FW: /8s and filtering

Ejay Hire ejay.hire at isdn.net
Tue Dec 10 19:08:44 UTC 2002

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


-----Original Message-----
From: N [mailto:nathan at stonekitty.net] 
Sent: Tuesday, December 10, 2002 1:01 PM
To: Forrest
Cc: nanog at merit.edu
Subject: Re: FW: /8s and filtering

comments inline

On Tue, Dec 10, 2002 at 12:36:39PM -0600, Forrest wrote:
> > 
> > >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 
> > >space.
> > 
> > 	It doesn't matter. For all practical purposes, basement
> > only 
> > 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~

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