Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 75)

route table. For example, if the Layer 3 switch has a path learned from OSPF and a path learned from
RIP for a given destination, only the path with the lower administrative distance enters the IP route
table.
Here are the default administrative distances on the BrocadeLayer 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. Refer to 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 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 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.
• 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 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
IP Configuration
FastIron Ethernet Switch Layer 3 Routing Configuration Guide 75
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