When youre doing overlay networking, i.e., you have tunnels from one
virtual machine in a DC to another in another DC, then can i consider a
tunnel between the two virtual machines as a "physical link" that exists in
a regular network?
I am wondering on what possibly can be the difference between a tunnel
being considered as a link and a true physical link.
I could run routing algorithms on both. The tunnel would only be considered
as an interface. Or i could run BFD on both.
Once difference that i can think of is that while you can send multiple
frames together on a tunnel (for example if there are ECMP paths within the
tunnel), you may not be able to send multiple frames at the same time on a
physical link. Anything else?
There are certain protocols and mechanisms tied to a physical medium or MAC layer. If you are doing L3 tunneling you lose those options, if you are doing L2 tunneling you may lose less of them depending how transparent the tunnel is.
Things like Ethernet pause frames or 802.3ah instead of BFD. So from a certain layer like L3+ it looks and behaves like a physical link but there are differences.
Perhaps I'm stating the obvious or something that's out of scope for what
you have in mind.
However, one property where they differ is that a physical link has a
physical capacity for carrying traffic (i.e., bandwidth).
A virtual interface inherits it's capacity from the physical link(s) that
underlie it. Some may be locally attached but most will be indirect.
Hence it's not possible to know how near it is to its limits.
Again, maybe outside what you're thinking about, but you asked "what can
possibly be the difference".