10. RIP uses cost 1 for any link.
Another example of RIP (Routing Information Protocol) that uses DVR algorithm
RIP uses cost 1 for any link.

11. Link State Routing Algorithm
Concept of link state routing – OSPF (Open Shortest Path First)
Link state information of each router is transmitted to all the other routers.
After each router builds up the topology for the network,
the router runs the Dijkstra’s shortest path algorithm to build up the routing table.

12. Path Vector Routing Algorithm
Initial routing tables in path vector routing –
BGP (Border Gateway Protocol)
BGP between border gateways, i.e., between ASes. (Path information, not cost, is exchanged.)

13. Stabilized tables for three autonomous systems
(AS1)
(AS2)
(AS3)
(AS4)
Packets from A1 to D4 does not go through AS1 – AS3 – AS4 that looks like a short cut.