It wouldn't be the first time that your redundant vendors end up sharing a conduit for a bunch of fiber somewhere. Guess where that backhoe will start digging?
Redundant vendors in the GP’s context referred to using multiple router vendors, eg Cisco and Juniper.
Using multiple connectivity vendors doesn’t guarantee path diversity. Demanding fibre maps and ensuring that your connectivity has separate points of entry into the building, doesn’t cross outside the building, and validating with your DC provider that your cross connects aren’t crossing either, guaranteed path diversity / redundancy.
Its a bit of both. Internationally I find I can't trust the network maps of the connectivity vendors and I'm better going for two separate companies (ones which are part of different subsea cables -- e.g. Wiocc on Eassy and Safaricom on TEAMS).
Of course I had one failure in Delhi which the provider blamed on 5 separate fibre cuts. Long distance circuits can run via areas where they can sustain multiple cuts across large amounts of area (regional flooding is a good one), and fixing isn't instant. This can be mittigated a little, but you still end up with circuit issues -- I had two fibre runs into Shetland the other month. Frist one was cut, c'est la vie. Second one was cut, had to use a very limited RF link. There's only so much you can do.
On the other hand I've just been given a BT Openreach plan which lists any pinch points of a new RO2 EAD install, I can see the closest the two get during transport is about 400m (aside from the end point of course, and experience has taught me I can trust it.
The GP was clearly talking about whole networks, not just the hardware vendors, if I read that different than the GP intended I'll wait for their correction.
One of the problems that I've seen in practice that with the degree of virtualization at play that it has at the same time become much more easy to in principle be guaranteed 100% independence and in practice it has become much harder to verify that this is the case because of all of the abstraction layers underneath the topology. One of my customers specializes in software that allows one to make such guarantees and this is a non-trivial problem, to put it mildly, especially when the situation becomes more dynamic due to outages from various causes.
In London I can literally follow the map from manhole to manhole, exchange to exchange. It's dark fibre so I can flash a light down it and a colleague can see it emerge at the other end. Now it's possible they don't follow the map and still make it to the other end, but it's pretty unlikely.
Sometimes of course you have to make judgement calls. From one location near Slough I have a BT EAD2 back to my building a few miles away. I know the route into my building, I can see the cables with my own eyes going in different directions. BT tell me which exchanges those cables goto, and provide me with a map into the field at a 1000:1 scale showing the cables coming in down a shared path. Sure it's possible BT are lying, but it's unlikely. Only use that location sporadically, and when I do it's a managed location, so I can accept the risk of a digger on the ground.
Another location in Norfolk, two BTNet lines, going to two different exchanges. They meet at the edge of the farm and go up the same trunk. That's fine, I can physically control the single point of failure there too, although if peering between BT and my network fails then I'm screwed, but I have a separate pinnacom circuit in a crunch.
Now obviously some failure become far harder to mitigate. A failure of the Thames Barrier would cause a hell of a lot of problems in Docklands, I'm not sure if any circuits in/out of places like telehouse, sovhouse, etc will remain. Cross that bridge etc. Whether my electricity provider will remain with a loss of the internet is another matter, so then it comes down to how much oil there in in the generators, and the generators of any repeaters on the routes of my network.
However the much easier to avoid is the problem of some shitty stacked switch the salesman says will always work.
I have to trust the dark fibre map provided, but I know exactly which way it ran, manhole to manhole. I had three cores, they shared the first 20 metres to the manhole, it's unlikely there would be a backhoe digging underneath the police van and pile of scaffolding that was parked in the shared conduit.
After that it went on different paths to three different buildings, which from each of those was then routed independently.
We take physical resilience seriously, as it isn't network engineers that do that part of the infrastructure. Enterprise network engineers then throw it all away by stacking their switches into a single point of logical failure.
(Still had a non-IP backup, but sometimes that breaks too -- just in different ways than the IP)
