BigIron RX Series Configuration Guide 78353-1002253-01Enabling next-hop recursion 26BigIron RX(config-bgp)# next-hop-enable-defaultSyntax: [no] next-hop-enable-defaultEnabling next-hop recursionFor each BGP4 route a BigIron RX learns, the device performs a route lookup to obtain the IPaddress of the route’s next hop. A BGP4 route becomes eligible for installation into the IP routetable 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 routepath.By default, the software performs only one lookup for a BGP route’s next-hop IP address. If thenext-hop lookup does not result in a valid next-hop IP address or the path to the next-hop IPaddress is a BGP path, the software considers the BGP route’s destination to be unreachable. Theroute 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 reachablethrough an IGP route, even though a hop farther away can be reached by the device through an IGProute. This can occur when the IGPs do not learn a complete set of IGP routes, resulting in thedevice learning about an internal route through IBGP instead of through an IGP. In this case, the IProute 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 recursivenext-hop lookups. When you enable recursive next-hop lookup, if the first lookup for a BGP routeresults in an IBGP path originated within the same Autonomous System (AS), rather than an IGPpath or static route path, the device performs a lookup on the next-hop gateway’s next-hop IPaddress. If this second lookup results in an IGP path, the software considers the BGP route to bevalid and thus eligible for installation in the IP route table. Otherwise, the device performs a lookupon the next-hop IP address of the next-hop gateway’s next hop, and so on, until one of the lookupsresults in an IGP route.NOTEYou must configure a static route or use an IGP to learn the route to the EBGP multihop peer.Example when recursive route lookups are disabledHere 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 network240.0.0.0/24.