Foundry Configuration Guide for the FESX, FSX, and FWSX
21 - 28 © Foundry Networks, Inc. December 2005
Example When Recursive Route Lookups Are Enabled
When recursive next-hop lookups are enabled, the Layer 3 Switch recursively looks up the next-hop gateways
along the route until the Layer 3 Switch finds an IGP route to the BGP route’s destination. Here is an example.
The first lookup results in an IBGP route, to network 102.0.0.0/24:
Since the route to 102.0.0.1/24 is not an IGP route, the Layer 3 Switch 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 Layer 3 Switch
next performs a lookup for 102.0.0.1’s next-hop gateway, 10.0.0.1:
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:
FastIron SuperX Router# show ip bgp route
Total number of BGP Routes: 5
Status A:AGGREGATE B:BEST b:NOT-INSTALLED-BEST C:CONFED_EBGP D:DAMPED
H:HISTORY I:IBGP L:LOCAL M:MULTIPATH S:SUPPRESSED
Prefix Next Hop Metric LocPrf Weight Status
1 0.0.0.0/0 10.1.0.2 0 100 0 BI
AS_PATH: 65001 4355 701 80
2 102.0.0.0/24 10.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 1
3 104.0.0.0/24 10.1.0.2 0 100 0 BI
AS_PATH: 65001 4355 701 1 189
4 240.0.0.0/24 102.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 3356 7170 1455
5 250.0.0.0/24 209.157.24.1 1 100 0 I
AS_PATH: 65001 4355 701
FastIron SuperX Router# show ip route 102.0.0.1
Total number of IP routes: 38
Network Address NetMask Gateway Port Cost Type
102.0.0.0 255.255.255.0 10.0.0.1 1/1 1 B
AS_PATH: 65001 4355 1
FastIron SuperX Router# 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
H:HISTORY I:IBGP L:LOCAL M:MULTIPATH S:SUPPRESSED
Prefix Next Hop Metric LocPrf Weight Status
1 102.0.0.0/24 10.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 1
FastIron SuperX Router# show ip route 10.0.0.1
Total number of IP routes: 38
Network Address NetMask Gateway Port Cost Type
10.0.0.0 255.255.255.0 0.0.0.0 1/1 1 D
AS_PATH: 65001 4355 1
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