Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 - Manual (Page 329)

Brocade ICX 6650 Layer 3 Routing Configuration Guide 311
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Optional BGP4 configuration tasks
It is possible for the BGP route table to contain a route whose next-hop IP address is not reachable
through an IGP route, even though a hop farther away can be reached by the Layer 3 switch through
an IGP route. This can occur when the IGPs do not learn a complete set of IGP routes, resulting in
the Layer 3 switch learning about an internal route through IBGP instead of through an IGP. In this
case, the IP route table does not contain a route that can be used to reach the BGP route
destination.
To enable the Layer 3 switch to find the IGP route to a BGP route next-hop gateway, enable
recursive next-hop lookups. When you enable recursive next-hop lookup, if the first lookup for a
BGP route results in an IBGP path originated within the same Autonomous System (AS), rather than
an IGP path or static route path, the Layer 3 switch performs a lookup on the next-hop gateway
next-hop IP address. If this second lookup results in an IGP path, the software considers the BGP
route to be valid and thus eligible for installation in the IP route table. Otherwise, the Layer 3 switch
performs a lookup on the next-hop IP address of the next-hop gateway next hop, and so on, until
one of the lookups results in an IGP route.
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 Brocade 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
192.168.0.0/24.
In this example, the Layer 3 switch cannot reach 192.168.0.0/24, 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 next-hop gateway
(192.168.10.1/24).
Brocade#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
2 192.168.0.0/24 10.0.0.1 1 100 0 BI
AS_PATH: 65001 4355 1
3 192.168.10.0/24 10.1.0.2 0 100 0 BI
AS_PATH: 65001 4355 701 1 189
4 192.168.50.0/24 10.0.0.1 1 100 0 I
AS_PATH: 65001 4355 3356 7170 1455
5 192.168.19.0/24 10.157.24.1 1 100 0 I
AS_PATH: 65001 4355 701