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

[Pete Templin]

On 4/30/20 12:30 AM, Mark Tinka wrote:
> On 29/Apr/20 17:04, Mike 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.
> When we moved to IS-IS in 2007, we decided to set our reference
> bandwidth to 1Tbps (not in the routers, but as a concept).
>
> My recommendation would be not to set the reference in the network.
> Rather, have it as an architecture concept and deliberately design
> metrics off of your reference. 1Tbps seemed reasonable in 2007. Who
> knows, maybe 10Tbps is reasonable in 2020 (with the downside being that
> your metrics will be very high for very slow links).
>
> Mark.

My recommendation would be to set the default such that most or all of 
the links would default to a metric higher than what your designed 
metrics would be. Let the defaults protect you so that if a metric is 
missed, the default behavior is not to let the new link become instantly 
"best".

pt