Foundry Networks Switch and Router Installation And Configuration Manual page 716

Foundry Switch and Router Installation and Configuration Guide
The Layer 3 Switch re-advertises a learned best BGP4 route to the Layer 3 Switch's neighbors even when the
software does not also select that route for installation in the IP route table. The best BGP4 routes is the BGP4
path that the software selects based on comparison of the paths' BGP4 route parameters. See "How BGP4
Selects a Path for a Route" on page 19-3.
When selecting a route from among different sources (BGP4, OSPF, RIP, static routes, and so on), the software
compares the routes on the basis of each route's administrative distance. The Layer 3 Switch 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.
NOTE: In software release 05.0.00 and later, the software will replace a statically configured default route with a
learned default route if the learned route's administrative distance is lower than the statically configured default
route's distance. However, the default administrative distance for static routes is changed to 1 in software release
05.2.00, so only directly-connected routes are preferred over static routes when the default administrative
distances for the routes are used.
Here are the default administrative distances on the Foundry Layer 3 Switch:
• Directly connected – 0 (this value is not configurable)
• Static – 1 (applies to all static routes, including default routes)
• EBGP – 20
• OSPF – 110
• RIP – 120
• 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. The administrative
distances are configured in different places in the software.
• 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, see "Modify Administrative Distance" on page 17-36.
• To change the default administrative distance for RIP, see "Changing the Administrative Distance" on
page 16-6.
• To change the default administrative distance for static routes, see "Configuring Static Routes" on page 15-
39.
You can change the default EBGP, IBGP, and Local BGP administrative distances using either of the following
methods.
USING THE CLI
To change the default administrative distances for EBGP, IBGP, and Local BGP, enter a command such as the
following:
BigIron(config-bgp-router)# distance 180 160 40
Syntax: distance <external-distance> <internal-distance> <local-distance>
The <external-distance> sets the EBGP distance and can be a value from 1 – 255.
The <internal-distance> sets the IBGP distance and can be a value from 1 – 255.
The <local-distance> sets the Local BGP distance and can be a value from 1 – 255.
19 - 34 December 2000