Preventing Load balancing in EIGRP

Question

In a recent project, I have been told to consider four Hubs (two in HQ, and two in DR), where each branch will have EIGRP neighborships with all four, as part of a DMVPN based WAN. I have tested this in my lab and found that the branch learns the same routes from all the hubs with the same metric, causing it to load balance traffic which is undesirable. As per my knowledge, EIGRP has 3 ways of metric manipulations :

1. Delay change
2. B/W
3. Offset list to directly manipulate the composite metric

However, since DMVPN utilises a GRE multipoint concept, I cannot make any changes to B/W or Delay on the Spoke side (and all servers lie in the DCs behind the hubs). Offset lists works well, but don't seem to be a good solution (uses an access-list as a reference which needs to be changed again once customer has more servers).

Another idea I have is to just send a default route to the spokes, with summary-metric set and then use that metric to influence traffic.

I would appreciate any other suggestions, and if there are none, can anyone confirm if I am thinking in the right direction (pros and cons of the default route method).

Answer 1

If you simply want to prevent load balancing, then you can use the maximum-paths command in EIGRP to do that. Just set it to 1:

router eigrp 1
 maximim-paths 1

EIGRP Commands:

maximum-paths (EIGRP)

To control the maximum number of parallel routes that the Enhanced Interior Gateway Routing Protocol (EIGRP) can support, use the maximum-paths command in the appropriate configuration mode. To remove the maximum-paths command from the configuration file and restore the system to its default condition with respect to the routing protocol, use the no form of this command.

maximum-paths maximum

no maximum-paths

Answer 2

"As per my knowledge, EIGRP has 3 ways of metric manipulations : Delay change, B/W, and Offset list to directly manipulate the composite metric"

There is another feature of EIGRP to consider: External Routes. One trick I have used to influence EIGRP Primary and Secondary paths over DMVPN is to use a dedicated EIGRP AS for the secondary path and then redistribute it into the primary AS. Redistributed routes will be injected into the EIGRP process as External Routes. Externals Routes will have an AD (Administrative Distance) of 190 by default and therefore be less preferred. This is not always optimal nor can it always be applied in all use-cases but it is something to consider.

"However, since DMVPN utilizes a gre multipoint concept, I cannot make any changes to B/W or Delay on the Spoke side ( and all servers lie in the DCs behind the hubs)."

It sounds like you are using a single Tunnel interface to define both DMVPNs at the Spoke-End. Split your DMVPN at the Spoke-End into two tunnels and you can influence EIGRP path selection with tunnel Bandwidth or Delay.