Load balance in EIGRP

[wfqk]

Hi two routers A and B are connected to each other by two cables. RouterA with e1/1 (1.1.1.10/31) and e1/2(1.1.1.20/31) and RouterB with e1/1(1.1.1.11/31) and e1/2(1.1.1.21/31). then two router C and D are connected to them like this :

C ----- A ======B ------D, we connect them each other by eigrp. My question is when routes go from C to D, how the load balance happen between A and B? Thank you

[Joseph W. Doherty]

If the links between A and B have the same EIGRP metrics, then EIGRP will do ECMP (equal cost multiple path). I.e. basically flows will be round-robin across the links. NB: by default, EIGRP does ECMP.

If the links between A and B have different EIGRP metrics, EIGRP will, by default, use the link with the best. However, EIGRP can be configured to support unequal cost. If so configured, it will round-robin flows in proportion to their costs. For example, if you have two links and one is twice as "good", it will get twice as many flows directed to it.

For any of the above load-balancing, by default, actual traffic load, of individual flows, is ignored, so, especially in the short term, actual link loading may not reflect the "expected" load sharing.

[wfqk]

Thank you very much for your excellent explanation. 

BTW, is it possible there are any loop happening between A and B? and how to prevent it if that happens? 

[Joseph W. Doherty]

"BTW, is it possible there are any loop happening between A and B? and how to prevent it if that happens?"

There are situations that can lead to loops in L3, but don't believe it likely, or perhaps even possible, with EIGRP for two links between A and B.

[Deepak Kumar]

Hi,

EIGRP having equal and unequal load balancing feature. Equal loading balancing is by default enabled and it will only work if you have the same matrix on both links. If the matrix calculation is not same then you can enable the unequal load balancing feature under the eigrp configuration. To accomplish unequal-cost load balancing, the variance command is used. 

EIGRP is a routing protocol whose strategy to prevent routing loops is based upon what is known as the EIGRP Feasibility Condition. EIGRP also makes use of another concept called Split Horizon. Basically it is a loop-free routing protocol until a network admin or software will not make any issue.