EIGRP Summary Address

[darrenriley5]

I've noticed on some of our VTI tunnel interfaces there is an additional line of config. This seems to have been added in error.

ip summary-address eigrp 100 0.0.0.0 0.0.0.0 180

Just wondered what this is doing and if it will have an adverse affect? Below is the config of my tunnel. Everything seems to be working fine.

Tunnel1

bandwidth 2000

ip address 10.162.32.189 255.255.255.252

ip mtu 1374

ip virtual-reassembly

ip route-cache flow

ip tcp adjust-mss 1334

ip summary-address eigrp 100 0.0.0.0 0.0.0.0 180

tunnel source 10.66.0.300

tunnel destination 10.162.128.252

tunnel mode ipsec ipv4

tunnel sequence-datagrams

tunnel checksum

tunnel protection ipsec profile VTI

[Peter Paluch]

Hi,

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.

Best regards,

Peter

[Giuseppe Larosa]

Hello Darren,

that line creates a default route to be advertised out the interface.

What is very important for a working network is last parameter:

180 means ad 180.

EIGRP creates a local default route to null0 with AD 180.

if you don't place this the default route to null0 is installed in routing table of local node and would create a black hole.

on remote site you should see only a 0.0.0./0 default route learned by EIGRP.

A similar effect could be achieved using a distribute list that calls a prefix list like

ip prefix-list only-default permit 0.0.0.0/0

router eigrp 100

distribute-list prefix only-default tunnel1 out

this could work if an eigrp default route already exists in EIGRP topology table.

Hope to help

Giuseppe

[Peter Paluch]

Giuseppe,

We seem to be almost "fighting" for answering the topics :) I hope that I have not offended you. I enjoy reading your suggestions and I learn a lot!

Best regards,

Peter

[darrenriley5]

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?

Many Thanks

[Giuseppe Larosa]

Hello Darren,

yes this confirms the summary routes are in use.

You should be fine and the reason for using them as you noted is to minimize EIGRP topology size on remote sites routers.

Peter:

you are wellcome, this is a friendly environment and learning is a never ending process.

Hope to help

Giuseppe

[Richard Burts]

Darren

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.

HTH

Rick

[Peter Paluch]

Hello,

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 :)

Best regards,

Peter