AS Number Registrations

Shane Kerr shane at
Wed Aug 30 04:23:04 EDT 2000

> The number of AS numbers ARIN issues has increased steadily over the last
> three years.  Members have expressed concern about this large increase in
> AS number registrations.
> 	Issued in 1998 -- 915
> 	Issued in 1999 -- 1,685
> 	Issued in 2000 -- 1,596 (through July)
> The vast majority of these AS numbers are issued to newly multi-homed
> sites.  Is this increase in AS number registrations a concern to the
> community and should ARIN policies be changed, as a result?

Note the current assignments on the IANA page:

At 3000 issued a year, the AS numbers will exhaust the "Held by IANA" 
in 4 years, and the "Reserved by IANA" in 15 or so.  Given a modest 50% 
increase in number issued each year, they'll all be gone in 2004 or 2005.

As was/is the case with IPv4, the options of dealing with number 
shortage are: 

1. Change the amount of numbers (e.g. IPv6)

   I'm not aware of any planned BGP protocol that extends the space of AS 
   numbers beyond 16-bits.  Even should such a protocol exist, it getting 
   it standardised and installed across the Internet before 2004 seems an 
   ambitious goal.

2. Change the way numbers are used (e.g. NAT, CIDR)

   I think this is both the best approach, as well as being the only 
   one that ARIN (or any other AS number assigning registry, i.e. RIPE 
   NCC or APNIC) can actually implement.

The IETF BCP document, AKA RFC 1930 (ug...think I need more TLA's?) 
describes the situations in when an AS is needed.  As mentioned in 
ARIN's e-mail, the vast majority of new AS numbers are issued to newly 
multi-homed sites.  This is described as a valid reason to need an AS 
number in this document.

As a side note, I think it imperative that ARIN does nothing that might
discourage sites from becoming multi-homed.  Removing single points of 
failure is key to insuring reliability.

I think the basic problem is that the BCP may not be 100% correct in 
stating that a site needs an AS number to be multi-homed.  It is 
possible to advertise a route from multiple origin AS numbers.  I 
suggest that this should be the recommendation of ARIN regarding 
multi-homed sites.

A multi-homed customer with two (or more) upstream providers certainly 
needs to exchange routing information with the providers, but there is 
no reason from the customer point of view why this protocol needs to 
be BGP (or rather, why it can't be internal BGP or some other internal 
routing protocol).  This protocol need not even be the same for all 
upstream providers!

The potential drawback of this scenario is when the upstream providers 
inject the route into the Internet.  What other sites on the Internet 
will see is a single route, originating from several AS numbers.  I am 
fairly sure that this is not a problem with current Internet routers.  
Indeed, many networks are currently advertised in this fashion today.  
I have been told that there is resistance to the idea of advertising a 
route from multiple AS numbers, but I'm not sure what the concerns are.

There may be other potential problems, e.g. with efficiency or with 
failure recovery time.  Individuals with more routing expertise than me 
will need to address these issues.  :(


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