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Objectives
After completing this chapter you will be able to:
 Understand how routers operate
 Describe Distance Vector and Link State Algorithms
 Describe hierarchies in IP routing
 Outline the advantages and disadvantages of routing
Path Taken by an IP Packet in an
Internetwork
Destination Host (Protocol Address)
Destination Host
(Physical address)
Destination Host (Protocol Address)
Router 1 (Physical address)
Destination Host (Protocol Address)
Router 2 (Physical address)
1
2
3
A
2
3
B
C
IP Version 4 Message Header
Version
IHL
Type of
Service
Identification
Time to Live Protocol
Total Length
Flags Fragment Offset
Header Checksum
Source Address
Destination address
Options (variable)
DATA (variable)
Padding
IP header is normally
20 bytes long
Default Gateway
IP Address
160.30.100.10
Subnet Mask
255.255.255.0
Default Gateway 160.30.100.1
IP Address
160.30.100.20
Subnet Mask
255.255.255.0
Default Gateway 160.30.100.1
160.30.100.1
160.30.200.1
255.255.255.0
255.255.255.0
IP Address
160.30.200.10
Subnet Mask
255.255.255.0
Default Gateway 160.30.200.1
Proxy ARP
IP Address
Subnet Mask
Default Gateway
IP Address
Subnet Mask
Default Gateway
160.30.100.20
255.255.0.0
160.30.100.1
160.30.200.1
160.30.100.10
255.255.0.0
255.255.255.0
255.255.255.0
IP Address
Subnet Mask
Default Gateway
160.30.200.10
255.255.0.0
Routing Tables
A
C
2
1
3
Routing Table
Network Interface Metric
A
1
10
C
2
65
B
3
65
B
Routing Algorithms
 Distance Vector Algorithms
- calculate the best path to each destination separately,
usually trying to find a path that minimises a simple
metric, such as the number of hop counts to the
destination.
Example is Routing Information Protocol (RIP).
 Link State Algorithms
- Instead of advertising a list of distances to each known
destination, a router running link-state algorithm
advertises the states of its local network links.
Example is Open Shortest Path First (OSPF).
Commonly used Metrics in IP Routing
 Path Length / Hop Count
 Reliability
 Delay
 Bandwidth
 Load
 Communications Cost
Distance Vector Algorithm
1
A
2
B
2
1
3
3
C
4
4
5
5
6
D
6
E
IP Routing Protocol Hierarchies
Router 1
Router 6
H2
Router 8
Router 2
Router 3
Router 7
Area 1
Area 2
Router 4
Area 0
(Backbone)
Router 9
Router 10
Router 11
Router 5
H1
Router 12
Router 13
Area 3
Neighbour Acquisition, Neighbour Reachability and
Network Reachability
Acquisition Request / Hello
Neighbour
Acquisition
Acquisition Reply / Hello
Hello
Neighbour
Reachability
Hello
Polling
Network
Reachability
List
Advantages of Routers
 Flexible - can differentiate between paths using metrics.
 Can load share over redundant paths.
 Network Devices understand routers - they understand
congestion messages.
 Easier to administer and control because each segment
has a different address.
 Provide a protective firewall.
Disadvantages of Routers
 Because routers are protocol-dependent devices, they
require software for each protocol that they run.
 Routers make movement of network devices between
network segments more difficult.
 Some low level protocols, for example, DEC LAT and
NetBIOS cannot be routed because they do not contain a
network layer address.