Spanning tree gets a boost

A lot has changed with Ethernet since it was invented more than 25 years ago.

From 10Mbps shared media repeaters and bridges back then, we now have switched 100Mbps and gigabit Ethernet.

But one aspect of the technology has not changed: the Spanning Tree Algorithm for preventing loops and recovering bridged LANs when a network topology changes.

That, however, also will soon change. The IEEE is working on an enhancement to Spanning Tree that aims to reduce network recovery time from tens of seconds to less than 10 seconds.

The work is known as “Rapid Reconfiguration,” or 802.1w-or just Fast Spanning Tree. Fast Spanning Tree is intended to alleviate data loss and session timeouts when large, switched Ethernet LANs recover following a topology change or a device failure.

Currently, the Spanning Tree Algorithm reconfigures bridged Ethernet networks in 30 to 60 seconds after a failure or change in link status. That amount of time was picked to enable all devices to relearn the net topology and avoid loops-frames that were retransmitted over and over again because bridges could not figure out their path or destination.

“I put in a timer so that before you’re allowed to turn on a link, you had to wait enough time to make sure everyone would have a chance to learn a new topology,” says Radia Perlman, distinguished engineer at Sun Microsystems Inc. and inventor of the Spanning Tree Algorithm. “It was an attempt to be very conservative and to presume that loops are worse than temporarily not being able to get from one place to another.”

Thirty to 60 seconds was an acceptable amount of time 15 to 20 years ago, when bridged Ethernet LANs were relatively small compared to today, and ran at 10Mbps over shared media. But in today’s large switched Ethernet networks-which include fast and gigabit Ethernet, and support multimedia applications-that amount of recovery time can drop sessions and lose data.

“With the Spanning Tree we have today, you’ve got to relearn the topology, calculate your cost to the root bridge, flush your databases and your learning tables, and then you have to listen and learn the network again,” says Jim McCarron, product technology manager at 3Com. “The times associated with doing that [are going to cause] some protocols to time out.”

If delay-sensitive traffic such as voice and video are added to the data network, the recovery time becomes even more of a problem.

Fast Spanning Tree is designed to reconfigure the network in less than 10 seconds. But the standard won’t be finalized for another year. In the meantime, vendors such as 3Com, Cisco and Foundry Networks are offering proprietary fixes to speed up Ethernet reconfiguration.

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 Spanning 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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