RIP Default Route

This topic is to discuss the following lesson:

Hi Rene,

Looking at R1’s configuration, you have not involved/advertised network towards ISP in RIP process. So how it will send RIP updates on ISP connecting interface?

Hello Swapnil

RIP is an Interior Gateway Routing Protocol (IGRP) which means that it functions within an organization’s or an enterprise network. RIP and other IGRPs (such as OSPF or EIGRP) are not regularly used by ISPs and their internal networks as they usually use BGP.

So R1 will not share its routing information with the ISP router nor with the ISP router share its information with R1. (If sharing of routes between ISP and R1 routers was enabled, it would most likely use BGP).

However, in order for R1 to route correctly to 4.4.4.4 (and to any other destination on the Internet), it will used a default route as configured in the lesson:

R1(config)#ip route 0.0.0.0 0.0.0.0 192.168.14.4

The ISP1 router in turn will also have to have a route to the internal networks of the enterprise. Something like:

ip route 192.16812.0 255.255.254.0 192.168.14.1

Since there is no routing protocol (such as BGP) configured between the R1 and ISP1 routers, these must be configured statically.

I hope this has been helpful!

Laz

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Hi,

In the lesson, it is stated that R1 sens RIP updates toward ISP also, but under ‘router rip’ there is no 192.168.14.0 so it shouldn’t send? Could you check again?

R1(config)#router rip
R1(config-router)#version 2
R1(config-router)#no auto-summary 
R1(config-router)#network 192.168.12.0
R1(config-router)#network 192.168.13.0

regards,

Hello Murat

When a router is configured to fun the RIP routing protocol, it sends out RIP updates from ALL of its interfaces, regardless of which networks are being advertised. This means that RIP updates will be sent to the ISP, but will be ignored. For this reason, it is a good idea to make such interfaces passive, to eliminate needless RIP updates.

Keep in mind that the network command simply states which networks will be advertised. It does not affect which interfaces will send RIP updates. It is the passive command that disables the (default) behaviour of sending out updates.

I hope this has been helpful!

Laz

Hello,

Just a few comments:

On the topology picture ISP1 should have .4 on Gi0/1.

Also the behavior for RIP is a bit different than OSPF related to default-information originate.
Default route is injected in RIP updates although there is no default route configured on DUT that does default-information originate. Not sure yet if this is a bug or wanted behavior…

So the note “The default-information originate command only works if you have a default route in your own routing table.” is not mandatory.

Many thanks,
Stefanita

Hello Stuat

Thanks, I’ll let Rene know.

Concerning the default-information originate command, I just labbed it up to confirm, and yes, RIPv2 does indeed send a default route when the command is initiated. Specifically, if you implement the command on R2, for example, R1 will insert a default route to R2 even if there is no default route configured in R2.

For this reason, if you want to redistribute a statically assigned route, including a statically assigned default route, you should preferably use the redistribute static command. That way, using my previous example, R2 will only send a default route to R1 if it exists, which is probably safer. If the default route is learned from another routing protocol, then the redistribute ospf, or eigrp can be used instead.

Now compared to OSPF, if you issue this command, a default route will be sent only if the always keyword is added to the end of the command, which will essentially be the same behaviour as that of the default-information originate command of RIP.

I’ll let Rene know to make clarifications.

I hope this has been helpful!

Laz

Hello @staut_stefanita,

I removed this note. I haven’t checked but sometimes things change between IOS versions. It’s possible that on IOS 12.4, the default route was required but on IOS 15 not anymore.

Thanks for letting us know!

Rene

Just a short question, it took me a while to really understand default routes: Is a static default route the same as a default route? (aren’t default routes always static?)

Hello Marit

A default route is the route that is used to route packets when there is no specific match to any other route in the routing table. This is also known as the gateway of last resort. It is defined by using the 0.0.0.0 0.0.0.0 designation which essentially matches any IPv4 address.

Now a default route can be statically assigned simply using the command:

ip route 0.0.0.0 0.0.0.0 X.Y.Z.A

where X.Y.Z.A is the IP address of the next hop used for the gateway of last resort. However, a default route can also be learned dynamically via a routing protocol. You can configure a router to share it’s own default route (whether statically defined or dynamically learned) with other routers using a routing protocol.

For OSPF or RIP, this is done using the default-information originate command in the router configuration. For EIGRP you have some additional options which are further described in the EIGRP Default network Route lesson.

I hope this has been helpful!

Laz

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Ah, yes of course, I get it now, thanks :slight_smile:

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Hello Laz,

Your statement above(When a router is configured to fun the RIP routing protocol, it sends out RIP updates from ALL of its interfaces, regardless of which networks are being advertised) appears to contradict the statement in the Rip Configuration Lesson.

We use the router rip command to go to the RIP configuration. Next step is to use the network command which does two things:

  1. All networks that fall in the range of the network command will be advertised in RIP to other routers.
  2. RIP updates will be sent on the interface that falls in the range of the network command.

Please advise

Hello Edwin

You are absolutely correct, and I labbed it up to confirm. RIP does only send and receive updates on interfaces with networks within the range of the network command’s network. In actuality, the note in the lesson that states that R1 will send RIP updates to the ISP router is inaccurate. I’ll let Rene know to change that…

Thanks, and as always, I hope this has been helpful!

Laz