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EIGRP network statement

Posted on 2014-03-27
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Last Modified: 2014-03-28
My Cisco router has 2 interfaces, 10.10.10.1/24 and 10.10.20.1/24. I am using EIGRP. Now if I advertise as:

router eigrp 99
network 10.0.0.0
no auto-summary

How will the neighbor see 10.10.10.0/24 and 10.10.20.0/24 through the 10.0.0.0 advertise by this router.

Thanks
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Question by:leblanc
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by:Ken Boone
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So this is a common misunderstanding about the network statement on most routing protocols - rip, ospf, eigrp  

The network statement in these routing protocols does not announce that network from these routing protocols.  Now in BGP that is exactly what it does, but not in the interior protocols.

What the network statement of 10.0.0.0 does is say if any interface on this router can fall under the realm of 10.0.0.0 /8 then allow that interface to participate in in the EIGRP process by exchanging routes with neighbors as well as advertise the network that is associated to that particular interface.

So network 10.0.0.0 will match those two interfaces of 10.10.10.0/24 and 10.10.20.0/24 and will advertise those two networks to the neighbors.  

So the network statement is NOT telling the route to advertise what is stated in the network statement but rather tells the router to check all of your interfaces against this network statement and if it falls within the match then advertise that interfaces network and let that interface participate in the EIGRP process.

Hope that helps!
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by:leblanc
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Clear as crystal... Now you mention that it is not the same in BGP. In other words, if you have network 10.0.0.0, then it will advertise the whole /8 network. Do I understand that correctly?
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by:Ken Boone
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well in BGP you use the network command in order to advertise a specific prefix.    So if you did network 10.0.0.0 mask 255.0.0.0 then yes that would advertise BGP to its neighbor.

See BGP works off of a TCP connection between two ip addresses.  It doesn't work off of the idea that an interface is involved with sending multicast or broadcast packets to its peers.

That is why there is a difference.  Typically in interior routing protocols your neighbors are directly connected.  That is not necessarily the case with BGP.
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by:leblanc
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About BGP, I have 40 subnets under the 10.0.0.0/8 network. Somebody told me that for security reason, I should advertise those 40 subnets, rather than just 10.0.0.0/8. Is that true? I don't think I agree with that. Thoughts?
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Well I am assuming that if you are doing BGP with a 10 net then this is for an MPLS type network - not an internet connection.

I don't think it is so much of a security issue but rather a preference.  It is not so much a security issues because you are advertising on a private network given my assumption in the first sentence.  If all of those 40 subnets are behind one location, then advertising it as a /8 saves on the routing table at all those remote routers - however, with 40 subnets this is negligible.  

I prefer to advertise all the nets in that example, because then I know my entire routing process is up and running and if I don't see particular routes I know where the source of my problem is.

I think it is easier to troubleshoot when you have all of your routes in the table.  If you summarize and then a remote can't reach one of the subnets, you look and hey the remote has the /8 route whereas if it doesn't have the /24 then you know for some reason that particular /24 has an issue.  Its really a preference.  

Now when you are using 40 subnets and advertise a /8 it can also cause problems down the road, because essentially you just used up the whole class A address at that one spot, so if later you want to use more subnets elsewhere you need to do something different.

Hope that helps.
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by:leblanc
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That makes a lot of sense what you just explained. Thank you.
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