BGP route update propagation questions

I'm trying to get a better feel for the dynamics of some
"maybe-necessary" BGP routing traffic, and had a few questions:

Under what circumstances will BGP send an update (of any sort)
to a peer when there is an internal failure that does _not_
result in the complete isolation of a prefix?

For example:

    __ AS1-rtr1 ---*-- AS2-rtr1 --\
       / | -- AS3
  10/8 -X- AS1-rtr2 ---*-- AS2-rtr2 --/
        >--link goes down at X

In this case, AS1 is announcing 10/8 to its peer, AS2.
An internal link within AS1 goes down. Are there any
circumstances under which AS2 will announce some kind of
change to AS3? My thoughts:

  * AS1 might announce a MED change to AS2, and AS2 might
    propagate that to AS3 in some unknown way (manually?)
  * AS2 would see the path to AS1 change, and would change
    its announced MED accordingly
  * For some reason, AS1 might announce a withdraw on 10/8,
    and then re-announce it. Is this possible, and under
    what circumstances would this happen?
  * Other things? I feel like the answers might depend on
    networks using ibgp with route reflectors vs. full mesh,
    or using a different interior routing protocol.

In a related situation, let's say AS1 did a 'clear ip bgp'
on AS1-rtr2, which was the preferred link for AS2 to get to 10/8.
Would AS2 propagate any internal BGP announcements to AS3?
(Same kinds of reasons as above, but this time AS1-rtr2
actually stopped announcing anything to AS2)

I'm not certain of the right framework in which to think about
the answers to these questions, alas. Clues from the operational
side would be very welcome indeed.


I know that at least one vendor had a bug whereby when
bestpath changed and the med also changed it did not always
propogate the med change.

  - jared