Appendix H: OSPF Equal Cost Multipath
In Open Shortest Path First (OSPF), if different subnet destinations in a network are reachable through multiple equal-cost paths, the router chooses a single path for each destination subnet. (A trunk is considered a path even though a trunk consists of multiple links).
With OSPF Equal Cost Multipath (OSPF-ECMP), routers support optional load-sharing across redundant paths to the same subnet. All traffic for different hosts in the same subnet may go through a different next-hop router. Multiple paths are balanced, based on destination IP address.
HP ProCurve’s OSPF-ECMP feature is interoperable with OSPF-ECMP implementations from various vendors including Cisco, 3Com, and Extreme. The HP ProCurve implementation supports up to four ECMP paths.
The example shows that there are three equal-cost, next-hop paths from Router A to the destination subnets with load-sharing across redundant paths. Router A can route packets based on destination IP address as follows:
Destination IP | Next hop |
10.1.1.1 | Router C |
| |
10.1.1.2 | Router B |
| |
10.1.2.3 | Router B |
| |
10.32.1.1 | Router B |
| |
10.32.1.2 | Router C |
| |
10.32.1.3 | Router B |
| |
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