Foundry Networks Switch and Router Installation And Configuration Manual page 606

Foundry Switch and Router Installation and Configuration Guide
Disable or Re-enable Load Sharing
Foundry routers can load share among up to eight equal-cost IP routes to a destination. By default, IP load
sharing is enabled. The default is 4 equal-cost paths but you can specify from 2 – 8 paths.
The router software can use the route information it learns through OSPF to determine the paths and costs.
Figure 17.8 shows an example of an OSPF network containing multiple paths to a destination (in this case, R1).
H1
H2
H3
H4
Figure 17.8 Example OSPF network with four equal-cost paths
In the example in Figure 17.8, the Foundry router has four paths to R1:
• BI->R3
• BI->R4
• BI->R5
• BI->R6
Normally, the Foundry router will choose the path to the R1 with the lower metric. For example, if R3's metric is
1400 and R4's metric is 600, the Foundry router will always choose R4.
However, suppose the metric is the same for all four routers in this example. If the costs are the same, the router
now has four equal-cost paths to R1. To allow the router to load share among the equal cost routes, enable IP
load sharing. The software supports four equal-cost OSPF paths by default when you enable load sharing. You
can specify from 2 – 8 paths.
NOTE: The Foundry router is not source routing in these examples. The router is concerned only with the paths
to the next-hop routers, not the entire paths to the destination hosts.
OSPF load sharing is enabled by default when IP load sharing is enabled. To configure IP load sharing
parameters, see "Configuring IP Load Sharing" on page 15-51.
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R1
OSPF Area 0
R3
R4 R4
R5
R6
BigIron
BI
December 2000