FW: /8s and filtering

Tue Dec 10 19:33:32 UTC 2002

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

	Brian

On Tue, 10 Dec 2002, Harsha Narayan wrote:

> Hello,
>  But how will such an ISP justify this to the RIR?
>
> Thanks,
> Harsha.
>
> On Tue, 10 Dec 2002, Brian wrote:
>
> > 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.
> >
> > 	Brian
> >
> >
> > On Tue, 10 Dec 2002, Harsha Narayan wrote:
> >
> > >
> > > Hello,
> > >  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?
> > >
> > > Harsha.
> > >
> > > On Tue, 10 Dec 2002, Ejay Hire wrote:
> > >
> > > >
> > > > 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
> > > > /24.
> > > >
> > > > -ej
> > > >
> > > > -----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
> > > > multihomers
> > > > > > 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
> > > > else
> > > > > 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
> > > > down
> > > > > 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
> > > > your
> > > > > upstream provider.  With the filtering that seems to be in place, it
> > > > seems
> > > > > like the only way you can truly multihome with a /23 is if it happens
> > > > to
> > > > > 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
> > > > boundaries.
> > > >
> > > > > What seems to be needed is perhaps a /8 set aside by the RIR
> > > > specifically
> > > > > 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.
> > > >
> > > > --
> > > > ,N
> > > >
> > > > ~Nathan - routing & switching dude/fly-boy/sport biker - San Jose CA~
> > > >
> > >
> >
>