# CCNP - EIGRP Load Balancing

Oct 12th, 2007

In EIGRP, a feasible successor is a neighbor whose reported distance is less than the feasible distance.

As stated above if a path elected as a FS, its RD should be less than the FD. That's ok, but what if the Reported Distance is equal to feasible distance, in such a case can this path be a Feasible Successor?

For instance, assume that you have 5 path to the destination network with the following values;

Path 1: FD-40, RD-20

Path 2: FD-40, RD-20

Path 3: FD-20, RD-10

Path 4: FD:50, RD-30

Path 5: FD:30, RD-10

Path 3 is Successor, and Path 5 is Feasible Successor, if RD must be less (and not equal)than the FD.

However, Path 1 and 2 RD's are equal to Path 3's FD, which is 20. Are these paths will be elected Feasible Successor and will participate in load balancing?

(because if a path is not a feasible successor, the path is not used in load balancing)

If the RD is equal to the FD, then the route is rejected.

The reason is clear: if the RD is the same as the FD, then in all probability the neighbor is connected to the next hop in exactly the same way as you are. So who is to say who has the closer route and can therefore offer an alternative route to whom? You might end up offering an alternate route to each other, which would be a loop.

Kevin Dorrell

Luxembourg

Overall Rating: 5 (1 ratings)

## Replies

Kevin Dorrell Fri, 10/12/2007 - 02:14

If the RD is equal to the FD, then the route is rejected.

The reason is clear: if the RD is the same as the FD, then in all probability the neighbor is connected to the next hop in exactly the same way as you are. So who is to say who has the closer route and can therefore offer an alternative route to whom? You might end up offering an alternate route to each other, which would be a loop.

Kevin Dorrell

Luxembourg

sgregg Sat, 10/13/2007 - 10:16

I dont see this question as load balancing with EIGRP so much as insuring route redundancy and best path selection.

I would note that for load balancing of which EIGRP can actually use 4 routes by default and up to 6 routes at once, you will likely need to configure the variance command in order For EIGRP to find the routes of equal quality. Look up the EIGRP variance command for more details.

-Shawn

LordFlasheart Sun, 10/14/2007 - 07:43

Re: I would note that for load balancing of which EIGRP can actually use 4 routes by default and up to 6 routes at once, you will likely need to configure the variance command in order For EIGRP to find the routes of equal quality. Look up the EIGRP variance command for more details.

Not any more. The default remains at 4 but the maximum number of paths configurable is now 16.

If anybody out there is taking the exam soon then that is worth noting.

Cheers,

Chris

sgregg Mon, 10/15/2007 - 15:33

Wow 16, really. I just did my CCNP at the beginning of this year with the new curriculum and I dont recall them mentioning this at that time. My how quickly things change in the IT world. Do you have a reference of what IOS version the support of 16 path load balancing for EIGRP starts? Inquiring minds would like to know.

thx,

S

LordFlasheart Tue, 10/16/2007 - 23:26

Hi.

I don't have the book in front of me but in the Authorized Self-Study Guide published in Jan 2007, all routing protocols that support load-balancing now have a maximum of 16 paths as of IOS 12.4 I believe.

HTH,

Chris