Date: Sat, 4 Dec 2004 12:17:22 +0100
From: Iljitsch van Beijnum
So now people have to renumber their AS when they start selling
transit? Not such a great idea...
Yeah. They'll have to tell their upstreams "here's our new ASN". No
downstreams will be affected -- by definition. Hopefully routers will
be friendlier to ASN changes; else one can use confederations.
How is this half as bad as renumbering even a /22 of IPv4 space? I'll
grant that it's imperfect, and there'd be the occasional large leaf with
many routers to reconfigure, but leaves tend to be _small_ networks.
This is not what the 32 bit AS draft proposes. (From memory, so I might
get some of the small details wrong.) The idea is that the new 32 bit
AS path is a new transitive attribute, which should be carried by
existing BGP implementations. However, the 16 bit AS path is still
there as well, with all the 16 bit incompatible ASes replaced by a
Close, except I initially suggested 16-bit ASNs be reserved for transit
So all of this should work with existing implementations except that
they don't see the full picture so AS path filtering on 32 bit ASes
won't work. Basic operation shouldn't be a problem, though.
Correct. Existing software would see the "special" 16-bit AS for all
leaves. Newer software would understand $attribute and supply the
correct AS path.
Note that I suggested starting to give out 32 bit AS numbers to new 32
bit compatible leaf sites while giving out 16 bit AS numbers to transit
ASes as a way to ease in to all of this with the least amount of
operational trouble. But at some point we'll run out of 16 bit AS
numbers and 32 bit leaf networks will become transit networks, so
I suppose there could be in excess of 65431 transit networks. I think
that's why Owen suggested reserving, say, 2^20 ASNs for transit in
32-bit space. That's probably wise, and at that point 16-bit ASNs would
be totally unsuitable. Until then, though...
people should upgrade at some point or live with the reduced filtering
capabilities. And new ASes can't get around 32 bit support if their AS
number isn't 16 bit safe, of course.
No matter _what_ the implementation details of expanded ASN space,
people must upgrade or lose <varying amounts of> capabilities.
What would you like to optimize for?
Application of Dijkstra's algorithm. Perform full SPF calculations on
transit networks. Leaves carry "my upstream" attributes with metrics a
la DPA; such information is combined with relevant transits' metrics.
Although not directly akin to OSPF stub areas and NSSAs, the basic
premise is the same.