On 8/23/25 11:57, Saku Ytti via NANOG wrote:
On Sat, 23 Aug 2025 at 18:54, nanog--- via NANOG <[email protected]> wrote:
on second thought, the real reason is that link-state protocols are distributed
algorithms which require all nodes to execute the same algorithm on the same
data, so there's no room to apply policy that wasn't baked into the design of
the protocol.
The above dances around the opposite side of a conversation John Scudder
and I were having about the properties of the system we've evolved. A
BGP rib-out is effectively the output of a hidden state machine for your
entire network.
It doesn't really matter for sending direction which egress they
choose, as long as it doesn't loop. So even in this SPT future, I can
choose longer upstream over shorter by local policy, just like today.
The big difference is, that the receiver cannot cherry pick which
prefixes to receive in which eBGP, you have to be able to receive all
prefixes on all eBGP with a given ASN. And these consistent
announcements are not today always used, and would need to be replaced
by registering multiple ASN.
This hits part of the above as well. What you lose through traditional
link state type mechanisms is the ability to do policy. Operators like
their policies for all sorts of reasons. In order to implement something
that resembles the hop by hop policy stuff you can do in BGP in
something that is link-state, it becomes necessary to distribute a
portion of that policy into the link state distribution machinery and
run it as part of your calculations for a large number of hops.
The easy way to picture some of the impacts of that is consider what
it'd take to distribute "at the boundary of AS X->Y, don't distribute
prefix P".
Traditional valley-free routing starts to require careful management of
large metrics.
Etc.
It gets very gross, very quickly. You touch on some of the issues.
-- Jeff
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