Brocade Communications Systems FastIron SX 1600 Configuration Manual Download

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Configuring IP load sharing

The IP route table can contain more than one path to a given destination. When this occurs, the Layer 3 switch selects the path with the
lowest cost as the path for forwarding traffic to the destination. If the IP route table contains more than one path to a destination and the
paths each have the lowest cost, then the Layer 3 switch uses IP load sharing to select a path to the destination.

IP load sharing uses a hashing algorithm based on the source IP address, destination IP address, and protocol field in the IP header, TCP,
and UDP information.

NOTE

IP load sharing is also called "Equal-Cost Multi-Path (ECMP) load sharing or just ECMP.

NOTE

IP load sharing is based on next-hop routing, and not on source routing.

NOTE

The term "path" refers to the next-hop router to a destination, not to the entire route to a destination. Thus, when the software
compares multiple equal-cost paths, the software is comparing paths that use different next-hop routers, with equal costs, to the
same destination.In many contexts, the terms "route" and "path" mean the same thing. The term "path" is used in this section to
refer to an individual next-hop router to a destination, while the term "route" refers collectively to the multiple paths to the
destination. Load sharing applies when the IP route table contains multiple, equal-cost paths to a destination.

NOTE

Brocade devices also perform load sharing among the ports in aggregate links. Refer to "Trunk group load sharing" in the

FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide

How multiple equal-cost paths enter the IP route table

IP load sharing applies to equal-cost paths in the IP route table. Routes that are eligible for load sharing can enter the routing table from
any of the following routing protocols:

•

IP static routes

•

Routes learned through OSPF

•

Routes learned through BGP4

Administrative distance for each IP route

The administrative distance is a unique value associated with each type (source) of IP route. Each path has an administrative distance.
The administrative distance is not used when performing IP load sharing, but the administrative distance is used when evaluating multiple
equal-cost paths to the same destination from different sources, such as between static IP routes, OSPF, and BGP4.

The value of the administrative distance is determined by the source of the route. The Layer 3 switch is configured with a unique
administrative distance value for each IP route source.

When the software receives multiple paths to the same destination and the paths are from different sources, the software compares the
administrative distances of the paths and selects the path with the lowest administrative distance. The software then places the path with
the lowest administrative distance in the IP route table. For example, if the Layer 3 switch has a path learned from OSPF and a path
learned from IBGP for a given destination, only the path with the lower administrative distance enters the IP route table.

Brocade Communications Systems FastIron SX 1600, Configuration Manual

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