Emerging standard to speed up Ethernet reconfigs

By Jim Duffy, Network World |  Networking

3Com's offering is called MultiPoint Link Aggregation; Cisco's are BackboneFast, UplinkFast and PortFast; and Foundry's is called FastSpan. These are all designed to ensure network resiliency by quickly reconfiguring the network when devices are added, or when ports, uplinks or switches are disabled.

The only problem: lack of interoperability. That's where Fast Spanning Tree comes in.

Fast Spanning Tree would allow users to, say, build their large Ethernet networks with Cisco switches at the core, and Cabletron or 3Com switches at the edge, and have millisecond reconfiguration if a link failed. The emerging standard is also intended to be backward-- compatible with Spanning Tree so topology calculations under Spanning Tree can be retained while reconfiguration is sped up.

"We covered the bases fairly well to make sure it's minimal disruption going forward," says John Roese, chief technology officer at Cabletron and a participant in the work on 802.1w.

Sun's Perlman acknowledged only a basic understanding of Fast Spanning Tree; she's not involved in the actual work on it. "It's not a large change to the [Spanning Tree] algorithm," she says. "If it really does work, then it certainly makes it a lot nicer."

The work on 802.1w is not without its critics, however. Nortel officials say Fast Span-ning Tree is merely a "band-aid" until the standard for link aggregation -- 802.3ad -- is firmed up. The link aggregation standard will quicken Ethernet reconfiguration even more than Fast Spanning Tree by allowing switches to use all available paths on aggregated links, and to balance traffic loads across those links, Nortel officials say.

"We've got this 15-year-old protocol that we're all trying to band-aid to make it work in an environment that it was never really designed for," says Lindsay Newell, a senior product marketing manager at Nortel. "The limitation with Fast Spanning Tree is that you can only ever use half the available bandwidth [on a dual-homed switch]. With link aggregation, you can use both paths at the same time."

The 802.3ad standard is expected to be finalized in the first quarter of 2000.

Some users say they don't have a pressing need for Fast Spanning Tree. "We haven't run into too many problems where we have loops or when new devices come up," says Kurtis Lindemann, network specialist at Ohio State University's Fisher College of Business in Columbus. "It's not a major issue for us right now. It's not something I'm anxious about."

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