IP Routing
Protocols
A routing
protocol is a set of rules or standard that determines how routers on a
network communicate and exchange information with each other, enabling them
to select best routes to a remote network, Each router has
priority 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 maintain a routing table, which is a
list of networks and possible routes 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 (Dynamic)
There are
several dynamic routing protocols for IP. Here are some of the more common
dynamic routing protocols for routing IP packets:
- RIP (Routing Information
Protocol)
- IGRP (Interior Gateway Routing
Protocol)
- EIGRP (Enhanced Interior
Gateway Routing Protocol)
- OSPF (Open Shortest Path First)
- IS-IS (Intermediate
System-to-Intermediate System)
- BGP (Border Gateway Protocol
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.
Routing Information Protocol - 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 operates a hold down timer of 180 seconds, transmits
full updates every 30 seconds.
RIP implements 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 variable length subnet masks (VLSM). in essence, RIP version 1 uses only classful routing, which means that all devices on the network use the same subnet mask.
Example
Topology of a network configured with a routing protocol.
All
routers were configured with a dynamic routing protocol RIP v1.
RIP v1 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
In this example, lets assume R2 and R3 have been properly
configured. We are just going to configure R1 for this purpose, we use the network
address IP of 192.168.1.0/27 for the FastEthernet and 192.168.1.96/30 for the Serial interface.
The R1 configuration goes like this:
R1(config)#interface fa0/0
R1(config-if)#ip address 192.168.1.1 255.255.255.224
R1(config-if)#no shutdown
R1(config-if)# interface se0/0/0
R1(config-if)#ip address 192.168.1.97 255.255.255.252
R1(config-if)#no
shutdown
R1(config-if)#router
rip
R1(config-router)#network 192.168.1.0
R1(config-router)#end
R1#
The above configuration was only for you to remember that
you need a classful network statement, which is what is required in RIP v1
configuration.
If you want to stop RIP updates from being propagated on the
serial interface, (but the serial interface can still receive RIP updates), use
the following configuration command:
R1config t
R1(config)#router rip
R1(config-router)#network 192.168.1.0
R1(config-router)#passive-interface se0/0/0
R1(config-router)end#
R1#
