Brocade Communications Systems FastIron SX 800 Configuration Manual page 383

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.
device(config-bgp-router)# 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.
device# show ip bgp route
Total number of BGP Routes: 5
Status A:AGGREGATE B:BEST b:NOT-INSTALLED-BEST C:CONFED_EBGP D:DAMPED
E:EBGP H:HISTORY I:IBGP L:LOCAL M:MULTIPATH m:NOT-INSTALLED-MULTIPATH
S:SUPPRESSED F:FILTERED s:STALE
Prefix
1 0.0.0.0/0
AS_PATH: 65001 4355 701 80
2 10.10.0.0/24
AS_PATH: 65001 4355 1
3 10.40.0.0/24
AS_PATH: 65001 4355 701 1 189
4 10.0.0.0/24
AS_PATH: 65001 4355 3356 7170 1455
5 10.25.0.0/24
AS_PATH: 65001 4355 701
In this example, the device cannot reach 10.0.0.0/24, 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 (10.0.0.1/24) is shown here.
device# show ip route 10.0.0.1
Total number of IP routes: 37
Network Address
10.0.0.0
FastIron Ethernet Switch Layer 3 Routing
53-1003627-04
Next Hop MED
10.1.0.2 0
10.0.0.1 1
10.1.0.2 0
10.0.0.1 1
10.157.24.1 1
NetMask Gateway
10.255.255.255
LocPrf Weight Status
100 0 BI
100 0 BI
100 0 BI
100 0 I
100 0 I
Port Cost Type
10.0.0.1 1/1 1
Optional BGP4 configuration tasks
B
383