OSPFv3 Overview
Configuring OSPFv3
page 2-12
OmniSwitch AOS Release 7 Advanced Routing Configuration Guide
March 2011
Equal Cost Multi-Path (ECMP) Routing
Using information from its continuously updated databases, OSPFv3 calculates the shortest path to a given
destination. Shortest path is determined from metric values at each hop along a path. At times, two or more
paths to the same destination will have the same metric cost.
In the network illustration below, there are two paths from Source router A to Destination router B. One
path traverses two hops at routers X and Y and the second path traverses two hops at M and N. If the total
cost through X and Y to B is the same as the cost via M and N to B, then these two paths have equal cost.
In this version of OSPFv3 both paths will be stored and used to transmit data.
Multiple Equal Cost Paths
Delivery of packets along equal paths is based on flows rather than a round-robin scheme. Equal cost is
determined based on standard routing metrics. However, other variables, such as line speed, are not
considered. So it is possible for OSPFv3 to decide two paths have an equal cost even though one may
contain faster links than another.
Source (A)
Destination (B)
X
Y
M
N
A-> X-> Y-> B = A-> M-> N-> B
