many providers filter based on the classful origin of the space. If the
block is out of what was once class a or b space, the likelihood of a /24
getting filtered out is fairly high. My previous employer did that.
Brian "Sonic" Whalen
Success = Preparation + Opportunity
On Mon, 7 May 2001, Chuck Larrieu wrote:
> Seems rather presumptuous of Cisco to speak for every ISP in the world....
>
> In order to limit the number of routes being advertised on the internet, I
> believe it was considered "best current practice" to limit prefix length to
> /19 or shorter. ( can't find the RFC at the moment, but I recall it being
> referenced several times in various threads on the NANOG list. )
>
> Obviously, with well over 100K routes in "the internet routing table" there
> are a great number of longer prefixes being advertised, no doubt in great
> part because of the number of companies that are "connected to multiple
> ISP's so they can load balance across the internet"
>
> Prefix advertising my be influenced by peering arrangements, downstream and
> upstream agreements, and customer requirements. Generally, once holes are
> punched through CIDR blocks, what can anyone do?
>
> When someone makes a statement like you attribute to Cisco, one must always
> follow up with specifics to determine what is really meant. Not all routes
> seen in one provider's network routing tables are necessarily present in
the
> tables of another provider.
>
> Chuck
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
> Murphy, Brennan
> Sent: Monday, May 07, 2001 4:01 PM
> To: '[EMAIL PROTECTED]'; Charlie Winckless; Murphy, Brennan
> Cc: [EMAIL PROTECTED]
> Subject: RE: real world BGP question
>
> Cisco told me today that a /24 drawn from Class C space
> has a better chance of being propogated throughout the Internet
> than a /24 taken from Class B space. Anyone disagree with that?
> Can anyone recommend a good source of info on this. Ive checked
> Halabi.
>
> I came across a good reference during my quest www.traceroute.org
> Unfortunately, it doesnt offer plain answers to my questions.
>
>
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
> Sent: Friday, April 27, 2001 1:58 PM
> To: Charlie Winckless; 'Murphy, Brennan'
> Cc: [EMAIL PROTECTED]
> Subject: Re: real world BGP question
>
>
> Currently on a US basis a /24 would generaly work. Internationaly (Europe)
> most providers would filter out anywhing longer then /20.
>
>
> ----- Original Message -----
> From: "Charlie Winckless"
> To: "'Murphy, Brennan'"
> Cc:
> Sent: Friday, April 27, 2001 1:18 PM
> Subject: RE: real world BGP question
>
>
> > I used to work for VERIO. At that time they would not
> > router smaller than /19 on their backbone.
> >
> > This may have changed.
> >
> > > -----Original Message-----
> > > From: Murphy, Brennan [mailto:[EMAIL PROTECTED]]
> > > Sent: Friday, April 27, 2001 10:46 AM
> > > To: 'Michelle T'; '[EMAIL PROTECTED]'
> > > Subject: RE: real world BGP question
> > >
> > >
> > > I guess that is my real question: what is the longest prefix that
> > > is exchanged among/between major carriers.
> > >
> > > The real world example here is what if you had 4 server farms
> > > answering
> > > to one DNS name: ftp.foo.com You have Round Robin DNS running
> > > round trip times to match a user with their nearest server farm....
> > > so it sends back the closest/fastest IP. The question is, how
> > > big do those
> > > subnets for the server farms have to be in order to be maximally
> > > advertised throughout the internet?
> > >
> > > So, I've seen two answers in this thread /20-21 or /24. I wonder
> > > where I could find the real answer? Maybe Halabi has a link in the
> > > back of his book to an organization that maintains info such as
> > > this.
> > >
> > > Any more input is greatly appreciated. Thanks to all who have
> > > responded.
> > > I figured this question was a relavant BGP question relating
> > > our studies
> > > to an actual scenario.
> > >
> > >
> > >
> > > -----Original Message-----
> > > From: Michelle T [mailto:[EMAIL PROTECTED]]
> > > Sent: Friday, April 27, 2001 12:06 PM
> > > To: Murphy, Brennan; [EMAIL PROTECTED]
> > > Subject: RE: real world BGP question
> > >
> > >
> > > /24 is the longest prefix you will see accepted by nearly any
> > > carrier out
> > > there. Many will only accept /20 or /21. All perform
> > > aggregation to some
> > > degree, though exception routing is allowed to send the /24's
> > > (/23, /22,
> > > etc) out to the ISP peers when the customer is multi-homed two diverse
> > > carriers.
> > >
> > > I can tell you that I work for a Tier 1 ISP and we accept
> > > longer prefixes
> > > for many customers who are multi-homed just to us. They use
> > > the various
> > > subnets as a simple method of controlling inbound traffic
> > > distribution, to
> > > enact policy, etc...
> > >
> > > Many times we see multi-homed (dual-ISP) customers advertise
> > > an aggregate
> > > /16 or longer and also advertise /24's for the same reaason (policy,
> > > distribution, etc).
> > >
> > > Michelle Truman
> > >
> > >
> > > -----Original Message-----
> > > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> > > Murphy, Brennan
> > > Sent: Friday, April 27, 2001 10:28 AM
> > > To: '[EMAIL PROTECTED]'
> > > Subject: real world BGP question
> > >
> > >
> > > What is the smallest subnet that major carriers will exchange with one
> > > another? /24..../26.../27?? I know that the real issue is
> > > the size of
> > > the route table.
> > >
> > > I'm just wondering about the reallity of scenarios that
> > > Habali describes
> > > where an institution advertises an aggregate with specific subnets.
> > >
> > > I know that when you're multi-homed to a carrier, that carrier will
> > > sometimes
> > > take your /26 and /27 nets to help route inbound traffic but
> > > that carrier
> > > will not advertise those nets to its neighbors.....at least
> > > thats what I've
> > > heard.
> > >
> > > Anyone have any real world experience with this? Or is there a URL
> > > I could read up on?
> > >
> > > Thanks,
> > >
> > > BM
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > > **Please read:http://www.groupstudy.com/list/posting.html
> > **Please read:http://www.groupstudy.com/list/posting.html
> **Please read:http://www.groupstudy.com/list/posting.html
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