which networks to advertise

[carl_townshend]

I have set up 3 vlans on my switch, with ip subnets 192.10.1.0, 192.10.2.0 and 192.10.3.0, should I advertise these in eigrp as all seoperate or just 192.10.0.0 network ?

[ankurbhasin]

Hi Carl,

Let the newtork statement be as simple as possible

router eigrp

network 192.10.0.0

will take care of all.

When the network command is configured for an EIGRP routing process, the router matches one or more local interfaces. The network command matches only local interfaces that are configured with addresses that are within the same subnet as the address that has been configured with the network command. The router then establishes neighbors through the matched interfaces.

So 192.168.0.0 will match 192.168.1.0 and 192.168.2.0 and 192.168.3.0

Incase you have another network 192.168.4.0 and you do not want it to be advertised then it is advisable to advertise specific interfaces otherwise it will also adertise 192.168.4.0

HTH, if yes please rate the post.

Ankur

[Richard Burts]

I am afraid that Ankur is incorrect in his response. The network statement in EIGRP operates in a classful fashion (it differentiates class A, class B, and class C addresses). network 192.10.0.0 is a class C network and it will NOT match 192.10.1.0 or 192.10.2.0 or 192.10.3.0.

I agree with Ankur's advice to be as simple as possible. But when you have 3 class C addresses you will need 3 network statements under EIGRP.

HTH

Rick

[sundar.palaniappan]

Rick is correct with one small exception. One network statement (see below) can accomplish this by using the wild card bits.

router eigrp 1

network 192.10.0.0 0.0.3.255

HTH,

Sundar

*Please rate all helpful posts.

[pkhatri]

Sundar,

I'd have to go with Rick here. The network statement you have will also include 192.168.10.0/24 which is not part of Carl's requirements - he only wanted to match on 192.168.10.1-3/24

Paresh

[sundar.palaniappan]

That would be true only if he has 192.10.0.0/24 configured on an interface. There's no mention of that in the original post.

My point was you don't need an individual network statement for every class C network to activate EIGRP on an interface. You could use the wild card bits to simplify the config.

Sundar

[sundar.palaniappan]

As a matter of fact, if you want to simplify the config and make it precise this should accomplish both of that.

router eigrp 1

network 192.10.2.0 0.0.1.255

network 192.10.1.0 0.0.0.255

hehe, we saved one line of config :-)

HTH,

Sundar

[carl_townshend]

can you tell me why you put the one in on the mask ?

[pkhatri]

Carl,

When you use the 'network 192.10.2.0 0.0.1.255', with the one on the mask, it tells the router to match on either a 1 or a 0 in the last bit of the third octet. As a result, that will match both 192.168.2.0 and 192.168.3.0.

Pls do remember to rate posts.

Paresh

[ankurbhasin]

Hi Rick,Sundar,Paresh,

Thanks for correcting me.One always learn.

WHY I got this concept cause in network which I had seen it was class B network which was subnetted but advertised as 172.16.0.0 which gave me a wrong concept but got it now. MY 5 points to all.

Can you people clear me bit more on this

For suppose I have 172.16.10.10/24 on my interface and if I configure my network statement with 172.16.0.0 keeping auto summarisation disabled it should advertise complete subnet right or I am wrong here.

Thanks anyways!!

Regards,

Ankur

[pkhatri]

Ankur,

The network statement only serves to determine what interfaces EIGRP runs on ... even if you configure a /16 such as 172.16.0.0, that does not mean that auto-summarisation to that network will take place. Therefore, in your example, EIGRP will only advertise 172.16.10.0/24 - it will not autosummarise it to 172.16.0.0/16. However, any other interfaces that fall within 172.16.0.0/16 will be enabled for EIGRP and will have their subnets advertised.

Paresh

[ankurbhasin]

Hi Paresh,

Thanks for your quick response.

I am still in doubt I tested in lab and it is right what rick explained.

But I am thinking in my mind again and again that WHY 192.168.0.0 will not cover 192.168.10.x/24 and 192.168.20.x/24 keeping auto summarisation disabled.

Network statement 192.168.0.0 which as per me means cover everything beyond 192.168

Regards,

Ankur

[pkhatri]

Ankur,

When you specify a network statement under EIGRP, it defaults to being interpreted in a classful context. Now, 192.168.x.x is a Class C network, which means that a network statement of 'network 192.168.0.0' will only cover 192.168.0.0/24. In fact, if the first octet is anywhere between 192 and 223, the same reasoning will apply.

Any networks in the 128-192 range are considered to be class B, and will be interpreted as having a /16 mask.

Hope that makes it a bit clearer.

Paresh

[ankurbhasin]

Hi Paresh,

Thanks for the example. Understood now but let me take a last example to give me full understanding.

I have 192.168.10.10/25 on one of my interface and my network statement is 192.168.10.0 and no wild card statement it will cover everything in 192.168.10.0 network right but because auto summarization is disabled it will advertise with /25 correct.

Regards,

Ankur

[pkhatri]

Hey Ankur,

That is absolutely correct.

What you should also keep in mind is that even if auto-summarisation is in effect, it will only auto-summarise if you have more than one classful network statement configured. In that case, updates for network A sent out out an interface belonging to network B will be auto-summarised to A's classful boundary.

Paresh