How can I find out the source of a route in EIGRP

[CSCO10203269]

From an EIGRP troubleshooting perspective, how can I find out the source of a specific route advertisement in the routing table?  With BGP, you can trace a route advertisement down to a specific BGP process ID. Provided a unique BGP process ID is configured on each device in your network, you can determine the source of a route advertisement. EIGRP doesn't seem to have a way to determine that.

[Peter Paluch]

Hello,

EIGRP does have its Router ID but its usage seems to vary between different IOS versions.

For redistributed routes, the originator of the redistributed route is always added as an informational attribute in EIGRP and sent in EIGRP updates along the distributed route. You may read more about it here:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800949ab.shtml

In recent IOS versions, it seems that this Router ID is being added also to internal networks but this behavior is not consistent - I have not seen older IOSes doing it. This must be a recent change. I have noticed the presence of originator's Router ID on internal EIGRP routes on 3560 Catalyst multilayer switches running 12.2(58)SE1 and newer, and I suspect the same would go also for router IOSes around version 15.0 and newer.

Is there any internal Cisco specialist on EIGRP around who could provide more detail on this? Thanks in forward!

Best regards,

Peter

[CSCO10203269]

Peter, thanks for the response and the link.  I'm in a new environment that's running EIGRP exclusively throughout the network -- previous environment was BGP(WAN) / OSPF(LAN) -- and I'm trying to make head/tails of how things are setup.

[Richard Burts]

Perhaps I am not entirely clear about what you are trying to accomplish. But it seems to me that there is a fairly simple answer. If you look in the routing table you can find the address of the next hop/advertising router. You may be able to find additional information about the prefix and the advertising router by looking in the EIGRP topology table. Once you know the address of the advertising router then you can go to that router and look up the prefix. You can do this hop by hop until you get to the origin of the advertisement.

There is not a simple, single command way to find the information. But hop by hop you can find the source of a specific route advertisement.

HTH

Rick

[CSCO10203269]

I'm trying to learn the environment by discovering the advertising router of each subnet. The MPLS routing tables aren't much help since the next hop is the interface to the provider. I've found the topology tables to be more helpful 'provided' its an EIGRP external route entry I'm searching for. If its an EIGRP entry, all the topology table provides is the IP address of the provider's PE router.  Hop by hop is the only way I know to do this, I was just wondering if there was an EIGRP command I haven't used yet.

Thanks

[Richard Burts]

It would be nice if there were a command that would do that. But I do not believe that such a command has been implemented in EIGRP.

HTH

Rick

[dslice]

We added the router-id to internal routes a couple of years ago when we changed the packet format to support some new capabilities. Unfortunately, we can't send the new tlvs to peers that don't understand it, so we have to strip it out when sending to older peers. Since those changes didn't make it into all branches (it was an enhancement after all) not all releases got the change.

Sent from Cisco Technical Support iPad App

[Peter Paluch]

Hello Donald,

Aaah, I see. Thank you for updating! Is it possible to be more specific about the new capabilities themselves that drove this new packet format?

Best regards,

Peter

[dslice]

We had to add new TLVs to support wide metrics (bandwidth above 2G) and some other possibly not-yet-announced features. I don't know what has been made public so I would prefer to leave it at that.  Suffice it to say that while we were adding the new capabilities, we wanted to include the internal router-id for multiple reasons.  Troubleshooting is one (as the original poster mentioned) as well as possible tagging/filtering capabilities.

[Peter Paluch]

Hello Donald,

Thank you very much for this inside info!

Best regards,

Peter

[nseshan]

Hi Peter,

I have a similar question, although it is not entirely pertaining to the intricacies of EIGRP. My question is that if i wish to filter a few prefixes using route-maps, coming into a router running EIGRP with its neighbors, how do i match the route-source in the route-map, if i wish to filter by the source of the route? Or is there any other way to do it if the EIGRP router ID has not been configured for some reason?

Regards,

Narayanan

[Nandan Mathure]

Narayanan,

You can use a "distance 255 SOURCE WILDCARDMASK ACCESS:IST' under the router eigrp AS.

Here Source = Source of routing updates WILDCARD = can be 0.0.0.0 and access-list will define which routes are to be filtered.

Note: distance of 255 makes a route invalid.

Let me know if this helped. Rate useful posts.Thanks,

Nandan Mathure

[dslice]

Narayanan,

It really depends on what you mean by the source of the route.  Is the route an EIGRP internal that you're wanting to filter based on the originating router or immediate peer? Or an external that you're wanting to filter based on where the route was redistributed from?  Please clarify.  Also, the router-id exists whether you've defined it or not.  Manually defining the router-id means that you pick the value to use explicitly instead of letting the algorithm do it for you. 

Nandan, the distance command you described only works for internal routes and the immediate peer.  I don't understand Narayanan's question well enough to know if that is sufficient, but wanted to make sure you were aware of the limitations.

[nseshan]

Hi Donald,

My situation is one wherein, there are two routers in an EIGRP domain, R1 & R2. R1 is also part of an OSPF domain and also a CE for an MPLS cloud. The objective here is to tag all the routes sourced by R2 (mostly via the network statement, hence being internal routes), so as to send them with a tag into MP-BGP over the MPLS cloud. The issue i am facing is that, in case R2 is also linearly connected to another router R3, and i still have to match routes sourced by R2 via the network command, how can i do it?

I ask this as in case of the three linear routers, the forwarding address and the next-hop IP for all routes sourced by R3 and R2 will be the same! Only when R2 is the edge of the domain, will this problem not affect the route-matching/tagging. I hope i have clarified the situation?

Regards,

Narayanan

[Mohamed Sobair]

Just to add to the wonderful posts by all the posters here, my knowledge about the Eigrp Router-ID is not to basically trace the source of the route rather than Preventing Routing Loops that could be caused by External Eigrp Prefixes. plus of course filtering capabilities.

I am aware that it has been added as a feature to the internal Eigrp routes recently but not all IOSs supports it as mentioned by Donald.

Regards,

Mohamed