[j-nsp] OSPF reference-bandwidth 1T

adamv0025 at netconsultings.com adamv0025 at netconsultings.com
Fri Jan 18 10:58:10 EST 2019

> Thomas Bellman
> Sent: Thursday, January 17, 2019 3:48 PM
> On 2019-01-16 16:41 MET, Saku Ytti wrote:
> > No one should be using bandwidth based metrics, it's quite
> > non-sensical. I would recommend that if you have only few egress
> > points for given prefix, adopt role based metric P-PE, P-P-city,
> > P-P-country etc. If you have many egress options for given prefix
> > latency based metric might be better bet.
> You are obviously talking about a service-provider perspective here, since
> you are talking about P and PE.  Not an unreasonable assumption on this list
> of course, but I don't see any indication of what kind of network Event Script
> is running
> Would you advise avoiding bandwidth-based metrics in e.g. datacenter or
> campus networks as well?
> (I am myself running a mostly DC network, with a little bit of campus network
> on the side, and we use bandwidth-based metrics in our OSPF.
> But we have standardized on using 3 Tbit/s as our "reference bandwidth",
> and Junos doesn't allow us to set that, so we set explicit metrics.)
It makes me think where in Clos or Benes fabric would I need to resort to bw-based metric style.
The very point of Clos graphs is that all edges and vertices of a given layer are built equal and in Benes graph all edges and vertices are the same.
If you have shortcut/backdoor links in these then you're in trouble already.

One major problem with reference BW is that you'll ultimately run out of whatever, reasonable at the time, value you'll set there.   


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