[NANOG] Re: question about peering relationships

[William Herrin via NANOG <nanog () lists nanog org>]

On Mon, Apr 7, 2025 at 8:10 AM Elmar K. Bins via NANOG
<nanog () lists nanog org> wrote:
> nanog () lists nanog org (Sriram, Kotikalapudi (Fed) via NANOG) wrote:
> > Does the following ever happen in reality? Do you think it is strange and unlikely?
> > The lateral (i.e., non-transit) peer of an AS is also the transit provider of the AS's transit provider.  Example: 
> > AS A has AS B as a transit provider and AS C as a lateral peer, and AS C is a transit provider of AS B.
>
> Yes, and it's very often a mess traffic-engineering wise...

Hi Elmar,

Would you mind discussing this further and offering examples of some
of the traffic engineering challenges?

AS A should see AS C's origin and customer routes from both AS C
directly (peer) and AS B (provider). Unless AS A is playing local pref
games, the peering routes have shorter AS paths and are preferred -- a
sensible outcome.

AS C should see AS A's routes both from AS A directly (peer) and AS B
(customer). Unless AS C is playing local pref games, the peering
routes have shorter AS paths and are preferred  -- a sensible outcome.

AS B should see AS A's routes both from AS A directly (customer) and
AS B (provider). Unless AS B is playing local pref games, the customer
routes have shorter AS paths and are preferred -- a sensible outcome.

AS C does not see AS B's routes via its lateral peering link with AS A
because on a peering link the AS only sends its origin and customer
routes. Thus AS C sends AS B's packets to its customer, AS B  -- a
sensible outcome.

Now, if one of the networks is playing local pref games, which they
shouldn't be doing, then they may misroute packets the long way around
the planet. But that's their fault for playing local pref games.

Regards,
Bill Herrin



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