[c-nsp] ospf auto-cost reference-bandwidth on modern gigabit networks

[Saku Ytti]

On Thu, 30 Apr 2020 at 12:31, Robert Raszuk <robert at raszuk.net> wrote:

> It is like GPS putting all cars on the big and congested highway when you have a totally empty asphalt side road next to it :)
>
> The BW based IGP metric mapping comes from times of F/R, 64 kbps satellite uplinks and zyxel DTE/DCE V.35 devices.

I'm skeptical SPT topology ever was remotely typical where it made
sense. It may work approximately, but that's not because it's good
SPT, that's because acceptable outcome has large overlap with possible
implementations.



Two examples spring to mind

1) 10GE network, where you have 1G tails redundantly connected, you
don't want the 1G tail to transit any of the 10GE traffic. You
actually want overload or similar, the dual-connected 1GE is never
desirable to transit.

2) 10GE core, with 1G islands attached to it, 1G will transit 1G
inside 1G island, but you don't want 1G island to transit 10GE core.
So you actually want a penalty on the island-border, so traffic
doesn't cross it. (i.e. role based).

In both above cases, people want distance-vector, with constraints,
and BW based may approximate correct behaviour, but correct behaviour
is actually very different from BW SPT.

I think an interesting exercise is, why don't we see BW based ISIS
topologies, but we do see OSPF? I think it is status quo bias, because
vendors offer OSPF with this default, is must make sense.



-- 
  ++ytti