Dell PowerConnect B-RX Configuration Manual page 855

Example when recursive route lookups are enabled
When recursive next-hop lookups are enabled, the device recursively looks up the next-hop
gateways along the route until the device finds an IGP route to the BGP route's destination. Here is
an example.
BigIron RX# show ip bgp route
Total number of BGP Routes: 5
Status A:AGGREGATE B:BEST b:NOT-INSTALLED-BEST C:CONFED_EBGP D:DAMPED
1
2
3
4
5
The first lookup results in an IBGP route, to network 102.0.0.0/24.
BigIron RX# show ip route 102.0.0.1
Total number of IP routes: 38
Since the route to 102.0.0.1/24 is not an IGP route, the device cannot reach the next hop through
IP, and thus cannot use the BGP route. In this case, since recursive next-hop lookups are enabled,
the device next performs a lookup for 102.0.0.1's next-hop gateway, 10.0.0.1.
BigIron RX# show ip bgp route 102.0.0.0
Number of BGP Routes matching display condition : 1
Status A:AGGREGATE B:BEST b:NOT-INSTALLED-BEST C:CONFED_EBGP D:DAMPED
1
The next-hop IP address for 102.0.0.1 is not an IGP route, which means the BGP route's destination
still cannot be reached through IP. The recursive next-hop lookup feature performs a lookup on
10.0.0.1's next-hop gateway:
BigIron RX# show ip route 10.0.0.1
Total number of IP routes: 38
This lookup results in an IGP route. In fact, this route is a directly-connected route. As a result, the
BGP route's destination is now reachable through IGP, which means the BGP route is eligible for
installation in the IP route table. Here is the BGP route in the IP route table.
BigIron RX# show ip route 240.0.0.0/24
Total number of IP routes: 38
BigIron RX Series Configuration Guide
53-1001810-01
H:HISTORY I:IBGP L:LOCAL M:MULTIPATH S:SUPPRESSED
Prefix Next Hop
0.0.0.0/0 10.1.0.2
AS_PATH: 65001 4355 701 80
102.0.0.0/24 10.0.0.1
AS_PATH: 65001 4355 1
104.0.0.0/24 10.1.0.2
AS_PATH: 65001 4355 701 1 189
240.0.0.0/24 102.0.0.1
AS_PATH: 65001 4355 3356 7170 1455
250.0.0.0/24 209.157.24.1
AS_PATH: 65001 4355 701
Network Address Gateway
102.0.0.0 10.0.0.1
AS_PATH: 65001 4355 1
H:HISTORY I:IBGP L:LOCAL M:MULTIPATH S:SUPPRESSED
Prefix Next Hop
102.0.0.0/24 10.0.0.1
AS_PATH: 65001 4355 1
Network Address Gateway
10.0.0.0 0.0.0.0
AS_PATH: 65001 4355 1
Network Address Gateway
240.0.0.0 10.0.0.1
AS_PATH: 65001 4355 1
Enabling next-hop recursion
Metric LocPrf
0 100
1 100
0 100
1 100
1 100
Port Cost Type
1/1 1 B
Metric LocPrf
1 100
Port Cost Type
1/1 1 D
Port Cost
1/1 1
26
Weight Status
0 BI
0 BI
0 BI
0 BI
0 I
Weight Status
0 BI
Type
B
783