Category: TRILL

The emerging Ethernet bridging technologies have been hyped to an extent where the lines between them completely blurred, resulting in statements like “we need DCB and TRILL for FCoE”. Actually, none of that is true, but let’s focus on DCB and TRILL first.

Someone had a “borderless data center mobility” dream a few years ago and managed to infect a few other people, resulting in a networking industry pandemic that is usually exhibited by the following “facts”:

• Unhindered Virtual Machine mobility across the globe is the absolute prerequisite for any business agility. Wrong. There are other field-proven solutions and although inter-site VM mobility has been demonstrated, it’s still a half-baked idea and has many caveats.
• You can only reach that Holy Grail by extending your layer-2 domains across vast distances. Totally wrong. It would be easier to fix L3 routing and signaling protocols than to invent completely new technologies trying to fix L2 problems. Users of Microsoft NLB are might disagree ... in which case I wish them luck in scaling their architecture.
• Large-scale bridging is absolutely mandatory if you want to build cloud solutions with tens of thousands of servers. Not sure about that. Google is there, Facebook, Twitter and Amazon are (at least) close, large web hosting providers have been around for years ... and yet they somehow managed to survive with existing technologies and good network designs.

Just today XKCD published a very relevant comic, so I can skip my usually sarcastic comments and focus on the plethora of emerging large-scale bridging standards and implementations. Let’s walk through them:

A comment by Brad Hedlund has sent me studying the differences between TRILL and 802.1aq and one of the first articles I’ve stumbled upon was a nice overview which claimed that the protocols are very similar (as they both use IS-IS to select shortest path across the network). After studying whatever sparse information there is on 802.1aq and the obligatory headache, I’ve figured out that the two proposals have completely different forwarding paradigms. To claim they’re similar is the same as saying DECnet phase V and MPLS Traffic Engineering are similar because they both use IS-IS.

Based on the readers’ comments on my “Bridging and Routing: is there a difference?” post (thanks you!), here are a few more differences between bridging and routing:

Cost. Layer-2 switches are almost always cheaper than layer-3 (usually combined layer-2/3) switches. There are numerous reasons for the cost difference, including:

Peter John Hill made an interesting observation in a comment to one of my blog posts; he wrote “TRILL really is routing at layer 2.”

He’s partially right – TRILL uses a routing protocol (IS-IS) and the TRILL protocol used to forward Ethernet frames (TRILL data frames) definitely has all the attributes of a layer-3 protocol:

• TRILL data frames have layer-3 addresses (RBridge nickname);
• They have a hop count;
• Layer-2 next-hop is always the MAC address of the next-hop RBridge;
• As the TRILL data frames are propagated between RBridges, the outer MAC header changes.

In his comment to one of my TRILL posts, Petr Lapukhov has asked the fundamental question: “how is bridging different from routing?”. It’s impossible to give a concise answer (let alone something as succinct as 42) as the various kludges and workarounds (including bridges and their IBM variants) have totally muddied the waters. However, let’s be pragmatic and compare Ethernet bridging with IP (or CLNS) routing. Throughout this article, bridging refers to transparent bridging as defined by the IEEE 802.1 series of standards.

Design scope. IP was designed to support global packet switching network infrastructure. Ethernet bridging was designed to emulate a single shared cable. Various design decisions made in IP or Ethernet bridging were always skewed by these perspectives: scalability versus transparency.

Tired of losing half of your bandwidth to spanning tree? TRILL will solve all your problems, bring the world peace and make better coffee than Starbucks (hint: the second claim is fake and the third one is not so hard to achieve).

Undoubtedly TRILL is an interesting technology that can alleviate the spanning tree limitations. Unfortunately I’ve seen a very similar technology being heavily misused in the past (resulting in some fantastic failures) and remain skeptical about the deployment of TRILL. My worst case scenario: TRILL will make it too simple to deploy plug-and-pray bridged (vendors will call them “switched”) networks with no underlying design that will grow beyond control and implode.

Greg Ferro has kindly invited me to be a guest author on his excellent blog Etherealmind.com and I simply had to spill my thoughts on TRILL in the TRILL: It’s a DéJà-Vu All Over Again article after they’ve been discussing it during one of the Packet Pushers podcast.