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Confussion over multi-homing

  • From: David Lott
  • Date: Thu Sep 14 13:46:02 2000

I've read the current policy on ARIN's allocation of space and I must
admit that I'm still confused.

First, allow me to state the assumptions that I'm under.  I understand
the policy to state that if a business needs to multi-home and requires
less space than a /20, then they should request this space from their
ISP.  I also understand that there are filters at the /20 boundaries in
order to minimize the size of the routing table.

Question:  Doesn't this break multi-homing for end users that need less
than a /20?

For example, assume that the end user is connected to two regional ISPs
(ISP-A and ISP-B).  Neither of which have agreements with each other.
However, they do share a common backbone with a national provider we
will call ISP-Z.  If ISP-Z has filters at /20 for both of the ISPs that
it is connected to, then ISP-A address space will be the only space
listened to on the ISP-A to ISP-Z link.  The same would be true for the
ISP-B address space only being listed on the ISP-B to ISP-Z link.

This creates a situation where address space from ISP-B would not be
advertised through ISP-A and in effect, breaks multi-homing.  Consider a

remote site attempting to reach the web server at the end user.  DNS
resolves the address to ISP-B address space.  Also assume that the link
between the end user and ISP-B is down.  As the packet enters the
national carrier ISP-Z's network, at some point the router will have to
decide to send the pack on.  If ISP-B is still advertising the remaining

portion of their network (say at the /20 boundary) then ISP-Z will
forward the packet to ISP-B.  This is normal and proper for a single
homed address space.  However, if the end user had their own micro
allocation, their address space would be advertised to both ISP-A and to

ISP-B and in turn to the national carrier.  As such, the destination
network route would be dropped from the advertisement coming out of
ISP-B and the only remaining route would be via ISP-A and the packet
would still get there - if the end user had a micro allocation as per
previous policy.

Also, let us further look a situation where ISP-B is down.  When the
national carrier detects ISP-B is down it will remove that particular
route from it's table.  In the old way of doing things with micro
allocation to multi-homed end users, ISP-A would advertise the address
space from the end user.  It is my understanding that under the current
policy, ISP-A would have to advertise the address space allocated to the

end user from ISP-B.  If the address is less than a /20 and if the
national carrier is filtering on a /20, wouldn't that cause the update
to be dropped and thus not added to the routing table for the national

I guess my confusion could be cleared up if someone could describe how,
under the /20 policy, an end user requiring multi-homing and less than a

/20 allocation would be able to survive one of their two ISPs going down

(remember the AT&T and MCI outages?).


PS.  I have sent this email to the ARIN policy list but have not
received any
useful responses.

David Lott
    VP of Operations
     MSN Communications
     (303) 347-8303