Rip Routing a Distance Vector Protocol

Distance Vector Protocols: RIP routing

Routing update, classfull and classless routings and IP routing protocols have been discussed previously in the IP routing guide. Now this article discusses the distance vector routing protocols: RIP routing. Check here for
the basic concept of the link state routing protocols.

RIP routing and other distance vector protocols advertise routing information by sending routing updates, out the interfaces on a router. These updates contain a series of entries, with each entry representing a subnet and a metric. The metric represents how good the route is from that router’s perspective, with a smaller number being a better route.

Any routers that receive a copy of a distance vector routing update receive that information and possibly add some routes to their routing table. The receiving router adds the routes only if the routing update described a route to a subnet that it did not already know about or if it described a route that already was known, but the newly learned route has a better (lower) metric.

Figure below shows basic distance vector routing updates with resulting learned route.

In the figure above, router A advertises the route (that includes only the subnet number and a metric) to its LAN subnet to Router B. router B updates its routing table with more information than the routing update itself and receives the update from its serial 0 interface. So Router B considers Serial0 to be the correct outgoing interface for subnet 10.102.100.0. The update came from IP address 10.102.101.1, so Router B considers that IP address to be the next-hop IP address.

Also, if the distance vector update does not include the subnet mask, as in the figure, Router B assumes that Router A uses the same mask that it does. As it turns out, these routers would not support VLSM because if Router A used a different subnet mask than Router B, B would make a wrong assumption about the mask. The fact that the routing protocol in this example does not transmit mask information also makes it a classful routing protocol. For these examples, assume that all routers are using the same subnet mask in this network—specifically, 255.255.255.0.

The following are characteristics of distance vector routing and concept:

• Even though without a routing protocol, Routers add directly connected subnets to their routing tables.
• Routers send routing updates out their interfaces to advertise the routes that this router already knows. These routes include directly connected routes as well as routes learned from other routers.
• Routers listen for routing updates from their neighbors so that they can learn new routes.
• The routing information includes the subnet number and a metric. The metric defines how good the route is; lower metric routes are considered better routes.
• When possible, routers use broadcasts or multicasts to send routing updates. By using a broadcast or multicast packet, all neighbors on a LAN can receive the same routing information in a single update.
• If a router learns multiple routes to the same subnet, the router chooses the best route based on the metric.
• Routers send periodic full updates and expect to receive periodic updates from neighboring routers.
• Failure to receive updates from a neighbor in a timely manner results in the removal of the routes previously learned from that neighbor.
• A router assumes that, for a route advertised by Router X, the next-hop router in that route is Router X.

RIP version 1 Routing

RIP v1 routing is an easy tool to use for comparison with the other routing protocols. RIP uses hop count for a metric. That means, from an individual router’s perspective, if there are two routers between itself and a subnet, its metric for that subnet is 2.

By examining the above figure, router B’s metrics for its locally attached subnets are both 0 because there are no routers between B and those subnets.

Similarly, Router A’s metric for 172.101.100.0 is 0. Because Router B separates Router A from subnet 172.101.100.0, Router A’s metric for subnet 172.101.103.0 is 1. Finally, Router C’s metric for subnet 172.101.103.0 is 2 because two routers separate it from that subnet.

The following lists some of RIP-1 routing features compared to other routing protocols:

• Based on distance vector Logic
• Uses hop count for the metric
• Full Routing updates are sent every 30 seconds
• Convergence time can take 3 to 5 minutes – very slow
• It’s a classfull protocol since it does not support VLSM

RIP Version 2 Routing

RIP Version 2 (RIP-2) routing has several enhancements to the original RIP protocol. RIP-2 still uses distance vector logic; uses hop count for the metric; sends full, periodic updates; and still converges relatively slowly. Compared to RIP-1 routing, RIP-2 does support for VLSM like other link-state routing proctocols such as EIGRP and OSPF, making it a classless routing protocol, with RIP-2 including the subnet mask for each subnet in the routing updates.

The following table outlines the improvements made to RIP with the creation of RIP-2.

The most important feature comparing the two RIP versions is that RIP-2 supports VLSM. Today, RIP-1 was in the past. If you choose RIP routing, RIP-2 is more functional. If you want a routing protocol that uses a public standard and you want to avoid the complexity of link-state protocols, RIP-2 is your best choice.

References: Cisco and CCNA exam books

See also:

• Designing the IP address
• Route summarizations – calculation guidelines
• Basic guidelines in configuring router password
• Windows XP network – best practice
• General guidelines in networking tutorial