Dell PowerEdge M820 Dell PowerConnect M6220/M6348/M8024 Switches Configuration - Page 89
Using Equal Cost Multipath, Forwarding Without ECMP, Next Hop with Two Static Routes
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Using Equal Cost Multipath The equal cost multipath (ECMP) feature allows a router to use more than one next hop to forward packets to a given destination prefix. It can be used to promote a more optimal use of network resources and bandwidth. A router that does not use ECMP forwards all packets to a given destination through a single next hop. This next hop may be chosen from among several next hops that provide equally good routes to the destination. For example, in Figure 4-7, Router A sends all traffic to destinations in Network D through next hop NH1, even though the route through NH2 is equally good. Forwarding all traffic via NH1 may cause Link A to be overloaded while Link B is not used at all. Figure 4-7. Forwarding Without ECMP With ECMP, Router A can forward traffic to some destinations in Network D via Link A and traffic to other destinations in Network D via Link B, thereby taking advantage of the bandwidth of both links. A hash algorithm is applied to the destination IP addresses to provide a mechanism for selecting among the available ECMP paths. ECMP routes may be configured statically or learned dynamically. If a user configures multiple static routes to the same destination but with different next hops, then those routes will be treated as a single route with two next hops. For example, given the network in Figure 4-8, if the user configures the following two static routes on Router A, the routing table will contain a single route to 20.0.0.0/8: Figure 4-8. Next Hop with Two Static Routes Routing Configuration 89