North American Network Operators Group

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Re: 16-bit ASN kludge

  • From: Edward B. Dreger
  • Date: Sat Dec 04 15:06:08 2004

IvB> Date: Sat, 4 Dec 2004 12:17:22 +0100
IvB> From: Iljitsch van Beijnum

IvB> So now people have to renumber their AS when they start selling
IvB> 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.

IvB> This is not what the 32 bit AS draft proposes. (From memory, so I might
IvB> get some of the small details wrong.) The idea is that the new 32 bit
IvB> AS path is a new transitive attribute, which should be carried by
IvB> existing BGP implementations. However, the 16 bit AS path is still
IvB> there as well, with all the 16 bit incompatible ASes replaced by a
IvB> "special" AS.

Close, except I initially suggested 16-bit ASNs be reserved for transit

IvB> So all of this should work with existing implementations except that
IvB> they don't see the full picture so AS path filtering on 32 bit ASes
IvB> 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.

IvB> Note that I suggested starting to give out 32 bit AS numbers to new 32
IvB> bit compatible leaf sites while giving out 16 bit AS numbers to transit
IvB> ASes as a way to ease in to all of this with the least amount of
IvB> operational trouble. But at some point we'll run out of 16 bit AS
IvB> 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...

IvB> people should upgrade at some point or live with the reduced filtering
IvB> capabilities. And new ASes can't get around 32 bit support if their AS
IvB> 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.

IvB> 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.

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