Configuring IP
December 2005 © Foundry Networks, Inc. 16 - 41
This example shows two routes. Both of the routes are directly attached, as indicated in the Type column.
However, one of the routes is shown as type “*D”, with an asterisk (*). The asterisk indicates that this route is a
candidate default network route.
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.
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On X-Series devices, IP load sharing uses a hashing algorithm based on the source IP address, destination IP
address, and protocol field in the IP header.
You can enable a Layer 3 Switch to load balance across up to six equal-cost paths. The default maximum number
of equal-cost load sharing paths is four.
NOTE: IP load sharing is not based on source routing, only on next-hop 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. Most of the user documentation uses the
term “route” throughout. 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: Foundry devices also perform load sharing among the ports in aggregate links. See “Trunk Group Load
Sharing” on page 10-6.
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
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 route table. For example, if the
1.IP load sharing is also called “Equal-Cost Multi-Path (ECMP)” load sharing or just “ECMP”
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