RIP Distance Vector Routing Protocol

RIP is a distance vector routing protocol and the simplest routing protocol to start with. We’ll start by paying attention to the distance vector class. What does the name distance vector mean?

• Distance: How far away, in routing world we use metrics which we just discussed.
• Vector: Which direction, in routing world we care about which interface and the IP-address of the next router to send it to.

I don’t know if you ever go cycling but here in The Netherlands we have some nice so called mushroom signposts telling you which way to go and how far (in kilometers) the destination is. The same principle applies to distance vector routing protocols.

If you don’t like cycling you’ll like the highway signs better:

Enough about cycling and highways let’s see how distance vector routing protocols operate.

In this picture we have three routers and we are running a distance vector routing protocol (RIP). As we start our routers they build a routing table by default but the only thing they know are their directly connected interfaces. You can see that this information is in their routing table. In red you can see which interface and in green you can see the metric. RIP uses hop count as its metric which is nothing more than counting the number of routers (hops) you have to pass to get to your destination.

Now I’m going to enable distance vector routing, what will happen is that our routers will copy their routing table to their directly connected neighbor. R1 will copy its routing table to R2. R2 will copy its routing table to R3 and the other way around.

If a router receives information about a network it doesn’t know about yet, it will add this information to its routing table:

Take a look at R1 and you will see that it has learned about the 192.168.23.0 /24 and 3.3.3.0 /24 network from R2. You see that it has added the interface (Fa1/0) how to reach these networks (that’s the vector part) and you see that it has added the metric (hop count) for these networks (that’s the distance part).

192.168.23.0 /24 is one hop away, 3.3.3.0 /24 is two hops away.

Awesome! You also see that R2 and R3 have filled their routing tables.

Every 30 seconds our routers will send a full copy of their routing table to their neighbors who can update their own routing table.

So far so good, our routers are working and we know the destination to all of our networks…distance vector routing protocols are vulnerable to some problems however. Let me show you what can go wrong:

The FastEthernet 1/0 link on R3 is going down, so it will change its routing table. Its status went from 0 to Down.

Every 30 seconds our routers send a full copy of their routing table to their neighbors and it just happens to be that it’s time for R2 to send a copy. R2 sends his full routing table towards R3. What do you think will happen?

R3 gets the routing table from R2 and will see that R2 is advertising the 3.3.3.0 /24 network with a hop count of 1. That’s excellent is what R3 thinks….a hop count of 1 is better than having a network that is down. R3 will add this information to its routing table.

A few seconds later it’s time for R3 to send its routing table to its neighbor R2. R2 will come to the following conclusion:

“I can reach 3.3.3.0 /24 by going to R3 and my hop count used to be 1. I’m receiving a routing table from R3 now and it now says that the hop count is 2…I need to update myself to include this change”.

The hop count on R2 is now 3, it received 2 from R3 plus it adds the hop towards R3.

R2 will also send a copy of its routing table towards R1 who will update itself as well.

Do you see where this is going? These routers are going to keep updating themselves to infinity.

What will happen when we send an IP packet to the 3.3.3.0 /24 network?

Look at the routing table of R2 and R3, they are pointing to each other. Ladies and Gentleman…we have a routing loop! That’s not a good thing, IP packets have a TTL (Time to Live) field however so they won’t loop around forever like Ethernet Frames do.

To prevent routers from updating themselves over and over again we have a maximum. For RIP this is a hop count of 16. 16 is considered unreachable so the maximum number of hops you can have is 15.

This problem is called counting to infinity.

There is something else we use to counter the counting to infinity problem. In our example R3 advertised the 3.3.3.0 /24 network towards R2. How useful is it that R2 advertises this network towards R3?

That’s like telling someone a joke that you just learned from that person…not very effective (unless the joke is extremely good/lame 

In routing it’s not very effective so whatever you learn from your neighbor you are not going to advertise back to him. We call this split horizon.

Something else we do is that once a network goes down (3.3.3.0 /24 in our example) the router will send a triggered update immediately to update its neighbors.

The triggered update will contain the network that is down and an infinite metric (16 in the case of RIP). Sending a update for this network with an infinite metric is called route poisoning.

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To make sure R3 does not update itself via some other router / path in the network, R2 will send a poison reverse in response to the route poison it has received from R3.

I just explained to you what split horizon is…”don’t advertise whatever you learned from your neighbor back to them”.

“Route poisoning overrules split horizon”

There is one more thing we use with distance vector routing protocols. When R2 and R1 find out that our 3.3.3.0 /24 network is down they will also start a holddown timer. This holddown timer will run for 180 seconds and it does the following:

• If we receive information about the 3.3.3.0 /24 network from another router with the same or worse metric than we currently have, we ignore this information.
• If we receive information about the 3.3.3.0 /24 network from another router with a better metric, we stop the holddown timer and update our routing table with this new information.
• If we don’t receive anything and the holddown timer elapses we remove this network from the routing table.

What do you think? That’s plenty of information about RIP and distance vector routing protocols right?

Let me finish this lesson by giving you an overview between RIP version 1 and version 2. They are similar with a few differences:

RIP	Version 1	Version 2
Class	Classful	Classless
Addressing Type	Broadcast	Multicast
Support manual summaries	No	Yes
Authentication support	No	Yes

RIP version 1 is classful so it does not send a subnet mask along with its routing updates, version 2 is classless so it will send the subnet mask along. RIP version 1 sends everything using broadcasts compared to RIP version 2 which uses multicast (you can also use unicast btw).

That’s all I have for you now. In another lesson I will show you all the RIP commands you need to know for CCNA! I hope you enjoyed this lesson, if it was helpful to you please leave a comment!