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. Phil -----Original Message----- From: "Glen Kent" <glen.kent@gmail.com> Sent: 11/19/2014 2:04 AM To: "nanog@nanog.org" <nanog@nanog.org> Subject: Overlay as a link Hi, 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? Glen