[j-nsp] Inhibiting external announcements of routes for which a larger announcement exists

[Tore Anderson]

Hi,

apologies for the bad subject line - couldn't think of a way to condence 
my question into one line in a good ay.  Let me explain what I'm trying 
to do:

I've got 87.238.32/19 allocated from the RIPE NCC, and I intend to split 
it between our existing Norwegian site and our up-and-coming Swedish 
one.  Most likely I will leave 87.238.32/20 and 87.238.48/21 for 
Norway, and have 87.238.56/21 for Sweden.  We'll and have BGP speakers 
with transit providers and IX connections in both countries.

When everything is working fine I'd like to announce the /19 in both 
places, as the link between the sites should be high-speed enough to 
handle it.  However, should the link between the two sites fail, I'd 
like to immediately stop announcing the /19, and instead start 
announcing the Norwegian /20+/21 in Norway and the Swedish /21 in 
Sweden, so that traffic destined for Norway won't enter my AS in Sweden 
and vice verca.  I expect the backup link between the sites not to be 
fast enough to support that kind of traffic.

It should be simple enough to accomplish this by creating aggregate 
routes for the /20 and the /21s on the routers in their respective 
countries, and a /19 in both places that need all the /20 and /21s as 
contributing members to be active.  However that means that in a normal 
situation I'll announce /19 _and_ the longer prefixes at the same time, 
and I'd like not to pollute the global routing table with superfluous 
prefixes unless necessary (ie. if the link between the countries goes 
down).  I want a setup that inhibits the announcement of the /20 
and /21s to external neigbours if (and only if) the /19 is also 
announced to them at the same time.  Is that possible?

Regards,
-- 
Tore Anderson