Solved ICND2 Enabling EIGRP or OSPF on interfaces via network statements

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I read (though I haven't found confirmation from the Cisco site) that the best practice for advertising EIGRP networks is to really just advertise the interface on which that network is connected. So instead of figuring out the proper wildcard mask to use in advertising a network, just copy the IP address of the interface and give it s wildcard mask of '0.0.0.0'.
So the first step would be to perform a 'show ip interface brief', then enter the router EIGRP configuration mode. After which you enter your network statements for those interfaces you wish to advertise out.
Example: router EIGRP 100
network 192.168.16.2 0.0.0.0
network 172.14.0.1 0.0.0.0
This works great.

In the practice labs, I have use this configuration procedure for both EIGRP and OSPF and I cannot see the downside to doing it this way. I am wondering what is the drawback with using this procedure for either protocol. Thanks and Great work!

@Acky,

This is a way to see proof of concept when you need to do it quickly. It will work in production as well. But...there are a couple of drawbacks:

1. Exam wise, this will not do because it doesn't show you know wildcard masking. So even though it works it's not proof of concept of wildcard mask.

2. In production, it tends to be advertising the more than what you're normally trying to do when you use EIGRP or OSPF. It's Advertising too much when you do it.

Hi Acky,

This way of enabling EIGRP via matching the interface IP address is actually mentioned in the Official Cisco Cert Guide for the 200-105 exam. (Just finished working through it)

It is only briefly touched on as a way of enabling EIGRP on a specific interface, but you should get used to doing wildcard masks, as this will likely be what is used in the exam

Thanks,

I understand the need to know wildcard masking for the exam. I don't have any problems figuring out the wildcard mask. I just thought it was interesting that you could apply it in this manner and in production, I thought it would make it simpler.

Ronnie, you say it advertises more than needed. Wow! I am trying to get my head wrapped around that. Could you be more specific or provide examples. When you advertise using the interface- what subnet are you actually advertising? I am not sure what you mean- or how it does advertise more. Thanks for all your help.

@Acky said in ICND2 Enabling EIGRP or OSPF on interfaces via network statements:

When you advertise using the interface- what subnet are you actually advertising? I am not sure what you mean- or how it does advertise more. Thanks for all your help.

This is the key here. You need to ask yourself what subnet are you intending to advertise. IS

router eigrp 1
network 172.16.0.0 0.0.0.0
no auto-summary

the same as

router eigrp 1
network 172.16.96.0 0.0.0.0
no auto-summary

if it is, what if you didn't mean to advertise network 172.16.63.0 or what if you only wanted to advertise 172.16.96.0?

how does the all zero wildcard mask do that or is it doing it automagically?

Ronnie,

I understand what you're saying and can see the difference in your example. But what network are you advertising when you list the specific IP address of the interface?
Such as : router eigrp 1
network 172.16.96.1 0.0.0.0
no auto summary

Does the router figure out what subnet this is attached to and only limit it to that? Or are you saying it 'opens" it up to the classful address, or something else?
It the listing of the specific interface address with the network command that has me hung up.

Again, I know, it's the wildcard masks I should be focused on for the test, but I would like to understand this better.

Oh, and Lee- thanks, I did find the spot in the Official Cert Guide (R&S INCD2 200-105), page 249. you referenced. It didn't give any details as to the pros or cons.

Thanks for all the help.

@Acky,

I'm saying that this is something you can try.

router eigrp 1
network 172.16.0.0 0.0.0.0
no auto-summary

What networks will be advertised out?
What if I don't want 172.16.63.0/24 to be advertised out.
what if I only want 172.16.0.0/24 to be advertised out.
How would I configure this with the above configuration?

Hi Ronnie,

In your example,
What networks will be advertised out?
For network 172.16.0.0 0.0.0.0
I would say the whole class B network of 172.16.0.0 would be advertised.

What if I don't want 172.16.63.0/24 to be advertised out.
Then I would give a network statement like: network 172.16.64.0 0.0.0.255

What if I only want 172.16.0.0/24 to be advertised out.
How would I configure this with the above configuration?
I would then provide: network 172.16.0.0 0.0.0.255

thanks, Acky

@Acky said in ICND2 Enabling EIGRP or OSPF on interfaces via network statements:

Hi Ronnie,

In your example,
What networks will be advertised out?
For network 172.16.0.0 0.0.0.0
I would say the whole class B network of 172.16.0.0 would be advertised.

What if I don't want 172.16.63.0/24 to be advertised out.
Then I would give a network statement like: network 172.16.64.0 0.0.0.255

What if I only want 172.16.0.0/24 to be advertised out.
How would I configure this with the above configuration?
I would then provide: network 172.16.0.0 0.0.0.255

thanks, Acky

What happens when you have only the 32bit wildcard mask already listed?
Can you do either of the other two? for example

router eigrp 1
network 172.16.0.0 0.0.0.0
network 172.16.0.0 0.0.0.255

would I be able to use the original 32bit subnet mask or not...if I want to exclude the .63 network?

My point was not an exercise in wildcard mask ability as much as it was to get you see that even though the 32 bit can work. It doesn't work in every situation. So even though it works, if you're not wanting to configure each interface with eigrp or enter multiple network statements. It doesn't work if you have even one subnet in that range that you do not want advertised.

I'm probably not making myself clear; that is what I wanted you to see. We use the 32 bit wildmask all the time just to get things to work when answering a question or setting up a lab but this fails when I wanted to that 172.16.63.0/24 to be advertised out ospf instead of eigrp (or vice versa). Or when I only want a limited number of networks to be advertised, which is normally the case in production networks.

Ronnie,

OK, I can see in your example that the 172.16.0.0 0.0.0.0 statement is really "overriding" the other network statement.

But in your examples, you have always given a network or subnet ID in the statements. What I wasn't clear on was the fact that with a single interface address and a 32 bit wildcard mask in the network statement, that you would be advertising every network attached to that interface. Or more so, you would be advertising every network within that classful address, as well?

I can see where that could cause problems and/or limitations.

Thanks
Acky

@Acky,

I think I misread your post, I was assuming you were doing the network. My apologies.

When you issue the

router eigrp 1
**network 172.16.35.1 0.0.0.0**
no auto-summary

Do a show ip protocols on the router receiving the eigrp advertisement and the results you get are the following:

Routing Protocol is "eigrp 1"
  Outgoing update filter list for all interfaces is not set
  Incoming update filter list for all interfaces is not set
  Default networks flagged in outgoing updates
  Default networks accepted from incoming updates
  EIGRP-IPv4 Protocol for AS(1)
    Metric weight K1=1, K2=0, K3=1, K4=0, K5=0
    NSF-aware route hold timer is 240
    Router-ID: 172.16.36.1
    Topology : 0 (base) 
      Active Timer: 3 min
      Distance: internal 90 external 170
      Maximum path: 4
      Maximum hopcount 100
      Maximum metric variance 1
 
  Automatic Summarization: disabled
  Maximum path: 4
 ** Routing for Networks:
    172.16.35.1/32**
  Routing Information Sources:
    Gateway         Distance      Last Update
  Distance: internal 90 external 170

In your example, it would only advertise that one interface IP. I marked the relevant portion with **

My apologies for taking you down another rabbit hole.

Ronnie,

I did perform the lab on EIGRP and I did see the advertising of only one interface in the 'show IP Protocols' command, but I cannot find any difference when it comes to reaching anything on the network attached to that interface.

So, I am still at a loss for understanding why (besides understanding it for the exam) you wouldn't just advertise the interface since it's easier to implement and it still allows routing to connected network. When you advertise the one interface, it is advertising more than necessary?

So in production, you do not advertise- just the interface?

Acky

@Acky,

That is correct, it only advertises that single IP and not entire network. So, as I posted I read the question wrongly and responded wrongly.

The time it may advertise too much is if you use it as I posted originally.

Thanks again. Awesome site.

Acky

With no additional comments, I'm marking this post as solved.