[j-nsp] OSPF reference-bandwidth 1T

[James Bensley]

On Thu, 17 Jan 2019 at 18:09, Saku Ytti <saku at ytti.fi> wrote:
> It boggles my mind which network has _common case_ where
> bandwidth is most indicative of best SPT.

Hi Saku,

I've worked on several small networks where you don't have equal
bandwidth links in the network. I don't mean U/ECMP, I mean a ring
topology for example, and some links might be 10G and some 1G etc.
Maybe the top half of the ring from 9 o'clock moving clockwise round
to 3 o'clock is 10Gbps, or 20Gbps, and the bottom half from 3 o'clock
moving clockwise round to 9 o'clock is 10Gbps or 1Gbps. I want traffic
from the 3 o'clock PE to always go anticlockwise to get to 8 o'clock
depsiten being one hop further to reduce the traffic across the bottom
half of the ring.

Previosuly you said....

On Wed, 16 Jan 2019 at 15:42, Saku Ytti <saku at ytti.fi> wrote:
...
> No one should be using bandwidth based metrics, it's quite
> non-sensical.

But for any link between PoP A and PoP B the bandwidth is directly
related to the cost, i.e. 1Gbps from A to B costs < 10Gbps and 10Gbps
from A to B costs < 100Gbps etc. Having worked on small very small
ISPs with only 2 or 3 PEs and lucky 8 ball to get by, costs is
everything and you end with both both; links of varrying speeds and
links fo varrying MTUs (oh the joy!).

> P-P-country etc. If you have many egress options for given prefix
> latency based metric might be better bet.

Yeah, for larger networks with more money this works well. $dayjob has
a lot of realtime voice and video flying around so we use latency
based metrics and it works well but we also have our own transmission
infrastructure meaning that bandwidth isn't an factor for us. Not
everyone has that luxury.

Cheers,
James.