M-LAG, meanwhile, only supports limited architectures, Eastlake says. Also, M-LAG might have very limited multi-pathing capability and may still run Spanning Tree to be sure there are no problems in the case of errors, Eastlake believes.
"Proprietary alternatives for Spanning Tree have been around for years, which I take as a further proof that the quarter-century-old Spanning Tree protocol leaves something lacking for some modern applications," Eastlake says. "Such alternatives, besides being proprietary, typically only work for restricted topologies. TRILL works for general LAN topologies."
SPB works too, for Avaya. The vendor chose SPB for its data center switches due to its simplicity and Layer 2 focus.
"It's part of our carrier heritage to enable flattening (of the network)," says Steve Bandrowczak, vice president and general manager of Avaya Data Solutions. "It features ease of implementation, extension into storage ... It's the right path."
Avaya also believes SPB -- IEEE 802.1aq -- will have broader industry support. But Eastlake says it only works in an area consisting of all contiguous Shortest Path Bridges, and it is limited to point-to-point links.
And it is not yet full baked within the IEEE, Eastlake claims. Even though 802.1aq also uses the IS-IS routing protocol, it is to configure bridging mechanisms employed to forward frames. As such, it does not route, Eastlake says, so its multipath scalability is limited.
"That's why its support for Equal Cost MultiPath (ECMP) has been so limited and why its per-switch routing calculation overhead is exponentially higher than TRILL, making it less scalable than TRILL," he says.
Eastlake says IEEE 802.1 is looking to initiate a new project to try to add routing to SPB and overcome these "problems."
TRILL users pay the price for scale in terms of control, says Peter Ashwood-Smith, a contributing author on SPB.
"One mode of forwarding is like a shotgun blast of packets while the other mode of forwarding is like a rifle shot," he says. "TRILL does a shotgun blast while 802.1aq does a rifle shot. What we find talking to customers is that some want to spray packets and others want more control, and most want both depending on the type of traffic."
SPB also offers more predictability that TRILL, Ashwood-Smith says. Its rifle-type forwarding allows users to plan what is going to happen with offline tools.
"That's more difficult, and possibly impossible, with the shotgun type of forwarding," Ashwood-Smith says.
Also, TRILL might require more extensive equipment upgrades than SPB, he says. To support the shotgun-type forwarding, every card on every hop has to be upgraded, while an SPB implementation will only require upgrades to the ingress/egress cards, and will leave 40/100G Ethernet cards "untouched," Ashwood-Smith says.
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