Configuring BGP4
December 2005 © Foundry Networks, Inc. 21 - 29
This lookup results in an IGP route. In fact, this route is a directly-connected route. As a result, the BGP route’s
destination is now reachable through IGP, which means the BGP route is eligible for installation in the IP route
table. Here is the BGP route in the IP route table:
This Layer 3 Switch can use this route because the Layer 3 Switch has an IP route to the next-hop gateway.
Without recursive next-hop lookups, this route would not be in the IP route table.
Enabling Recursive Next-Hop Lookups
The recursive next-hop lookups feature is disabled by default. To enable recursive next-hop lookups, enter the
following command at the BGP configuration level of the CLI:
FastIron SuperX Router(config-bgp-router)# next-hop-recursion
Syntax: [no] next-hop-recursion
Changing Administrative Distances
BGP4 routers can learn about networks from various protocols, including the EBGP portion of BGP4 and IGPs
such as OSPF 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 Layer 3 Switch 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 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 21-4.
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. 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.
NOTE: 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, 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)
FastIron SuperX Router# show ip route 240.0.0.0/24
Total number of IP routes: 38
Network Address NetMask Gateway Port Cost Type
240.0.0.0 255.255.255.0 10.0.0.1 1/1 1 B
AS_PATH: 65001 4355 1
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