RIP updates

Hi everybody !

Is there someone who could help me to understand the RIP updates ?
How does the 'debug ip rip' command work ?

When is a network advertised and when is a subnet summarized ?

I'm not sure to have understood everything yet...

I don't "feel" what "major net" mean, for example...

Thanks in advance !

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BubbaJ

You should start with the basics first. Learn about the network classes. An IP address is really just a 32 bit number. The major classes are:

Class A - the fist bit is a 0
Class B - the first two bits are 10
Class C - the first 3 bits are 110

That means the 1st octet of each class looks like this:

Class A - 0 to 127
Class B - 128 to 191
Class C - 192 to 223

Class A is defined to use a network mask of 8 bits. That means the 1st octet represents the network number, and the last 3 represent the host number. NNN.HHH.HHH.HHH

Class B uses a network mask of 16 bits. This means that the 1st and 2nd octets combined are the network number. NNN.NNN.HHH.HHH

Class C uses a network mask of 24 bits. This means the 1st, 2nd, and 3rd octets are the network number. NNN.NNN.NNN.HHH

Subnetting is where you extend the network number into the host bits in order to create more networks with fewer hosts.

Nawakette

Thanks BubbaJ.

Here is an example that I've read in a book but I don't understand...

Imagine you have

Router 1 with FastEthernet0 = 192.168.14.1/255.255.255.0 + Serial0 = 192.168.15.1/255.255.255.0

Router 2 with FastEthernet0 = 192.168.16.1/255.255.255.0 + Serial0 = 192.168.15.2/255.255.255.0

For FastEthernet0 on Router1, (192.168.14.1), the networks in the neighborhood are 192.168.15.0/24 and 192.168.16.0/24.

Take the example of 192.168.15.0/24 :

I read that the rules were the following :

Are 192.168.14.0/24 and 192.168.15.0/24 on the same major network ?
For me, the answer is 'YES' because they are both Class C (192.X.X.X).

Have 192.168.14.0/24 and 192.168.15.0/24 the same subnet ?
For me, the answer is 'YES' because both have a subnet mask 255.255.255.0

In this case, we publish the NETWORK (and not the "subnet") 192.168.15.0/24.

Is it correct ?

Danman32

I agree. Be comfortable with single path static routes before moving on to dynamic routing. Otherwise you'll never figure out OSPF or EIGRP.

Nawakette

And if now we have the following ones :

Router 1 with FastEthernet0 = 172.16.5.2/255.255.255.0 + Serial0 = 172.20.88.2/255.255.255.0

Router 2 with FastEthernet0 = 172.16.2.1/255.255.255.0 + Serial0 = 172.20.88.1/255.255.255.0

For FastEthernet0 on Router 1, (172.16.5.2), the networks in the neighborhood are 172.20.88.0/24 and 172.16.2.0/24.

Take the example of 172.20.88.0/24 :

Are 172.20.88.0/24 and 172.16.5.0/24 on the same major network ?
For me, the answer is 'YES' because they are both Class B (172.X.X.X).

Have 172.20.88.0/24 and 172.16.5.0/24 the same subnet ?
For me, the answer is 'YES' because both have a subnet mask 255.255.255.0

In this case, we publish the NETWORK (and not the "subnet") 172.20.88.0/24.

But according to this link http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd6.shtml#topic1

they summarizes 172.16.5.0/24 and advertises the route 172.16.0.0/16

What does it mean ?
What have I missed ?

Thanks

BubbaJ

Nawakette wrote:

Are 192.168.14.0/24 and 192.168.15.0/24 on the same major network ?
For me, the answer is 'YES' because they are both Class C (192.X.X.X).

No. If you understand what I wrote before, you will see that any network that starts with 192 in th 1st octet uses 3 octets for the major network. The 1st 3 octects are different. They are, however, in the same network class (C).

Nawakette wrote:

Have 192.168.14.0/24 and 192.168.15.0/24 the same subnet ?
For me, the answer is 'YES' because both have a subnet mask 255.255.255.0

No. A subnet is an extension of the network into the host bits. Neither network is actually subnetted since they are Class C networks which natively use 24 bits as a network mask. On a Class C network it doesn't become a subnet until the mask is greater than 24 bits. 255.255.255.0 is only a network mask for a Class C network, but 255.255.255.128 (and 255.255.255.192, 255.255.255.224, 255.255.255.240, 255.255.255.248, 255.255.255.252, 255.255.255.254) is a subnet mask for a Class C network.

Nawakette

Okay, thanks, I think I've understood now

Example 1 :

Are 192.168.14.0/24 and 192.168.15.0/24 on the same major network ?
The answer is 'NO'.
They are not on the same major network (one is on 192.168.14.0 and the other on 192.168.15.0) even if they are of the same Class (C).

So, we have to summarize to the major network boundary. Therefore, we publish the NETWORK (and not the "subnet") 192.168.14.0.

Example 2 :

Are 172.16.5.0/24 and 172.20.88.0/24 on the same major network ?
The answer is 'NO'.
They are not on the same major network (one is on 172.16.0.0 and the other on (172.20.0.0) even if they are of the same Class (B).

So, we have to summarize to the major network boundary. Therefore, we publish the NETWORK (and not the "subnet") 172.16.0.0.

Hope I'm right now

BubbaJ

You are catching on.

Nawakette

Thank you for the help :P

Nawakette

Hello !

Could you please have a look at :

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd6.shtml#topic1

and tell me why they are looking for updates sent via Serial3/0 (172.20.88.2) but they are not looking for updates sent via the FastEhernet interface ?

According what I understood, we should have :

RIP: sending v1 update to 255.255.255.255 via FastEthernet0/0 (172.16.5.2)
RIP: build update entries
network 172.20.0.0 metric 1
subnet 172.16.2.0 metric 2

in addition to :

RIP: sending v1 update to 255.255.255.255 via Serial3/0 (172.20.88.2)
RIP: build update entries
network 172.16.0.0 metric 1

Where is my mistake ?

Idem for the updates received on Router2's Serial2/0
interface but not on the FastEthernet interface...

BubbaJ

What they are showing there is how RIP only looks at the major network. In this case each router thinks it owns the 172.16.0.0 network. RIP v1 has no way to send subnet information.

When one of the routers informs the other that it has a way to get to 172.16.0.0, the other router looks at that thinks that it has a better way (less hops).

Nawakette

I understand that the example I provided only explain how RIP looks at the major network...

But, what I don't understand is, in a general way, which interfaces will "calculate" and send the updates.
Only the Serial interface connected to the other router(s) ?
Or all the interfaces (Serial, Ethernet...) ?

It's not clear in my little brain

Thank you for your help !

BubbaJ

RIP uses the network xxx.xxx.xxx.xxx command under the router configuration to decide which interfaces to use. The command also tels RIP which networks to send, but only if there is an interface on the router with that network. RIP will also send any networks it has received from other RIP routers.

Is this what you are looking for?[/b]

Nawakette

This is an important thing that I didn't take into account

Now, it's more clear... even if there is still some dark things.

I suppose that I can't conclude anything about the Cisco website's example above because I've no information about the RIP configuration (no idea of which networks are configured in the router mode) ?

Suppose you have two routers, Router1 and Router2, configured like this :

Router1 has two interfaces :

FastEthernet0 = 192.168.14.0/255.255.255.0
Serial0 = 192.168.15.1/255.255.255.0

Router2 has two interfaces :

Serial0 = 192.168.15.2/255.255.255.0
FastEthernet0 = 192.168.16.1/255.255.255.0

Suppose you've also :

Router1(config)#router rip
Router1(config-router)#network 192.168.14.0
Router1(config-router)#network 192.168.15.0

Router2(config)#router rip
Router2(config-router)#network 192.168.15.0
Router2(config-router)#network 192.168.16.0

If I understood correctly, Router1 will send update concerning networks 192.168.14.0 and 192.168.15.0 and will use the interfaces FastEthernet0 and Serial0 in order to send it.

But what about network 192.168.16.0 ?

I'm sorry for my stupid questions and sorry to disturb you

BubbaJ

Nawakette wrote:

Suppose you have two routers, Router1 and Router2, configured like this :

Router1 has two interfaces :
FastEthernet0 = 192.168.14.0/255.255.255.0
Serial0 = 192.168.15.1/255.255.255.0

Router2 has two interfaces :
Serial0 = 192.168.15.2/255.255.255.0
FastEthernet0 = 192.168.16.1/255.255.255.0

Suppose you've also :
Router1(config)#router rip
Router1(config-router)#network 192.168.14.0
Router1(config-router)#network 192.168.15.0

Router2(config)#router rip
Router2(config-router)#network 192.168.15.0
Router2(config-router)#network 192.168.16.0

If I understood correctly, Router1 will send update concerning networks 192.168.14.0 and 192.168.15.0 and will use the interfaces FastEthernet0 and Serial0 in order to send it.

But what about network 192.168.16.0 ?

Router1 will learn about 192.168.16.0 from Router2. Router1 will send it, along with the two routes that it has, in an update through its ethernet interface. It will not, however, send it back through the interface from where it learned it. That's the split horizon thing.

Nawakette wrote:

I'm sorry for my stupid questions and sorry to disturb you

You are not disturbing me. This forum is for learning these things, and I can see that you are trying to learn.

Nawakette

Thanks for all BubbaJ. Your help is very usefull for me :P

I've still a last question concerning the RIP updates subject.

In the example found on the Cisco website
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093fd6.shtml , we have :

When Router 2 prepares to receive and update from Router 1, you need to identify the questions that need to be answered.

Again, remember that the reception of network 172.16.5.0/24 is of interest here. However, remember that when Router 1 sent the update the network was summarized to 172.16.0.0/16.

Here is the question you need to answer:
Is the network that receives updates (172.16.0.0/16) part of the same major network of 172.20.88.0, which is the address assigned to the interface that received the update?

--> The answer is 'No'.
Until there, I've understood...

But here I don't understand :

No: Do any subnets of this major network already exist in the routing table known from interfaces other than that which received the update?

Below, they say that the update has been ignored. So it means that "there is no subnet of this major network (172.20.0.0 ???) which already exist in the routing table known from interfaces other than that which received the update"...

Could you please explain ?

Yes: Ignore the update.

However, the routing table of Router 2 indicates that the update was ignored.
The only entry for any subnetwork or network on 172.16.0.0 is the one directly connected to Ethernet0.
The output of the show ip route command on Router 2 shows:

172.20.0.0/24 is subnetted, 1 subnets
C 172.20.88.0 is directly connected, Serial2/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.2.0 is directly connected, Ethernet0/0

BubbaJ

The second question is dealing with the 172.16.0.0 network. Both routers have a subnet for this network. Each router tries to tell the other about it, and each ignores the update from the other since each has a better route to the network.

They are step-by-step questions. The second answer is based on the question in the first question. The major network in question is 172.16.0.0, not the 172.20.0.0 network. If you look at it like that, it should make sense to you.

Nawakette

Thanks a lot BubbaJ