Appendix H: Ospf Equal Cost Multipath - HP 5400zl Series Technical Overview

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
10.1.1.1
10.1.1.2
10.1.2.3
10.32.1.1
10.32.1.2
10.32.1.3
Next hop
Router C
Router B
Router B
Router B
Router C
Router B
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