[c-nsp] BGP Path Selection and next-hop reachability (IGP vs BGP)

[Oliver Boehmer (oboehmer)]

> 
>On Sat, Dec 01, 2012 at 10:37:36PM +0000, Oliver Boehmer (oboehmer) wrote:
>> Ack.. BGP in IOS will use whatever valid route to resolve the next-hop,
>> including a BGP route (which could be perfectly valid in some specific
>> scenarios). 
>> You need to configure selective address tracking to avoid this to
>>happen.
>> With an appropriate "bgp nexthop route-map .." config, for example only
>> considering loopbacks or only OSPF or connected routes, BGP would
>>consider
>> 5.5.5.5 unreachable, and ignore this path for the best-path calculation.
>
>Mmmmh.  This sounds like interesting stuff, but unfortunately, my
>Google-fu
>is failing me today - all I turn up is the normal "set ip next-hop..."
>stuff inside "neighbour 1.2.3.4 route-map foo".
>
>Oli, can you point us to some documentation and/or examples for this?

Sorry, google for "selective address tracking", or the below sample config
which will only consider host routes as well as connected routes (for
direct eBGP neighbors) as valid ones.

router bgp ..
 address-family ...
  bgp nexthop route-map NHTmap
!
ip prefix-list LOOPBACKS 0.0.0.0/0 ge 32
!
route-map NHTmap permit 10
 match ip address prefix-list LOOPBACKS
route-map NHTmap permit 20
 match source-protocol connected