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

[Oliver Boehmer (oboehmer)]

On 02/12/2012 13:12, "Phil Mayers" <p.mayers at imperial.ac.uk> wrote:

>On 12/02/2012 12:09 PM, Phil Mayers wrote:
>> On 12/02/2012 12:05 PM, Oliver Boehmer (oboehmer) wrote:
>>
>>> 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
>>
>> Hmm. I hadn't seen that before. What's the difference between that and:
>
>Ignore that; I'm confusing two different things (route next-hop
>resolution versus peer address). Sorry for the noise.

that's actually a valid question: "neighbor ...  fall-over" (fast session
deactivation) watches the peer route and tears down the session if it goes
away. "bgp nexthop ..." (next-hop tracking) watches the nexthop and
declares the paths invalid. FSD is generally used for multihop eBGP
sessions, while NHT is generally used for iBGP where we don't want/need to
tear down the session.
Both features use the same "Address Tracking" infrastructure to get
notified of route changes.

	oli