Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 252)

238 Brocade Virtual ADX Switch and Router Guide
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Using the IP default route as a valid next-hop for a BGP4 route
8
The route-map map-name parameter specifies the name of the route map you want to use to set or
change BGP4 attributes for the network you are advertising. The route map must already be
configured.
For information about the other parameters, refer to “ “Defining route maps” on page 252.
Using the IP default route as a valid next-hop
for a BGP4 route
By default, the device does not use a default route to resolve a BGP4 next-hop route. If the IP route
lookup for the BGP4 next-hop does not result in a valid IGP route (including static or direct routes),
the BGP4 next-hop is considered to be unreachable and the BGP4 route is not used.
In some cases, such as when the device is acting as an edge device, you can allow the device to
use the default route as a valid next-hop. To do so, enter the following command at the BGP4
configuration level of the CLI.
Virtual ADX(config-bgp-router)# next-hop-enable-default
Syntax: [no] next-hop-enable-default
Enabling next-hop recursion
For each BGP4 route learned, the device performs a route lookup to obtain the IP address of the
next-hop for the route. A BGP4 route is eligible for addition in the IP route table only if the following
conditions are true:
• The lookup succeeds in obtaining a valid next-hop IP address for the route.
• The path to the next-hop IP address is an IGP path or a static route path.
By default, the software performs only one lookup for the next-hop IP address for the BGP4 route. If
the next-hop lookup does not result in a valid next-hop IP address, or the path to the next-hop IP
address is a BGP4 path, the software considers the BGP4 route destination to be unreachable. The
route is not eligible to be added to the IP route table.
The BGP4 route table can contain a route with a next-hop IP address that is not reachable through
an IGP route, even though the device can reach a hop farther away through an IGP route. This can
occur when the IGPs do not learn a complete set of IGP routes, so the device learns about an
internal route through IBGP instead of through an IGP. In this case, the IP route table will not
contain a route that can be used to reach the BGP4 route destination.
To enable the device to find the IGP route to the next-hop gateway for a BGP4 route, enable
recursive next-hop lookups. With this feature enabled, if the first lookup for a BGP4 route results in
an IBGP path that originated within the same AS, rather than an IGP path or static route path, the
device performs a lookup on the next-hop IP address for the next-hop gateway. If this second lookup
results in an IGP path, the software considers the BGP4 route to be valid and adds it to the IP route
table. Otherwise, the device performs another lookup on the next-hop IP address of the next-hop for
the next-hop gateway, and so on, until one of the lookups results in an IGP route.
NOTE
You must configure a static route or use an IGP to learn the route to the EBGP multihop peer.