Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 92)

78 Brocade Virtual ADX Switch and Router Guide
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Configuring IP parameters
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When the software receives multiple paths to the same destination and the paths are from
different sources, the software compares the administrative distances of the paths and selects the
path with the lowest distance. The software then places the path with the lowest administrative
distance in the IP route table.
Here are the default administrative distances on the Virtual ADX:
• Directly connected – 0 (this value is not configurable)
• Static IP route – 1 (applies to all static routes, including default routes and default network
routes)
• 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.
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 Virtual ADX 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
Virtual ADX 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 72.
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 load sharing
IP load sharing and load sharing for static 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.
Table 5 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 all Virtual ADX switches, load sharing for static IP routes is
enabled by default.