Re: Link-state EGP

[nanog--- via NANOG <nanog () lists nanog org>]

On 24 August 2025 08:34:51 CEST, Saku Ytti via NANOG <nanog () lists nanog org> wrote:
> On Sat, 23 Aug 2025 at 23:32, Jakob Heitz via NANOG
> <nanog () lists nanog org> wrote:
>
> > Losses:
> > Privacy. Telling your competitors what all your links and private peerings are may not be what you want.
> > You might not advertise all your prefixes to some of your neighbors, but you still need the link for other prefixes.
>
> This disjoint advertisement is a legitimate argument, but as explained
> elsewhere we could address it by registering more ASNs and moving the
> ASNs, not prefixes. Privacy appears to be the same argument for
> disjoint advertisements.
>
> > If you are only advertising the link, then any neighbor could send you traffic that you don't want to provide 
> > transit for. So you drop it. How does your neighbor know? You send him the routes for traffic you are willing to 
> > transit traffic for.
>
> Your links that you advertise are the ASme-ASyou you provide traffic
> for. You don't advertise links you don't carry traffic for. So I would
> advertise ASme-ASme, ASme-AScustomer + ASprovider-ASme to my upstream,
> but I would not advertise ASme-ASupstream to my upstream.

But link-state protocols are global shared state, gossip protocols and don't support split horizon. You have a customer 
with two upstreams but you hide something from your upstream; they'll find out about it anyway via your customer and 
their other upstream.

I don't know what you mean by "links you carry traffic for". All links are presumably intended to carry traffic. So you 
advertise all links.

Lying in a link-state routing protocol is a good way to create routing loops. They fundamentally rely on every node 
having an identical set of information and running an identical algorithm.

> My upstream similarly would advertise to their peers and upstream
> ASupstream-ASme.
>
> This would allow anyone to validate those paths, because they expect
> ASme to have ASprovider-ASme adjancency, and they expect ASprovider to
> corroborate that with having ASprovider-ASme adjacency. Both
> link-states are signed and singatures verifiable by some out-of-band
> mechanism.
>
>
>
> I do think that in an alternate reality, where we would have
> anticipated that BGP abuse and +1M prefixes we would have landed
> somewhere entirely different than where we are today. And in that
> reality whatever limitations that feature has, we would have learned
> to live with them and started to think they are requirements, because
> they are requirements there, because we can only. build solutions on
> top of those that work with that stack.
> I have full confidence we could have made this link-state based
> reality work, and the Internet would work just the same for Internet
> users. I have no confidence that it would be worthwhile.
> It would be different and whatever it enables would seem like
> requirements to us now, while they were just solutions we ended up
> with the limitations we had.
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