December 20, 2000, 2:29 PM — FRAMINGHAM, MASS. -- A lot has changed with Ethernet since it was invented over 25 years ago.
From 10M bit/sec shared media repeaters and bridges back then, we now have switched 100M bit/sec 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 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 10M bit/sec 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.
