Changing Administrative Distances - Dell PowerConnect B-RX Configuration Manual

Changing administrative distances

The BigIron RX can learn about networks from various protocols, including the EBGP portion of
BGP4 and IGPs such as OSPF, ISIS, and RIP. Consequently, the routes to a network may differ
depending on the protocol from which the routes were learned.
To select one route over another based on the source of the route information, the device can use
the administrative distances assigned to the sources. The administrative distance is a
protocol-independent metric that IP routers use to compare routes from different sources.
The device re-advertises a learned best BGP4 route to the device's neighbors even when the route
table manager does not also select that route for installation in the IP route table. The best BGP4
route is the BGP4 path that BGP selects based on comparison of the paths' BGP4 route
parameters. Refer to
When selecting a route from among different sources (BGP4, OSPF, RIP, ISIS, static routes, and so
on), the software compares the routes on the basis of each route's administrative distance. If the
administrative distance of the paths is lower than the administrative distance of paths from other
sources (such as static IP routes, RIP, or OSPF), the BGP4 paths are installed in the IP route table.
Here are the default administrative distances on the BigIron RX:
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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. The administrative distances are configured in different places in the software. The
device re-advertises a learned best BGP4 route to neighbors by default, regardless of whether the
route's administrative distance is lower than other routes from different route sources to the same
destination.
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To change the default administrative distances for EBGP, IBGP, and Local BGP, enter a command
such as the following.
BigIron RX(config-bgp)# distance 200 200 200
Syntax: distance <external-distance> <internal-distance> <local-distance>
BigIron RX Series Configuration Guide
53-1002253-01
"How BGP4 selects a path for a route"
Directly connected – 0 (this value is not configurable)
Static – 1 is the default and applies to all static routes, including default routes. This can be
assigned a different value.
EBGP – 20
OSPF – 110
ISIS – 115
RIP – 120
IBGP – 200
Local BGP – 200
Unknown – 255 (the router will not use this route)
To change the EBGP, IBGP, and Local BGP default administrative distances, see the
instructions in this section.
To change the default administrative distance for OSPF, RIP, ISIS, refer to
administrative distances"
To change the administrative distance for static routes, refer to
page 198
Changing administrative distances
on page 767.
on page 740.
"Changing
"Configuring static routes"
26
on
767