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

[James Bensley]

On Wed, 29 Apr 2020 at 16:10, Mike <mike+lists at yourtownonline.com> wrote:
>
> Hello,
>
>      Is there a recommended 'modern default' for ip ospf auto-cost
> reference-bandwidth, to account for the fact that modern networks have
> 1g and faster interfaces?
>
>     My core equipment all has 10G and 1G interfaces today, and it seems
> to me that if I set the reference-bandwidth to 100gbps, Im not losing
> anything, but gaining a useful distinction between the modern
> 1g,10g,40g,and 100g interfaces of today. I understand the necessity of
> setting this on all routers in the network, however, I do have some
> older equipment (cisco 7201) doing T1 aggregation that also has 1g
> interfaces with nothing higher, so I am wondering the practicality of
> maybe just skipping this default change on that and like gear? These
> devices are at the edge and further only have single uplink connections
> to the core so it would seem safe to not worry about this here.

Hi Mike,

To answer your first question, I’d just push it up to 1Tbps rather
than 100Gbps. You don’t know what’s around the corner, you say you
have 10Gbps in the network now, 2 years from now you may put your
first 100Gbps link in and 2 years after that upgrade to 2x100Gbps
LAGs. So, I’d just bump it up to 1Tbps and then you don’t have to
worry about it for a long time / ever again (based on your current
size). If you’re going to the effect of rolling this out now, you
don’t want to do it again in a few years.

There was also a thread on this last year on the J-NSP List, see here:
https://puck.nether.net/pipermail/juniper-nsp/2019-January/036942.html

I’ve worked on some networks with ref bandwidth set to 100Gbps and
1Tbps, it worked fine in all cases, and they were mixed vendor
networks too.

To answer your second question about leaving out some older routers,
that’s really down to your operational niche. If these 7201’s really
only have a single 1G link, on paper it’s fine, but if they have a
recent enough IOS version that supports a 1Tbps ref bandwidth, then
I’d say you should be aligning them to the rest of your network. It
will hurt you a year from now on that 3AM on-call call-out when you’ve
forgotten why and this 7201 has different costs to all your other
routers and you have to waste time bottoming out why, before looking
into whatever the real issue is.

Cheers,
James.