What are IP Routing Protocols?
A routing protocol is a set of rule(s) or standards that determines how routers on a network communicate and exchange information with each other, enabling them to select the best routes to a remote network.
Each router has priority knowledge only of networks attached to it directly. Routers running routing protocol share this information first, among immediate neighbours, then throughout the entire network. This way, routers gain insight and 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 maintains a routing table, which is a list of networks and possible routes known by the router. The routing table includes network addresses for its 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.
IP Routing Protocols (Dynamic)
There are several dynamic routing protocols for IP. They are grouped into Internal and External protocols according to their mode and where they are designed to operate (see diagram above).
IGP vs EGP
Routing protocols are grouped according to their behaviour or individual method of routing.
One option is to classify them based on where they operate within or between autonomous systems or networks.
An autonomous system (AS) characterizes a collection of network devices under a common administrator, an example of this is an internal network of an establishment or a network infrastructure of an ISP.
Another way of grouping is based on whether they exchange routes within an AS or between different autonomous systems:
Interior Gateway Protocols (IGP)
These class of protocols are used to route traffic within the organization network or, AS. They are designed for small, medium-sized, and large organizations, but their scalability has its limits. The protocols can offer very fast convergence time, and basic functionality is not difficult to configure.
The most commonly used IGPs in enterprises are Enhanced Interior Gateway Routing Protocol (EIGRP) and Open Shortest Path First (OSPF) as well as Routing Information Protocol (RIP). These lots are normally network-enabled within the service provider’s internal network, the routing protocol named Intermediate System-to-Intermediate System (IS-IS) is also commonly used.
Exterior Gateway Protocol (EGP).
These groups of protocols are designed for exchanging routes between
different autonomous systems (AS). Border Gateway Protocol (BGP) is the only EGP that
is used today. The main function of BGP is to exchange a huge number of routes
between different autonomous systems that are part of the largest network (the
Internet).
The figure above illustrates the differences between an IGP and an EGP.
The IGPs can be divided into the following groups:
Distance vector protocols: The distance vector routing approach determines the direction (vector) and distance (such as link cost or number of hops) to any link in the network.
Distance vector protocols use routers as guides along the path to the final destination. The guidance only displays direction and distance but does not reveal what the path is like. The only information that a router knows about a remote network is the distance or metric to reach this destination and which path or interface to use to get there.
Distance vector routing protocols do not have an actual map of the network topology. Early distance vector protocols, such as RIPv1 and IGRP, used only the periodic exchange of routing information for a topology change. Later versions of these distance vector protocols (EIGRP and RIPv2) implemented triggered updates to respond to topology changes.
Link-state protocols: The link-state approach uses the Shortest Path First (SPF) algorithm to create an abstract of the exact topology of the entire network or at least within its area. A link-state routing protocol is like having a complete map of the network topology. The map is used to determine the best path to a destination instead of using guides.
The guidance along the way from the source to the destination is not necessary because all link-state routers have an identical “map” of the network. A link-state router uses the link-state information to create a topology map and to select the best path to all destination networks in the topology. The OSPF and IS-IS protocols are examples of link-state routing protocols.
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 protocols. 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 an unreachable and undesirable route in it routing process. This hop count limits the size of the network that RIP operate.
RIP operates a hold-down timer of 180 seconds and transmits full updates every 30 seconds.
RIP implements the hold down, split horizon, and 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 deficiencies of RIP (RIPv1) is the inability 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
RIP v1
In this example, let’s 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 the RIP v1 configuration.
If you want to disable 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#
