Dell PowerConnect B-RX Configuration Manual page 286

7
Configuring forwarding parameters
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 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 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 device:
•
•
•
•
•
•
•
•
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:
•
•
210
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)
IP static route – The value you assign to the metric parameter when you configure the route.
The default metric is 1. Refer to
static routes to the same destination"
RIP – The number of next-hop routers to the destination.
"Configuring load balancing and redundancy using multiple
on page 203.
BigIron RX Series Configuration Guide
53-1002253-01