[j-nsp] Conditional Advertisemet?

[Felix Schüren]

Eric,
> Have a BGP configuration where I learn a /16 network from one eBGP
> neighbor and want to advertise a subset of that network, i.e. a /28, to
> a different eBGP neighbor.  Currently, I am able to advertise the subnet
> by configuring a static route which puts the /28 in table, however; this
> kills anything dynamic.  Is there a way with conditional advertisements
> or a generate route to dynamically advertise the /28 upon receipt of the
> /16?  Also, I don't want to rely on the BGP neighbor (who is advertising
> the /16) to give me the /28 instead.  
> 

as far as I am aware, there isn't.
Aggregate/generate routes will only look at more-specific routes as 
possible contributors, so an aggregate /28 would never even look at the 
/16. A Static route pointing to the ebgp peer IP would go away if the 
link died, but could not check whether the /16 is there. ...the only way 
I can see is to change things around - have your peer advertise the /28, 
  then create an aggregate route for the /16 (with a policy which only 
allows the /28 to contribute) to use in your network. Other than that, 
I'm stumped.

kind regards,

Felix

PS: I just had a NASTY that should not be implemented - have a routing 
instance import only interface routes + the /16 from your specific ebgp 
peer, and within the routing instance create a static route for the /28 
pointing to any IP (say, a.b.0.0) from the /16 with "resolve" as 
keyword. This should create the /28 only when the /16 is there (and the 
/16's next-hop is reachable from within the routing instance). After 
that apply policy on export to reset next-hop etc...