Equal Cost Multi-Path (Ecmp) Routing; Non Broadcast Ospf Routing; Graceful Restart On Stacks With Redundant Switches - Alcatel OmniSwitch 6600 Family Configuration Manual

Configuring OSPF

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.
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Source (A)
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 1-30

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
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A-> X-> Y-> B = A-> M-> N-> B
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M
Multiple Equal Cost Paths
for more information and setting up static neighbors.
March 2005
Y
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Destination (B)
N
OSPF Overview
page 1-13