That summary route will effectively make EIGRP to summarize any possible network advertised out that interface to a default route. In other words, it is a trick how to send a default route (and only a default route) in EIGRP through a particular interface. The "180" number in that command is an administrative distance and it is there to prevent that the automatically added discard route to 0.0.0.0/0 via Null0 will override your own existing correct default route.
If you are not using EIGRP you can safely remove that command.
Thanks very much, beginning to understand. Maybe this hasn't been added in error, maybe this is correct as we do use EIGRP. The config is used for our branch routers in a hub and spoke set up. The branch routers connect to two routers at Head Office using 4 VTI tunnels. We use the following command to load balance across them.
traffic-share min across-interfaces
I've checked the ip route table and as you said there is the following default route.
D*EX 0.0.0.0/0 [170/22802432] via 10.152.32.253, 1w1d, Tunnel328
[170/22802432] via 10.152.32.189, 1w1d, Tunnel326
On the two other tunnels the summary command hasn't been used so in my routing table I have entries for every network. I guess in a hub and spoke set up we should be using a summary address to keep the routing table down to a minimum. Is there anything to be wary of when using this?
In a hub and spoke network it is generally good practice to keep the routing table at the spoke as small as possible. The EIGRP summary 0.0.0.0 is an effective way to do this. The main thing to be wary about is the issue that Peter and Giuseppe have mentioned: when you configure the EIGRP summary 0.0.0.0 the router will put into the routing table a route for the prefix with next hop of null 0. The default administrative distance for this "discard" route is 5. This would result that if you have a real default route pointing somewhere that it would be replaced by the "discard" route that the summary creates. By supplying an administrative distance like 180 you let the real default route still be preferred.
Another aspect of the EIGRP summary 0.0.0.0 was mentioned in a presentation at the recent Cisco Live: using the EIGRP summary 0.0.0.0 will cause the router to advertise a default route, even if the router does not really have a working default route. This could potentially result in the spoke router sending packets to the hub which the hub can not route and must discard. For many of us the benefit of simplification of the routing table is worth it and the discarded packets are not a serious enough problem to outweigh the benefit of EIGRP summary 0.0.0.0. Wheher it is a concern in your environment is something that you need to resolve using your knowledge of your environment.
There indeed is a default route but notice that it is an EIGRP External route - it is marked as D EX and its administrative distance is 170. That does not align with the configured summary route - it would be an internal EIGRP route with the administrative distance of 90.
It seems that your central router actually redistributes the default route into EIGRP. Look for "redistribute" commands in its configuration. In that case, the configured summary does not re-summarize the redistributed 0.0.0.0/0 to itself, rather, it is sent as-is, therefore it is announced as external.
In this case, the configured summary command merely prevents more specific routes to be advertised via your tunnel to the other endpoint, however, as the default route is already imported into EIGRP, it would be announced so or so. You could achieve the very same effect in your case simply by using a distribute-list and allowing only the default route to be sent to your neighbor.
Of course, the golden rule here is: if it works, don't fix it :)
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
Im studying the 100-105 book by Odom and am currently on the topic of Port security. I purchased a used 2960 and I'm trying to follow a...
While deploying a number of 18xx/2802/3802 model access points (APs), which run AP-COS as their operating platform. It can be observed on some occasions that while many of their access points were able to join the fabric WLC withou...
I am going to design and build an LAN network under a tunnel underground with long distance between the switches.
I will have 2 Catalyst switches and 8 Industrial IE3000, and they will be connected with fiber.
For now I am planning on use Layer-2 s...