Peter John Hill made an interesting observation in a comment to my “The TRILLing brain split” post; 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.
The split personality Cisco has exposed at Cisco Live 2010 is amazing: on one hand you have the Data Center team touting the benefits of Routing at Layer 2 (an oxymoron if I’ve ever seen one), on the other hand you have Russ White extolling the virtues of good layer-3 design in the CCDE training (the quote I like most: “It all meets at Layer 3 ... that’s why CCDE is layer-3 centric”). If you’re confused, you’re not the only one
Read more ... (this time @ etherealmind.com)
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.