Brocade BigIron RX Series Configuration Guide - Manual (Page 937)

BigIron RX Series Configuration Guide 859
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Enabling next-hop recursion
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By default, the software performs only one lookup for a BGP route’s next-hop IP address. If the
next-hop lookup does not result in a valid next-hop IP address or the path to the next-hop IP
address is a BGP path, the software considers the BGP route’s destination to be unreachable. The
route is not eligible to be installed in the IP route table.
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 device through an IGP
route. This can occur when the IGPs do not learn a complete set of IGP routes, resulting in the
device 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’s destination.
To enable the device to find the IGP route to a BGP route’s 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 device performs a lookup on the next-hop gateway’s 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 device performs a lookup
on the next-hop IP address of the next-hop gateway’s next hop, 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.
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 BigIron RX 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
device. Here is the IP route table entry for the BGP route’s next-hop gateway (102.0.0.1/24).
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
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
BigIron RX# show ip route 102.0.0.1
Total number of IP routes: 37
Network Address Gateway Port Cost Type
102.0.0.0 10.0.0.1 1/1 1 B