Monday, April 5, 2010


Just a quick review, Distance Vector routing protocols are easy to configure but it does not have many features. Examples of DV protocols are RIP and IGRP. Link state on the other hand require more knowledge to properly configure, but offers a lot of features. Examples of LS protocols are OSPF and IS-IS. A hybrid protocol like EIGRP combines the best of both worlds but is proprietary.
DV is simple but is prone to some weaknesses. DV by nature sends the entire routing table at specific intervals. Due to this, it consumes a lot of bandwidth. Also, DV is prone to something known as the count-down to infinity.

Let's look at this simple set up: - A - - B - - C -

Now suppose that all the routers have all the routes. However, C suddenly loses the connection. C immediately flushes the network out of the table.

Suppose that B sends an update before C notifies it of the downed link, C will now receive the route as a route from router B. Initially B has the route from 1 hop away, so the route C has would be 2 hops away, going through B to C. Now, C would advertise that 2 hop route back to B and B's route would become 3 hops away, going to C to B to C. At this point you already have a routing loop.

DV protocols implement features to deal with loops. First of all, anything that is more than 16 hops away would be flushed.

When a network goes down, a triggered update would send a poisoned route with a hop count of 16 hops to remove the route from all tables.

Split horizon also tells routers not to send updates back to the same direction it was received from. In this case, B would not send the route to C.

Finally there is a hold down timer. Every time there is an update about a dropped route, the hold down timer is set. The hold down timer says that it would not believe any update about a route for a certain amount of time. This is to prevent invalid routes from entering back into the system before the poison propagation is complete.

In the exam, the measures can be summarized into:
1) Maximum Distance
2) Route Poisoning
3) Triggered Updates
4) Split Horizon
5) Hold Down Timer

Unlike DV, Link State routing protocols actually form neighbor relationships. After initial routing tables are exchanged, the routers send small, triggered updates. The two most popular Link State protocols are OSPF and IS-IS. Link State protocols maintain neighbor relationships through Hello packets.

Link State protocols are faster to converge and is unlikely to encounter routing loops. It forces you to design your network correctly. LS, however, has higher CPU utilization and has some technical complexities.

No comments :

Post a Comment