1. What Are Routers?
Routers are an intermediate system at the network layer that is used to connect networks together based on a common network layer protocol

2. Router types Router types Example Hardware router Software router
A device that performs routing as a dedicated function
Software router
A router that is not dedicated to performing routing only, but performs routing as one of multiple processes running on the router computer

3. Main routing components include:
Routing interface
Routing protocol
Routing table

4. What Are Routing Interfaces?
A routing interface is an interface over which IP packets are forwarded

5. What Are Routing Protocols?
A routing protocol is a set of messages that routers use to determine the appropriate path to forward data
RIP
OSPF
Designed for small to medium-size networks
Uses a routing table
Easier to configure and manage
Does not scale well
Designed for large to very large networks
Uses a link-state database
Complex to configure and manage
Operates efficiently in large networks

6. What Are Routing Tables?
A routing table is a series of entries called routes that contain information about the location of the network IDs in the internetwork

7. Interior Routing Protocols
The internet is a connection of many different networks. We have seen how to navigate around this big system
How do we make our routing decisions within a network?

8. Autonomous Systems Owned and administered by a single organization
Frequently a backbone system connecting a number of LANs (e.g., University)
Usually connected to the Internet at one or more points
Use routers to connect the LANS

9. Autonomous System
LAN 1
LAN 2
LAN 3
LAN 4
LAN 5
LAN 6
Backbone link

10. Large Autonomous System
The Autonomous System of a large organization could include many routers, and cover a wide geographical area
Telstra’s AS includes about 400 routing nodes

11. Routing Problem in AS
Much of the traffic generated within the AS would be addressed to stations within the AS
Traffic to or from the Internet needs to reach the correct LAN
If it is a WAN, it might be used by the Internet to conduct packets over long distances

12. Routing Problem in AS
Any of these situations reduce to the following problem. What is the “best” way to get a packet from router A to router B?
Router A is the first router to receive the packet
Router B is the router connected to the LAN where the destination station is connected or is the router which passes the packet onto its next hop

13. Routing Problem in AS We assign a weight to each link
To avoid congestion, or balance load, the weight is a function of the utilization of the link, eg u2
Usually, traffic will be different in each direction, in the same link
The same link must then have two different weights for the two directions

14. Weight Function
Why not use a weight function which is simply equal to the utilization?
An extremely simple example will show why - two routers and two links

15. Weight Function
Total traffic is 100 Mbps, bandwidth of each link is 100 Mbps
Might split traffic 50-50, or, say, 75-25
Sutilisation = = 1 (first case)
Sutilisation = = 1 (second case)
Cannot discriminate between these cases
Cannot tell us which is the “best”

16. Weight Function However, if weight is u2
Su2 = = = 0.5 (first case)
Su2 = = = (second case)
Prefer first case, ie (80% of 0.625)
Higher powers, eg u4, give stronger discrimination

17. Weight Function u4
Su4 = = = (first case)
Su4 = = = (second case)
Prefer first case, ie 50-50
First case gives total weight 39% of second case

18. One Solution Based on Bellman-Ford (distance vector) algorithm
Neighboring routers send each other their routing tables

19. RIP Protocol
Routing Information Protocol (RIP) uses weight of 1 for each link min hop count
Max hop count allowed is only 16 - after that, RIP assumes destination is unreachable
Uses elapsed time as cost function

20. Dijkstra’s (Link State) Algorithm
Every router has a map of the complete network
All link costs are available to every router
Router computes best route, independently of other routers
Routers use Dijkstra’s algorithm for this computation

21. Dijkstra’s Algorithm
Algorithm to find lowest cost from router ‘s’ to all other routers in the network
Algorithm proceeds by looking at routers directly connected to router “p”
Step 1. Set RC(s) = 0, mark this RC as permanent (route cost to router “s”). Set RC to all other routers = . They are temporary. Set p = s.

22. Dijkstra’s Algorithm
Step 2. For all routers, “x”, connected to router p, and which have temporary RCs, update the RC as follows RC = min(present RC, RC(p) + c(p,x)) where c(p,x) = cost of link from p to x
Step 3. Find minimum of all temporary RCs. Let this be RC(i)

23. Dijkstra’s Algorithm Step 4. Make this RC permanent. Set p = i
Step 5. If all RC are permanent, stop. Otherwise, return to step 2

24. Example Network 4 2 3 1 3 7 3 6 1 3 6 5 4 4 5 4

25. The Algorithm

26. Two Way Traffic
In practice, we will find different traffic flow in the two directions through one link
If the link cost is a measure of flow, speed or congestion, it should be different in the two directions
Thus a single link, as shown in diagrams so far, should be replaced with two directed links

27. Network with Directed Links4 1 2 2 1 5 4 1 3 2 4 7 3 6 4 5 3
c(1,2)  c(2,1)

28. Effect of Routing Tables on Traffic Pattern
A single change in a routing table produces a change in the traffic pattern
This produces a change in the link costs
Which may produce a change in a routing table, and so on
The system may settle down, or may continue in a “limit cycle”

29. Possibility of alternate routes
Routing table normally defines one route for any source/destination pair
Can achieve better utilisation of network if two (or more) routes are used
Effect is greater if network is small

30. Two Routes Keep Load Balanced

31. OSPF
The most popular link state protocol is OSPF (Open Shortest Path First)
Developed by the IETF
Capable of calculating more than one “best” path

32. Similar Protocols
Similar protocols are Intermediate Host to Intermediate Host (IS-IS)
IS-IS is designed for OSI networks
Extended Interior Gateway Routing Protocol (EIGRP)
Developed by Cisco before OSPF came out