Alcatel Carrier Internetworking Solutions 060187-10 REV D Equal Cost Multi-Path Ecmp Routing

Configuring OSPF OSPF Overview

Equal Cost Multi-Path (ECMP) Routing

Using information from its continuously updated databases, OSPF 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 OSPF both paths will be stored and used to transmit data.

XY

A-> X-> Y-> B = A-> M-> N-> B

Source (A) Destination (B)

M

N

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 OSPF to decide two paths have an equal cost even though one may contain faster links than another.

Non Broadcast OSPF Routing

OSPF can operate in two modes on non-broadcast networks: NBMA and point-to-multipoint. The inter- face type for the corresponding network segment should be set to non broadcast or point-to-point, respec- tively.

For non-broadcast networks neighbors should be statically configured. For NBMA neighbors the eligibil- ity option must be enabled for the neighboring router to participate in Designated Router (DR) election.

For the correct working of an OSPF NBMA network, a fully meshed network is mandatory. Also, the neighbor eligibility configuration for a router on every other router should match the routers interface priority configuration.

See “Configuring Static Neighbors” on page 1-30for more information and setting up static neighbors.

Graceful Restart on Stacks with Redundant Switches

OmniSwitch 6600 Family stacks with two or more switches can support redundancy where if the primary switch fails or goes offline for any reason, the secondary switch is instantly notified. The secondary switch automatically assumes the primary role. This switch between the primary and secondary switches is known as takeover.

OmniSwitch 6600 Family Advanced Routing Configuration Guide March 2005 page 1-13