On Wed, Apr 9, 2025 at 2:01 PM Matthew Petach <mpetach@netflight.com> wrote:
I don't even know what you're on about here. No aspect of the BGP protocol is remotely non-deterministic. Even when you use it badly.
Wondering if someone has a previous NANOG presentation or other white paper that covers multiple scenarios they can reference?
Is it your assertion that ISPs should leave routing decisions purely to the default BGP path selection algorithm, with no hints, nudges, or fingers on the scale to steer traffic flows?
Absolutely not. My position is that like fabled "goto," use of local pref should be considered harmful. That doesn't exclude the use of BGP's inbuilt tools like meds and prepends, nor does it exclude the use of optimizers capable of incorporating smart additional information into the selection algorithm when multiple paths are available. Local pref is not smart. It's a blunt instrument that hurts..
The routing table we are trying to manipulate is a rather blunt instrument. Packets are routed on the destination prefix only. BGP also does not tons of choices either especially when it has to use its limited tooling to affect the even more limited FIB. These are the tools we have. D <- C <- B ^-> A-^
If C accepts the peering route from A instead of the A customer route from B then C does not propagate A's route to D. Customer propagates to transit. Peer does not. A has to make a choice between having C as a peer and having C as an indirect transit provider. He can't have both. But unless C plays games with localprefs, A can trivially express his preference using prepends. And choices like this are what A signed on for when he decided to peer with C instead of buying transit.
Can you provide a white paper that covers your proposed 'BGP Improvements' in more detail? This could be a decent NANOG presentation too. I have always been under the impression that you can't have peering, transit, and customer relationships with the same upstream and expect things to work, with or without local preference. I know I am missing something in these conversations, and have not been able to glean a better understanding from this thread. Hoping someone can point me to a white paper or good NANOG presentation.