[siebenmann]

In a lot of environments, you can at least choose to restrict what networks can be used to manage equipment; sometimes this is forced on you because the equipment only has a single port it will use for management or must be set to be managed over a single VLAN. Even when it's not forced, you may want to restrict management access as a security measure. If you can't reach a piece of equipment with restricted management access over your management-enabled network or networks, for instance because a fiber link in the middle has failed, you can't manage it (well, remotely, you can usually go there physically to reset or reconfigure it).

You can cross-connect your out of band network to an in-band version of it (give it a VLAN tag, carry it across your regular infrastructure as a backup to its dedicated OOB links, have each location connect the VLAN to the dedicated OOB switches), but this gets increasingly complex as your OOB network itself gets complex (and you still need redundant OOB switches). As part of the complexity, this increases the chances an in-band failure affects your OOB network. For instance, if your OOB network is routed (because it's large), and you use your in-band routers as backup routing to the dedicated OOB routers, and you have an issue where the in-band routers start exporting a zillion routes to everyone they talk to (hi Rogers), you could crash your OOB network routers from the route flood. Oops. You can also do things like mis-configure switches and cross over VLANs, so that the VLAN'd version of your OOB network is suddenly being flooded with another VLAN's traffic.

(I am the author of the original article.)

[ralferoo]

We might be talking at cross-purposes a bit, but also it seems that you're considering a much larger scale than me, and also I hadn't really considered that some people might want to do data-intensive transfers on the management network, e.g. VM snapshots and backups.

Because of how I use it, I was only considering the management port as being for management, and it's separated for security. In the example in the article, there was a management network that was entirely separate from the main network, with a different provider etc. I guess you may have a direct premises-to-premises connection, but I was assuming it'd just be a backup internet connection with a VPN on top of that, so in theory and management network can connect to any other management network, unless its own uplink is severed. Of course, you need ISPs that ultimately have different upstreams.

In the situation that your management network uplink is down, I'd presume that was because of a temporary fault with that ISP, which is different to the provider for your main network uplink. You'd have to be pretty unlucky for that also to be down too. Sure, I can foresee a hypothetical situation where you completely trash the routes of your main network and then by some freak incident your management uplink is also severed. But I think the odds are low, because your aim should be to always have the main network working correctly anyway. If you maintain 99.9% uptime on your main network and your management uplink from another provider is also 99.9%, the likelihood of both being down is 0.0001%.

I'd also never, ever, ever, want a VLAN-based management network, unless that VLAN only exists on your internal routers and is separated up again into individual nets before it goes outside the server rooms. Otherwise, you've completely lost any security benefit of using an isolated network. OTOH, maintaining a parallel backup network on a VLAN that's completely independent to the management network, but which can be easily patched it by someone at that site if you need them to, isn't necessarily a bad thing.

But anyway, these are just my opinions, and it's been a long time since I was last responsible for maintaining a properly large network, so your experience is almost definitely going to be more useful and current than mine.

[siebenmann]

Because of our (work) situation, I was thinking of an OOB network with its own dedicated connections between sites, instead of the situation where you can plug each site into a 'management' Internet link with protection for your management traffic. However, once your management network gets into each site, the physical management network at that site needs to worry about redundancy if it's the only way to manage critical things there. You don't want to be locked out of a site's router or firewall or the like because a cheap switch on the management network had its power supply fail (and they're likely to be inexpensive because the management network is usually low usage and low port count).