Manual route summarization

jmc012

Guys
I'm trying to get manual summarization straight in my mind. Doing some experiments in packet tracer when I turn off auto summary with RIP 2 it looks like the main thing is I see is it will advertise all the subnets instead of just the entire thing like 192.1.1.0/24. Is that the main reason for using it, so in a discontiguous network it would keep routers from advertising the same routes? Or am I still confused?
Thanks
Jim

ITdude

Try this on for size:

When you summarize routes in RIP, IGRP, EIGRP, or OSPF, you're replacing a series of routes with a summary route and mask. With RIP, IGRP, and EIGRP, this lessens the size of the routing update packet itself therefore multiple routes are replaced with the summary route. The routes 8.0.0.0/8, 9.0.0.0/8, 10.0.0.0/8, and 11.0.0.0/8 can be summarized as 8.0.0.0 252.0.0.0. So, only the summary address will be found in the update packet.

It can also make the routing table smaller, and permit complete IP connectivity when done correctly. As above the four more-specific routes will be replaced by the single summary route. Since the entire routing table is parsed before the routing process is complete, keeping the routing table as small as possible does help speed the routing process.

HTH

jmc012

ITdude wrote: »

Try this on for size:

ITdude, thanks for the information! I think it is finally starting to sink in.

tech-airman

jmc012 wrote: »

Guys
I'm trying to get manual summarization straight in my mind. Doing some experiments in packet tracer when I turn off auto summary with RIP 2 it looks like the main thing is I see is it will advertise all the subnets instead of just the entire thing like 192.1.1.0/24. Is that the main reason for using it, so in a discontiguous network it would keep routers from advertising the same routes? Or am I still confused?
Thanks
Jim

jmc012,

Within the scope of RIP, there's two things related to summarization you need to consider: 1) Sending RIP updates and 2) Receiving RIP updates.

The default for RIPv1 is to send automatically summarized network updates. So let's say on a three interface router you have the following networks:

1. 192.168.1.0 255.255.255.0
2. 192.168.1.64 255.255.255.192
3. 192.168.1.128 255.255.255.192

So for a RIPv1 routing protocol update, only the following network will be advertized:

1. 192.168.1.0 255.255.255.0

That's because that network address is the automatically summarized network for 192.168.1.0, 192.168.1.64, and 192.168.1.128. But let's say you want to let the more specific subnetworks of 192.168.1.64 and 192.168.1.128 be advertised using RIP. That's where you need to configure the following:

Router#configuration terminal
Router(config)#router rip
Router(config-router)#no auto-summary
Router(config-router)#exit
Router(config)#exit
Router#

The "no auto-summary" part will DISable the automatic summarization that is sent using RIP because of the key word "no." So now that RIP sending updates has been covered, let's talk about RIP receiving updates.

By default, RIP routers listen for and receive both RIPv1 routing updates and RIPv2 routing updates. You can verify this by typing the following:

Router#show ip protocols

So in order to migrate a RIPv1 router fully to RIPv2, you should also specify that this RIP router will only listen for and accept RIPv2 routing updates by typing the following:

Router#configuration terminal
Router(config)#router rip
Router(config-router)#version 2
Router(config-router)#exit
Router(config)#exit
Router#

By configuring the above, you have now manually configured the RIP router to ONLY listen for and accept RIPv2 routing updates.

Did I help you understand manual summarization better?