Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 93)

Multi-Service IronWare Routing Configuration Guide 65
53-1003033-02
Enabling next-hop recursion
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.
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 BGP4 configuration level
of the CLI.
Brocade(config-bgp)# next-hop-recursion
Syntax: [no] next-hop-recursion
Example when recursive route lookups are disabled
The output here shows the results of an unsuccessful next-hop lookup for a BGP4 route. In this
case, next-hop recursive lookups are disabled. This example is for the BGP4 route to network
10.0.0.0/24.