Brocade TurboIron 24X Series Configuration Guide - Manual (Page 725)

Brocade TurboIron 24X Series Configuration Guide 691
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Optional configuration tasks
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.
TurboIron(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 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. Refer to
“How BGP4 selects a path for a route” on page 661.
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 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 administrative distance is lower than the statically configured default route 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.
The following default administrative distances are found on the 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)