Network redundancy and load balancing allow routers to distribute traffic across multiple equal-cost paths, maximizing bandwidth and providing automatic failover when a route goes down. OSPF supports up to four equal-cost paths by default and up to sixteen when configured manually, while EIGRP extends this further with unequal-cost load balancing across links of different speeds.
Network Redundancy & Load Balancing
One thing we want within our networks is to have some sort of redundancy. We want redundancy with our equipment, we want redundancy within our equipment, and we want redundancy between our equipment. So with these links, we want multiple routes to go to the same location, and these are for when one route dies and we want it to fail over to the other route. But there are some capabilities of doing load balancing. We have multiple routes going to the same location, so why do we not use multiple routes to get our data to that location, giving us more bandwidth?
Here is an example of a network that has two different routes that it can take to get to this network over here. This router right here can do load balancing to actually increase performance. What it will do is send some of the packets in this direction and other packets in this direction, giving it twice as much bandwidth to its end destination.
Now with OSPF, two things need to be the same. The administrative distance, which is pretty simple because administrative distance would be 110 on both of those for OSPF, and then also the metric, so the cost has to be the same whether it is going this direction or whether it is going this direction. As long as these two are the same, the metric and the AD are the same, your router will actually automatically load balance between these two different routes.
In fact it will actually load balance up to four different routes by default, provided that the AD and the metric are the same. By default OSPF, as long as the administrative distance and the metric are the same, can load balance up to four, and that is just by default. It can actually go up to 16 different routes that it can load balance between all of these different routes. Other routing protocols have similar limits to them and similar defaults to them.
So how can we change this? How can we utilize more than what the default is? If we go into the router ospf process ID and then type in maximum-paths and then the number of paths that we want to take, so let us say we want to do all 16 and we type in 16 there, then we would actually change this default to something much greater. Or less, possibly. We may want to do less, maybe we want to choose just one route, and we would just specify that the maximum number is one, so that way it will not load balance. So those are some ways that we can actually get more out of our different paths to the end destination.
Just a side note that EIGRP can actually do unequal cost load balancing. What that means is if these are 10 gig connections over here and these are one gig connections over here, it could utilize both of these connections, and it would utilize this one much more than it would utilize this connection. So that is one of the powers behind EIGRP: it can do unequal cost load balancing.
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