Re: maximum ipv4 bgp prefix length of /24 ?

[William Herrin <bill () herrin us>]

On Thu, Sep 28, 2023 at 2:25 PM VOLKAN SALİH <volkan.salih.06 () gmail com> wrote:
> I believe, ISPs should also allow ipv4 prefixes with length between /25-/27 instead of limiting maximum length to 
> /24..
>
> It is because BGP research and experiment networks can not get /24 due to high IPv4 prices, but they have to get an 
> IPv4 prefix to learn BGP in IPv4 world.

Hello,

Each BGP route advertised into the global table is expensive. Not for
the advertiser... for everyone else. See
https://bill.herrin.us/network/bgpcost.html
Caveat that the document is based on 2008 data. The numbers (though
probably not their shape) have changed.


> What do you think about this?

I think you'll convince the IETF to release the Class-E space before
you convince the ISPs to broadly honor sub-/24 prefixes.


> What could be done here?

In principle, a company could make a business out of announcing a
large block from a bunch of peering points and then tunneling (vpn)
parts of it back to customers with sub-/24 assignments. With a broad
enough selection of peering points, the routing would not be too
inefficient. And it would divorce the IP addresses from the last-mile
Internet infrastructure, allowing you to take your addresses with you
as long as you kept paying the tunnel company.

In practice... there's not enough money in it. If you could ante up
the cost, you could find a way to qualify for and acquire a full /24.


> Is it unacceptable; considering most big networks that do full-table-routing also use multi-core routers with lots of 
> RAM?

You're thinking of DRAM. But that's not the way it works. Some routers
use heavily parallel routing engines, each of which need enough dram
to hold the full forwarding information base and which can suffer from
CPU cache exhaustion even then. Others use an expensive kind of memory
called a TCAM that's very fast but both expensive and power hungry, so
generally not sized for huge numbers of tiny routes.

Regards,
Bill Herrin



-- 
William Herrin
bill () herrin us
https://bill.herrin.us/