Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 836)

806 Multi-Service IronWare Switching Configuration Guide
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Configuring IP load sharing
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Path cost
The cost parameter provides a basis of comparison for selecting among paths to a given
destination. Each path in the IP route table has a cost. When the IP route table contains multiple
paths to a destination, the Brocade device chooses the path with the lowest cost. When the IP
route table contains more than one path with the lowest cost to a destination, the Brocade device
uses IP load sharing to select one of the lowest-cost paths.
The source of a path’s cost value depends on the source of the path:
• IP static route – The value you assign to the metric parameter when you configure the route.
The default metric is 1. Refer to
“Configuring load balancing and redundancy using multiple
static routes to the same destination” on page 791.
• RIP – The number of next-hop routers to the destination.
• OSPF – The Path Cost associated with the path. The paths can come from any combination of
inter-area, intra-area, and external Link State Advertisements (LSAs).
• BGP4 – The path’s Multi-Exit Discriminator (MED) value.
NOTE
If the path is redistributed between two or more of the above sources before entering the IP route
table, the cost can increase during the redistribution due to settings in redistribution filters.
Static route, OSPF, and BGP4 load sharing
IP load sharing and load sharing for static routes, OSPF routes, and BGP4 routes are individually
configured. Multiple equal-cost paths for a destination can enter the IP route table only if the
source of the paths is configured to support multiple equal-cost paths. For example, if BGP4 allows
only one path with a given cost for a given destination, the BGP4 route table cannot contain
equal-cost paths to the destination. Consequently, the IP route table will not receive multiple
equal-cost paths from BGP4.
Table 95 lists the default and configurable maximum numbers of paths for each IP route source
that can provide equal-cost paths to the IP route table. The table also lists where to find
configuration information for the route source’s load sharing parameters.
The load sharing state for all the route sources is based on the state of IP load sharing. Since IP
load sharing is enabled by default on the Brocade device, load sharing for static IP routes, RIP
routes, OSPF routes, and BGP4 routes also is enabled by default.