Re: Aggregating channels...

["Arvid Grøtting" <arvidg@netfonds.no>]

Joe Abley <jabley@automagic.org> writes:

> In order to balance (share) world->ME traffic through the connections
> to both A and B it is necessary for A to leak the covered prefix
> coresponding to ME's A-derived PA netblock as well as A's supernet,
> so the world sees two paths for the same-length prefix. This is the
> additional clue requirement that Henning mentioned.

As many transit providers filter BGP routes quite aggressively, a PA
solution has the added advantage that if your specific route gets
filtered for having too long a netmask, the A-allocated supernet route
will still be out there.

Announce e.g. a /25, and it won't reach us as such, because both our
providers filter away anything more specific than a /24.  More
draconian policies have beed rumored to exist, and will probably
proliferate as the BGP routing tables grow.  (Filtering on the
minimum, or even the default, allocation size of each RIR-allocated /8
could become common soon enough.)

> Some ISPs are intolerant of long-prefix PA blocks being advertised
> through other ISPs, and even prohibit such behaviour in their terms
> and conditions. The solution in that case is either to acquire PI
> space (all three RIRs have protopolicies permitting small PI
> delegations to organisations for the purpose of multihoming) or to
> number out of PA space delegated from an ISP who is not intolerant
> of CIDR-abuse.

Or, indeed, to (threaten to) switch providers.

Do keep in mind that PA space stays with the provider, so you'll have
to renumber every time you switch away from the provider you got the
PA space from.  That's the disadvantage of PA space.  The disadvantage
of PI space is that it isn't guaranteed to be routable anywhere on the
Internet; Announce a (PI) /25, and it won't reach us at all.


-- 

Arvid