Brocade BigIron RX Series Configuration Guide User Manual
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BigIron RX Series Configuration Guide
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Configuring forwarding parameters
7
•
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’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.
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 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 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”
•
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.