Foundry Networks Switch and Router Installation And Configuration Manual page 494

Foundry Switch and Router Installation and Configuration Guide
Here are the default administrative distances on the Foundry Layer 3 Switch:
• Directly connected – 0 (this value is not configurable)
• Static IP route – 1 (applies to all static routes, including default routes and default network routes)
• Exterior Border Gateway Protocol (EBGP) – 20
• OSPF – 110
• RIP – 120
• Interior Gateway Protocol (IBGP) – 200
• Local BGP – 200
• Unknown – 255 (the router will not use this route)
Lower administrative distances are preferred over higher distances. For example, if the router receives routes for
the same network from OSPF and from RIP, the router will prefer the OSPF route by default.
NOTE: You can change the administrative distances individually. See the configuration chapter for the route
source for information.
Since the software selects only the path with the lowest administrative distance, and the administrative distance is
determined by the path's source, IP load sharing does not apply to paths from different route sources. IP load
sharing applies only when the IP route table contains multiple paths to the same destination, from the same IP
route source.
IP load sharing does not apply to paths that come from different sources.
Path Cost
The cost parameter provides a common basis of comparison for selecting from among multiple 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 Layer 3 Switch 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 Layer 3 Switch 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. See "Configuring Load Balancing and Redundancy Using Multiple Static Routes to the Same
Destination" on page 15-44.
• 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 15.6 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.
15 - 52 December 2000