EIGRP is able to load balance traffic across multiple paths, so yes, the traffic would be balanced across both routes. BUT, it would be balanced proportionately. In other words, if the metric for the first route is, say 100, and the metric for the second route is 20, then the ratio between those two metrics is 100:20, or 5:1. This means that the second route will carry five times as much traffic as the first route. (Remember, a higher metric or cost is a worse route).
The amount of traffic each route carries is proportional to the metric or cost of each route. The important thing to note here is that EIGRP is able to achieve this, while other routing protocols, such as OSPF does equal cost load balancing, regardless of the metric of each participating route.
In your sample case, both successor (R3) and feasible successor (R4) have been defined. What are we going to achieve with ‘variance’ command to load balancing between feasible successor ?
R4 AD is lower than R3 FD, so R4 will become feasible successor and add it to the routing table.
The variance command is used to tell a router if it should load-balance traffic across the successor and feasible successor. By default, in an unequal cost scenario, only the successor will be used to forward traffic.
By issuing the variance command, we are not changing the status of the FS or the successor. However, we are telling the router to add the FS to the routing table (assuming the variance command includes the appropriate ratio as described in the lesson). Thus, load balancing, based on the variance multiplier, will take place between those two paths.
This is an issue of terminology. A successor is defined as the best path to a particular destination. The only way you can have two successors is if they have exactly the same metric. So by definition, if you have two EIGRP successors, then equal cost load balancing will be performed between them.
Unequal traffic will only take place between a successor and a feasible successor, assuming the variance command is configured appropriately as in the lesson.
Now if you want to force traffic across two successors in such a way that one successor takes more traffic than the other, then you will have to modify the metric of one of those paths. This will make one of the successors a feasible successor, and thus you can use the variance command to perform unequal cost load balancing. You can adjust the metric by modifying the bandwidth, load, or delay on an interface which will change the metric accordingly.
I understand Eigrp will load balance up to 4 equal cost paths straight out of the box and we can do unequal cost load balancing with the varience command.
But I’m wondering does EIGRP load balance over 3 equal cost paths ?
I have three equal cost paths and I want to start routing over them. By default will it send one packet on each of the three links for each traffic flow ? or will only two links out of the three only be utilized ?
Thanks
Yes EIGRP will load balance over those three equal cost paths. The maximum-paths command under the EIGRP router configuration mode can be used to change the maximum number of paths that can be used for equal (or unequal) load balancing. Different platforms have different default and maximum values. Typically, the default is up to 4 paths, but you can change that on some platforms to 32.
The thing to note here is that this value states that you can have up to that many paths. So if you set it to 8, you can have 2, 3, 4, 5, 6, 7, or 8 paths via which load balancing can take place.
How the load balancing will take place depends on several factors. The traffic distribution among the paths depends on the type of switching that the router is using. With process-switching, the router will send one packet on each of the three links for each traffic flow. With fast-switching (default mode on most routers), the router will send the first packet of a new flow on the first link, the second packet on the second link, and the third packet on the third link, then repeat the cycle. What is actually used will also depend on the support and configuration of CEF. Take a look at this NetworkLessons note for more info.