Routing Protocols
A routing protocol is a set of rules or standard that determines how routers on a network communicate with each other and exchange information to enable them select best routes to a remote network, Each router has proirity knowledge only of networks attached to it directly. Routers running routing protocol shares this information first among immediate neighbors, then throughout the entire network. This way, routers gain an insight knowledge of the topology of the network.
Routing protocols perform several activities, including:
* Network discovery
* Updating and maintaining routing tables
The router which sits at the base of a network maitains a routing table, which is a list of networks known by the router. The routing table includes network addresses for its own interfaces, which are the directly connected networks, as well as network addresses for remote networks. A remote network is a network that can only be reached by forwarding the packet to another router.
Remote networks are added to the routing table in two ways:
i. By the network administrator manually configuring static routes.
ii. By implementing a dynamic routing protocol.
Dynamic Routing protocols are used by routers to share information about the reachability and status of remote networks.
IP Routing Protocols
There are several dynamic routing protocols for IP. Here are some of the more common dynamic routing protocols for routing IP packets:
Advantages of dynamic routing protocols
i. Dynamic routing protocols update and maintain the networks in their routing tables.
ii. Dynamic routing protocols not only make a best path determination to various networks, they will also determine a new best path if the initial path becomes unusable or there is a change in the topology.
iii. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator.
RIP
The Routing Information Protocol (RIP) is one of the Internet's first widely used routing protocol. It is still useful in local and medium area networks.
RIP is classified as a distance-vector routing protocol, which employs the hop count as a routing metric, The maximum number of hops allowed for RIP is 15. A hop count of 16 is considered an infinite distance viewing such distance as unreachable and undesirable route in it routing process. This hop count limits the size of network that RIP operate.
RIP opperates a hold down timer of 180 seconds, transmits full updates every 30 seconds.
RIP impliments the hold down, split horizon, route poisoning mechanisms to prevent incorrect routing information from being propagated. These and many more are some of the stability features of RIP.
RIP version 1
One of the deficiency of RIP (RIPv1) is the inability to to propagate periodic routing updates of subnet information, and also does not support varable length subnet masks (VLSM).
RIPv1 Characteristics
* A classful, Distance Vector (DV) routing protocol
* Routing Metric - Hop count
* Routes with hop count > 15 are unreachable
* updates are broadcast every 30 seconds
* Default administrative distance is 120
Example Topology of a network configured with a routing protocol.
All routers were configured with a dynamic routing protocol RIPv2. The figures below are routing tables of the three networks. The colour area is a learned route
Network America Routing table
America#show ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.1.1.0/30 is directly connected, Serial0/0/1
R 10.10.10.0/24 [120/1] via 172.16.10.6, 00:00:00, Serial0/0/0
172.16.0.0/30 is subnetted, 1 subnets
C 172.16.10.4 is directly connected, Serial0/0/0
C 192.168.20.0/24 is directly connected, FastEthernet0/0
R 209.165.200.0/24 [120/1] via 10.1.1.1, 00:00:28, Serial0/0/1
Network Iraq routing table
IRAQ#show ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
R 10.1.1.0/30 [120/1] via 172.16.10.5, 00:00:11, Serial0/0/0
C 10.10.10.0/24 is directly connected, FastEthernet0/0
172.16.0.0/30 is subnetted, 1 subnets
C 172.16.10.4 is directly connected, Serial0/0/0
R 192.168.20.0/24 [120/1] via 172.16.10.5, 00:00:11, Serial0/0/0
R 209.165.200.0/24 [120/2] via 172.16.10.5, 00:00:11, Serial0/0/0
London Network routing table
LONDON#show ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C 10.1.1.0/30 is directly connected, Serial0/0/0
R 10.10.10.0/24 [120/2] via 10.1.1.2, 00:00:06, Serial0/0/0
172.16.0.0/30 is subnetted, 1 subnets
R 172.16.10.4 [120/1] via 10.1.1.2, 00:00:06, Serial0/0/0
R 192.168.20.0/24 [120/1] via 10.1.1.2, 00:00:06, Serial0/0/0
C 209.165.200.0/24 is directly connected, FastEthernet0/0
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