Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 552)

524 Multi-Service IronWare Routing Configuration Guide
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Configuring BGP4+
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.
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+ address family
configuration level of the CLI.
Brocade(config-bgp-ipv6u)# 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
10.240.0.0/24.
In this example, the device cannot reach 10.240.0.0/24, because the next-hop IP address for the
route is an IBGP route instead of an IGP route, and is considered unreachable by the device. The IP
route table entry for the next-hop gateway for the BGP4+ route’s next-hop gateway (10.102.0.1/24)
is shown here.
Since the route to the next-hop gateway is a BGP4+ route, and not an IGP route, it cannot be used
to reach 10.240.0.0/24. In this case, the device tries to use the default route, if present, to reach
the subnet that contains the BGP4+ route next-hop gateway.
Example when recursive route lookups are enabled
When recursive next-hop lookups are enabled, the device continues to look up the next-hop
gateways along the route until the device finds an IGP route to the BGP4+ route destination.
The first lookup results in an IBGP route, to network 10.102.0.0/24.