The Case for FCoE Terminology
August 25, 2011 9 Comments
A previous post of mine (Jinkies! It’s an FCoE Mystery) talked about the need for some additional terminology in the FCoE world, specifically three different types of FCoE deployments. It’s generated a lot of comments, some which seem even longer than the actual post. I wanted to do a follow up, specifically regarding my reasoning for having the topology definitions.
FCoE, as a term, is very broad: It means that you’re taking a Fibre Channel frame and encapsulating it into an Ethernet frame. That’s it. There’s only one “FCoE” method in terms of this encapsulation. However, my point is that there are a number of very different ways you can go about moving those FCoE frames onto your Ethernet network.
Take this scenario: You’re presented with a switch. It has a nice sticker on it that says “FCoE switch”. Now what does that tell you about how you can fit it in your network?
Attention Cisco: You’re welcome
If you said it was a data center bridge (DCB) switch, you would then know that it’s a transit switch. No FCoE frames will be encap’d/decaped on that switch, but it supports at least PFC (priority flow control) so that FCoE frames can be guaranteed to be lossless.
Now, if you were told the FCoE switch is a FCF switch (has a full Fibre Channel stack), what does that tell you about how you can deploy it?
Still, almost nothing.
Take the example of a Cisco 6X00 Fabric Interconnect, the brains behind Cisco’s UCS server system. They are FCoE devices, and they are Fibre Channel Forwarders (FCFs). However, you can’t do what I would consider multi-hop FCoE. You can connect to a native Fibre Channel fabric, but not an FCoE fabric. That is, you can’t set up an FCoE ISL (Inter-Switch Link, but not the old Cisco pre-802.1Q VLAN tagging, it means something different in Fibre Channel) to another FCoE capable switch. This is why I added a third method to Ivan Pepelnjak’s sparse-mode (SMFCoE) and dense-mode (DMFCoE) definitions. (Note: That’s an embarassing number of acronyms).
So by having those three different distinctions (dense-mode/FCF, sparse-mode/DCB, one-hop/zero-hop) you can then tell immediately how you can deploy a FCoE switch in your network. Some switches will likely support multiple ways, but most right now are limited to one in how they’re deployed on your network.
I understand the concerns that both J Metz from Cisco and Erik Smith from EMC about adding complexity, but I think having these three different topology definitions can go a long way to help simplify discussions on FCoE topology, and in fact removes a lot of complexity (and mystery).
This morning I attended a webinar held by the Ethernet Alliance (based near me in Beaverton, Oregon) and I was happy to hear they also make a distinction between FCF FCoE switches and non-FCF FCoE switches. It really helps simplify things in terms of deployment.