RE: FW: /8s and filtering

Having a /24 doesn't indicate you are a network of any particular size,
ARIN ratified a policy that allows multihoming as justification for a


No, this is not the case. I enquired and it seems multihoming is not a
justification for a /24 in any RIR.

Does a network have to be able to fully utilize a /26 (25% of /24) in
order to multihome?


many isps will automatically give a /24 if their client is multihoming,
even if their usage is well below the 254 usable ips allocated by the
above block size.


But how will such an ISP justify this to the RIR?


I am not aware of ARIN taking such action.

To which policy are you referring?

Alec, ARIN AC member

Isn't it true that most bgp announcements with a mask longer than 24 bits
hit the proverbial bit bucket?


I was thinking about PI space. ARIN did in fact ratify a policy stating that a provider may allocate a customer a /24 of PA space if they are multihomed.

Alec, red-faced AC member

Did they ?
When ?

(I was involved with such a proposal, and it was turned down at the last ARIN meeting,
so I am curious if something else did get approved.)

Marshall Eubanks

Having a /24 doesn't indicate you are a network of any particular size,
ARIN ratified a policy that allows multihoming as justification for a


From: N []
Sent: Tuesday, December 10, 2002 1:01 PM
To: Forrest
Subject: Re: FW: /8s and filtering

comments inline

I was also curious about this - if I am a customer who wants to
multihome and can justify only a /24, I would go to an ISP which

has an

allocation from the Class C space rather than one from the Class A

  It doesn't matter. For all practical purposes, basement


care that their two or three providers have their route.

Maybe I'm missing something, but what good would it do for someone to
multihome if only their own providers accept their route, but nobody


does? I realize that their block should be still announced with their

ISP's larger aggregate, but what good does this do if your ISP goes


and can't announce the large aggregate.

For the assigned block to be part of the same aggregate(of both
providers), that implys that the providers sharing the responsibility
for the aggregate. It happens, but is rare. In this case, the providers
must accept more specific routes from each other, that is within the
space being aggregated. If they do not share specifics, one uplink down
will cause a large percentage ~50% for the customer. This scenario is
valid for load balancing, but redundancy is fragile. The only advantage
I see is no limit to prefix length. You can do this with a /28 if you
want... given the above caveats are addressed.

If you're a smaller organization, perhaps you'll only have a /23 from


upstream provider. With the filtering that seems to be in place, it


like the only way you can truly multihome with a /23 is if it happens


be in the old Class C space. Or is this wrong?

In today's VLSM world... the old classes have no bearing on filtering in
my experience. Prefix length discrimination knows no classfull

What seems to be needed is perhaps a /8 set aside by the RIR


to allocate to small organizations that wish to multihome that people
would accept /24 and shorter from.

There is value in the current filtering of longest prefixes... Allowing
anyone to multihome with BGP, using any network size, is going to double
our BGP tables overnight. Perhaps its good that you must be of some size
to participate in public BGP. Many providers offer redundancy that is
more appropriate for the smaller networks.


~Nathan - routing & switching dude/fly-boy/sport biker - San Jose CA~

T.M. Eubanks
Multicast Technologies, Inc.
10301 Democracy Lane, Suite 410
Fairfax, Virginia 22030
Phone : 703-293-9624 Fax : 703-293-9609
e-mail :

Test your network for multicast :
  Status of Multicast on the Web :

  The policy says that a /24 of PA space can be given if the
customer can show that it has an immediate requirement of 25% of the
/24 = /26.

That is the reason I asked if the multihomer has to be able to fully
utilize a /26=25% of /24 to be able to multihome.


Policy 2001-2 was ratified recently, which is separate and says that if the customer is multihomed he may receive a /24 with multihoming being the only justification.


My original question was how does this interact with the filtering
policies - especially when the Class C space is used up - there are only
three /8s left there.