This document discusses routing in computer networks. It explains that packet switches forward packets based on next hop information in routing tables rather than complete destination information. Routing is hierarchical, with destinations grouped, and next hops do not depend on the source, allowing for efficient routing. The routing process can be modeled using graphs to represent switches and connections and compute optimal routes through a wide area network using algorithms on the graph. Routing tables can be reduced in size by using default routes for destinations without explicit entries.

1. COMPUTER NETWORKSCOMPUTER NETWORKS
CS610
Lecture-16
Hammad Khalid Khan

2. Review Lecture 15
 Wide Area Networks
 Packet Switches
 Physical Addressing in a WAN
 Store and Forward
 Next Hop Forwarding

3. Choosing Next Hop
 Packet switch doesn't keep complete information about all
possible destination
 Just keeps next hop
 So, for each packet, packet switch looks up destination in
table and forwards through connection to next hop

4. Choosing Next Hop

5. Source Independence
 Next hop to destination does not depend on source of packet,
called Source Independence
 Allows fast, efficient routing
 Packet switch need not have complete information, just next
hop
– Reduces total information
– Increases dynamic robustness - network can continue to
function even if topology changes without notifying entire
network

6. Hierarchical Addressing and
Routing
The Process of forwarding the packets from one hop to
the next is known as Routing

7. Hierarchical Addressing and
Routing

8. Hierarchical Addressing and
Routing
 In particular, all destinations on same switch have same next
hop
 Thus, routing table can be collapsed:

9. Routing in a WAN
 More computers == more traffic
 Can add capacity to WAN by adding more links and packet
switches
 Packet switches need not have computers attached
 Interior switch - No attached computers
 Exterior switch - Attached computers

10. Routing in a WAN
 Both Interior and Exterior switches:
– Forward Packets
– Need Routing Tables
 Must have:
– Universal Routing - Next hop for each possible destination
– Optimal Routes - Next hop in table must be on shortest
path to destination


11. Modeling a WAN
 Use a graph:
– Nodes model switches
– Edges model direct connections between switches
 Captures essence of network, ignoring attached computers

12. Modeling a WAN

13. Route Computation with a Graph
 Can represent routing table with edges:
 Graph algorithms can be applied to find routes

14. Redundant Routing Information
 Notice duplication of information in routing table for node 1:
 Switch 1 has only one outgoing connection; all traffic must
traverse that connection

15. Default Routes
 Can collapse routing table entries with a Default Route
 If destination does not have an explicit routing table entry,
use the default route:

16. Default Routes

17. Summary
 Source Independence
 Hierarchical Addressing and Routing
 Routing in a WAN
 Default Routes