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PowerConnect B-Series FCX Configuration Guide 1011
53-1002266-01
Optional configuration tasks
30
Enabling next-hop recursion
For each BGP4 route a Layer 3 Switch learns, the Layer 3 Switch performs a route lookup to obtain
the IP address of the route next hop. A BGP4 route becomes eligible for installation into the IP route
table only if the following conditions are true:
• The lookup succeeds in obtaining a valid next-hop IP address for the route.
• The path to the next-hop IP address is an Interior Gateway Protocol (IGP) path or a static route
path.
By default, the software performs only one lookup for a BGP route 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 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 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 Dell 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.