Configuring BGP4
December 2005 © Foundry Networks, Inc. 21 - 27
NOTE: The software does not support using the default route to resolve a BGP4 route's next hop. Instead, you
must configure a static route or use an IGP to learn the route to the EBGP multihop peer.
Previous software releases support use of the default route to resolve routes learned from EBGP multihop
neighbors. However, even in this case Foundry recommends that you use a static route for the EBGP multihop
neighbor instead. In general, we recommend that you do not use the default route as the next hop for BGP4
routes, especially when there are two or more BGP4 neighbors. Using the default route can cause loops.
Example When Recursive Route Lookups Are Disabled
Here is an example of the results of an unsuccessful next-hop lookup for a BGP route. In this case, next-hop
recursive lookups are disabled. The example is for the BGP route to network 240.0.0.0/24.
In this example, the Layer 3 Switch cannot reach 240.0.0.0/24, because the next-hop IP address for the route is an
IBGP route instead of an IGP route, and thus is considered unreachable by the Layer 3 Switch. Here is the IP
route table entry for the BGP route’s next-hop gateway (102.0.0.1/24):
The route to the next-hop gateway is a BGP route, not an IGP route, and thus cannot be used to reach 240.0.0.0/
24. In this case, the Layer 3 Switch tries to use the default route, if present, to reach the sub-net that contains the
BGP route’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 I
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: 37
Network Address NetMask Gateway Port Cost Type
102.0.0.0 255.255.255.0 10.0.0.1 1/1 1 B
FastIron SuperX Router# show ip route 240.0.0.0/24
Total number of IP routes: 37
Network Address NetMask Gateway Port Cost Type
0.0.0.0 0.0.0.0 10.0.0.202 1/1 1 S
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