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CCNA, Route Summarization with RIP

I'm studying CCNA. The following setup does actually build RIP routes without problem. All 3 routers I typed following commands;

Config t
Router Rip
Network 172.16.0.0

I'm confused. How does this work when all 3 routers have different subnets, but having same same rip network statements with the same summarized network? I thought summarization is for combining routes of next hop's IP network, not router's for 2 or more hops away.

Can someone clarify it?
rip-sum.jpg
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crcsupport
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crcsupport
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2 Solutions
 
Cyclops3590Commented:
First off, you didn't specify "version 2" so with RIPv1 there's no summarization, everything is classful.

Second, RIP configuration, no matter version, is always configured with classful network statements.  A network statement just says two things
1) Turn on RIP for any interface with an IP that is in that classful network
2) Announce the connected route for the interface that RIP gets turned on

So while you have three routers and three different subnets, it'll work because of the above.  keep in mind RIPv1 doesn't do mask info which is why its never really used anywhere.  do a "sh ip route" to confirm what all you have.  ripv1 has issues depending on your exact ip scheme you use within your network

summarization is great when you have a nicely designed network with a hierarchical structure as to its IP scheme.

For example.  Let's say you have 10.0.0.0/8 space to assign.  You have 5 sites.  You assign as such.
10.1.0.0/16
10.2.0.0/16
10.3.0.0/16
10.4.0.0/16
10.5.0.0/16

Now you won't have each site be designed with a single subnet that can hold 65534 hosts.  That's just ludicrous and asking for problems.  That /16 space will be in turn divided further at that site as each site pleases.  However, If you're a host on 10.1.200.0/24 and want to get to 10.5.20.0/24.  Your gateway has not need to know the specific route to the /24 as anything in the 10.5.0.0/16 will go to site 5.  As such, you configure summarization so that site 5 only announces that one route.  This help keep the routing table smaller so less memory is used as well as routing decisions can be made faster.

so if you want yours to work type the following in the router rip

version 2
no auto-summary
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crcsupportAuthor Commented:
"1) Turn on RIP for any interface with an IP that is in that classful network
2) Announce the connected route for the interface that RIP gets turned on"

So, since rip is classful, the summarization network statement is correct for 3 routers having the same network statement.
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crcsupportAuthor Commented:
As long as most distant network is less than max hop 15, then I may assume RIP can deliver frame with its learned routes. So, let's say if 13 routers are all connected horizontally, then with same single statement 'network 172.16.0.0', all 13 routers will populate rip routes correctly. Correct?
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crcsupportAuthor Commented:
Reading your statement again, without 'version 2' statement, the 3 routers populate all routes correctly. PC0 can ping PC2, which means, summarization works with RIPv1. Why?
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Cyclops3590Commented:
yes, same statement.

it works (if I'm remembering this right) because you are using a contiguous network.  Specifics, I need to go back and refresh my memory as I never use RIPv1.  I'll get back to you on that one.  But there is some type of assumption I remember when using RIPv1 about the masks but it only really works with contiguous networks.  if you have you have your left and right networks in the same classful network and the center one in a different one then it wouldn't work because their discontiguous and the center network gets confused how to route to the correct network.
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Don JohnstonInstructorCommented:
RIP is a bit of a goofy protocol. Especially when configuring in a Cisco environment.

First, the network statement is classful. Doesn't matter if you're doing v1 or v2. If you enter "network 10.2.3.0", the IOS will automatically change it to "network 10.0.0.0". Which means that all interfaces with an IP address starting with 10. will be considered RIP networks.

Second, summarization is on by default. Except that it's summarization to the classful boundary. Unless you disable it (v2 only) with the "no auto-summary" command.  With summarization enabled, any subnets will be summarized to the classful boundary IF they are advertised out an interface that is not part of the subnet.  

For example, you have a router with two interfaces: E0 and E1. E0 has the IP address 10.1.2.3/24 and E1 is 172.16.1.1/24.  So if you look in the routing table, you will see:

10.1.2.0/24 directly connected to E0
172.16.1.0/24 directly connected to E1

But if you look at the updates the router is sending out E0, you will see 172.16.0.0.  Out E1, you'll see 10.0.0.0

Now if the E0 interface has an IP address of 172.16.2.2/24, then you would see the 172.16.1.0 network being advertised out that interface.
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crcsupportAuthor Commented:
It makes sense. So, RIP is always classful, user doesn't have to care about how it collects routes as long as it's inside max hop counts.
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Don JohnstonInstructorCommented:
Always is a strong word. :-)

It's classful when discussing the network statement in the configuration.

But with RIP version 2, the updates are not technically classful (if auto-summary is disabled).
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