[c-nsp] bgp query

[Mark Kent]

I couldn't decide whether to send this to nanog or here,
but decided on here by a coin toss.

Using cisco routers, here's a simplified diagram for my questions:

   {NSP1}<-->[AS1]<--C-->[AS2]<-->{NSP2}

Currently, AS2 is a customer of AS1.  AS2 announces some more specific
nets (/22 and /21) of a larger block (/18) owned/announced by AS1.
AS1 also "leaks" these more specific nets to NSP1.  Everything works.

Moving forward, AS2 and AS1 make a cooperative agreement, part of
which involves AS2 providing transit for AS1 to/from NSP2.  So now the
relationship is symmetric, except for the netblock issue.  AS2 still
has smaller chunks of the larger netblock owned by AS1.

Should we set things up so that more specific announcements are
suppressed, and only the one larger block is announced?

I'm concerned about what happens if circuit C dies.   
What mechanism would allow AS2 to revert to only announcing 
their subnets of the larger block?

I'm concluding that each side should announce both the whole block and
the sub-blocks, always.  But, I figured I should check and see if
there is a cleaner solution.

Thanks,
-mark