Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 835)

Multi-Service IronWare Switching Configuration Guide 805
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Configuring IP load sharing
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How multiple equal-cost paths enter the IP route table
IP load sharing applies to equal-cost paths in the IP route table. Routes eligible for load sharing can
enter the table from the following sources:
• IP static routes
• Routes learned through RIP, OSPF, and BGP4
Administrative distance
The administrative distance is a unique value associated with each type (source) of IP route. Each
path has an administrative distance. It is used when evaluating multiple equal-cost paths to the
same destination from different sources, such as RIP, OSPF and so on, but not used when
performing IP load sharing.
The value of the administrative distance is determined by the source of the route. The Brocade
device is configured with a unique administrative distance value for each IP route source.
When the software receives paths from different sources to the same destination, the software
compares their administrative distances, selects the one with the lowest distance, and puts it in
the IP route table. For example, if the Brocade device 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 Brocade device:
• 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 device will not use this route)
Lower administrative distances are preferred over higher distances. For example, if the device
receives routes for the same network from OSPF and from RIP, the device 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’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 paths
from the same IP route source to the same destination.