Dynamic Routing With RIP

8/14/2019 Dynamic Routing With RIP

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Dynamic routing with RIP

Takeaway: Although RIP routing can take up precious CPU cycles, it may in some

instances be your only option. RIP is a simple protocol to configure, and Todd Lammle is

here to explain the gory details.

The Routing Information Protocol (RIP) was the first dynamic routing protocol to be used in

an internetwork, so it was created and used primarily with UNIX hosts for the purpose of

sharing routing information.

In my last Daily Drill Down, Configuring static and default routing, I covered both default

and static routing, so now we're going to jump into dynamic RIP routing using Cisco routers.

The rumor mill

Like most early inceptions, RIP certainly does have its limitations! A biggie is that it's adistance vector routing protocol, meaning that it sends the complete routing table out all

active interfaces by default on periodic time intervals. This is known as routing by rumor

because a router that receives those broadcasts just believes the information, even though it

has no way to verify that information as true (that the routes really exist).

Why use RIP?

So why do we use RIP at all? Well, we probably shouldn't. But sadly, there are some routers

that don't run anything but RIP (or OSPF), so we're not always left with a choice. Maybe

lurking somewhere in your network is an old legacy router, say, a UNIX router. That being

the case, you just might be stuck supporting RIP on the network so this old horse can

participate in the routing updates.

Because IT spending budgets aren't always unlimited, this Daily Drill Down will show you

how to replace static routing and add dynamic routing using the RIP protocol. The good news

is that adding RIP routing on a Cisco router is really easy, to say the least. The bad news is

that what happens in the background is the problem! RIP sends out periodic updates every 30

seconds by default. This nasty habit can cause problems on already overly stressed serial

links. But no worriesfuture Daily Drill Downs will cover some solutions and strategies for

dealing with the bandwidth gremlins and evils caused by RIP routing.

Deleting the existing static routesTo show you how to configure RIP, I'm going to use the same figure I used in my last Daily

Drill Down, Configuring static and default routing (see Figure A). I'm not doing that

because I'm lazyit'll really help you understand if we build on our existing model! Plus,

before we begin, I'm going to show you how to remove our existing static routes and replace

all that with the dynamic routing protocol RIP.

Figure A
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Each router must have all three networks in the routing table in order to send packets through

the internetwork.

At this point, the routing tables look like this.

The 2500A router must know how to get to the 192.168.10.96 network, so a static route was

used telling the router to forward all packets to 192.168.10.66, which is the next hop.

Next,let's look at the 2500B router.

The 2500B router has a static route that describes how to get to subnet 32 and a default route

to the Internet.

Removing the static routes

First, before we can add RIP routing, we must remove the static routes, since the

administrative distance of a static route is one (1) by default. RIP uses an administrative

distance of 120 by default. If we added RIP without first removing the static routes, then RIP

routes would never be updated and placed in the routing table.

From the 2500A router, we need to remove the route to subnet 96. Here is how that will look.

The routing table now only shows the directly connected routes.

From the 2500B router, we'll remove the static route to subnet 32, but leave the default route

to the Internet.

Notice that the routing table now showsonly the two directly connected and the default

router to the Internet.

Configuring RIP routing

Now that the static routes have been removed, we can add the RIP routing protocol to each

router and allow the dynamic routing to find each network and place these into the routing
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tables of both routers.

From the 2500A router, the router rip command is used to turn on RIP routing.

The router rip command turns on RIP routing and places you at the config-router prompt. The

network command must be used to tell RIP which network you want advertised with RIP.Notice that this is a classful network address and a subnet address is not used. The router will

now find all the subnets and advertise each one throughout the internetwork. RIP is a classful

routing protocol, which means that every device on the network must use the same subnet

mask.

Let's add RIP to the 2500B router.

Now that both routers have been configured with the RIP routing protocol,let's take a look at

the routing tables.

The Rs represent a RIP-found route. The most interesting part of this output is the R*, whichindicates a default route. We have a default route configured on the 2500B router, which is

now being advertised by RIP as the networks default routeperfect since that's our Internet

connection!

Here's the routing table on the 2500B router.

This output tells us that the 32 subnet was found and is being advertised by the serial 0

interface of the 2500A router. The default route has been both found and advertised by RIP to

the 2500A router.

Verifying RIP routing

The show ip route command is probably the best command to use when verifying your

routers configuration of routing protocols, but there are a few more commands that are pretty

helpful, as well.

show ip protocols

The show ip protocols command provides routing protocol information, including the timers

and neighbors found. >Here is an example from the 2500A router.

All configured routing protocols on the router will be displayed with this command, but only

RIP is configured, so we only see RIP information. This command provides the update time,hold down and flush timers, as well as the neighbor's IP address and administrative distance.

Show protocols

Although the show protocols command displays no routing protocol information, it does

provide a list of all configured interfaces and the logical addresses that are configured on

each interface. This is truly very valuable information.

2500A#show protocols

Global values:

Internet Protocol routing is enabled

Ethernet0 is up, line protocol is up

Internet address is 192.168.10.33/27Serial0 is up, line protocol is up
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Internet address is 192.168.10.65/27

Serial1 is administratively down, line protocol is down

2500A#

Debug ip rip

The debug ip rip command is a great way to see RIP updates being sent and received on therouter. To turn off debugging, >use the no debug ip rip command or the undebug all

command.

>Let's take this apart piece by piece, because this is important information.

Notice that the 2500A router is sending out Ethernet0 an update telling about the path to

subnets 64 and 96. The metric is the hop count to each network. Since the 2500A router is

connected to the 64 network, it's advertising that route in one hop. However, the 96 subnet is

one hop away, so it will advertise that route as a hop count of 2. The default route is also

being advertised as two hops away.

>Let's take a look at the next output.

Notice that network 32 is only being advertised out serial 0. Why? Because the 2500B router

already knows about network 64 and 96. This is called the split-horizon rule, and it's used to

stop incorrect information from being advertised throughout the internetwork. If networks 64

and 96 were advertised out serial 0 of the 2500A router, then the 2500B router might consider

the 2500A router to get to the 96 subnet. That would be bad because it's incorrect; it's how

network loops can occur.

Okay, now >let's look at what we are receiving.

The 2500B router is advertising two routers to the 2500A router. One describes how to get to

the 96 subnet, and the other advertises the default route.

Suppressing RIP updates

But wait. Remember the Ethernet0 interface of 2500A advertising RIP networks? RIP is

advertised out all active interface routes every 30 seconds by default. So what if we don't

want RIP advertised out Ethernet0, or even worse, the serial 1 interface of the 2500B router

that's connected to the Internet? Can you say security problem? Do you really want to

advertise your network out to the Internet? We'll solve that little issue with the passive-

interface command.

>Here is an example of stopping RIP from being sent out the Ethernet0 interface of the

2500A router and the serial 1 and Ethernet0 interfaces of the 2500B router.

Now, let's take a look at>a debug ip rip command from the 2500B router.

The 2500B router is now sending routing information out serial 0 only. Okay, let's check>the

2500A router with the debug ip rip command.

Cool! The 2500A router is now advertising the routing table only out serial 0 and not out

Ethernet0.
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Conclusion

One man's trash is another man's treasureRIP routing is actually a good, fast routing

protocol in smaller networks, but if you configure larger networks, you can be in for some

serious trouble because RIP has a tendency to consume bandwidth and CPU processes!

Documents

Dynamic Routing With RIP