Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 74)

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.
7
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 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" section 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 table from any of the following sources:
• IP static routes
• Routes learned through RIP
• 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 RIP, OSPF and so on.
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 distance. The software then places the path with the lowest administrative distance in the IP
7 IP load sharing is also called "Equal-Cost Multi-Path (ECMP)" load sharing or just "ECMP"
Configuring IP load sharing
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