EIGRP Issues - Load Balance x Summary Routes

[fabiogalini]

Hello All!!!

I have a scenario where we have a layer 3 switch - SW1 - EIGRP peering with 2 routers - R1 and R2 - (dual homed) through a ethernet segment. There are too many networks behind those 2 routers and so there is the need to get them sumarized to the layer 3 switch. So far i looks pretty simple, but the main problem is the load balance once we have a subnet down behind one of those routers.

I have both summaries configured on the LAN interfaces of both routers so i see them load balancing on my SW1's routing table. But once we have a problem and a specific subnet is down behind just one of the routers i start to miss packets.

For SW1 no subnet is down because of the summary so it still continues to forward traffic to both routers. So If we have per packet load balance configured i miss half the packets to that destination. In the other hand, if i got per destination load balance configured i miss all of the packets to that given destination.

Does anyone sees a solution for that without taking summarization off the table?

Thanks.

[Richard Burts]

What you are describing is a normal behavior of EIGRP. When you summarize then the routers to whom you summarize lose knowledge of the detail, and that will lead them to continue to forward to the summarizing router even if a subnet behind the summary is actually down to that router.

As I see it you have several options to consider:

- you can keep the summary and live with the occasional loss of packets if a subnet goes down (consider it a cost of making the network more efficient).

- you can remove summarization. You eliminate the occasional packet loss but there is added complexity in the routing decision and it introduces additional instability into the routing process of the layer 3 switch.

- you can get the routers to communicate with each other and share the detail of what is in the subnets behind the summary. How you could do that depends on the topology of the network (which we do not know). One option to consider would be a GRE tunnel between the routers over the Ethernet. This way the EIGRP advertisement onto the Ethernet contains only the summary, but the routers can forward to each other packets for which the other router has the better (or only) path.

HTH

Rick

[fabiogalini]

Hello Richard,

The way I see, to sacrifice availability in exchange for efficiency doesn't look like a viable option for me. Also take summarization off is not desirable. GRE Tunnels looks like a good choice, and technical duable, however since the routers are not managed by me in the real world, that could bring some complications.

I was trying to find a solution within the LAN, but since it looks like there is no choice, Ill have to get in contact with my WAN provider and find a solution suitable for both environments.

Thanks for your comments!!

Does anyone else have any other guesses?

[Jose Jara]

Fabio,

I think the easiest solution would be to the change the config in the routers of your WAN provider. They should share between each other the prefixes for the case that one subnet is not available. If they have BGP towards the WAN, easiest thing would be with iBGP. Other option would be to send the summary plus the specifics, in this case all the devices in the ethernet segment would have all the details. In order to prevent the L3 switch to learn all the specific subnets, you may apply filtering allowing only the summaries. For example, using a leak-map (route-map LEAK permit 10.. ) in the ip summary-address command, would allow all the specifics. Then you would have to allow just the summaries, for example with a distribute-list.

Best Regards,

Jose.