BROCADE IP PRIMER

Chapter 9: Spanning Tree ProtocolThe two links are still part of STP. One will be in Forwarding. One will be inBlocking. The difference is that when the Forwarding link fails, the core switchwill converge the second link to Forwarding in four seconds (two seconds forListening; two seconds for Learning). It works very similar to Fast Port Span.This tool is to be used cautiously. With Fast Port Span, if you plug anotherswitch or a hub into the port (with Fast Port Span), the switch will detect it, andit will be automatically disabled. With Fast Uplink Span, the switches involvedare presuming that you know what you're doing. Make certain you are onlyusing this command for an edge switch. Anything else would cause the samepotential for network instability as altering the timers.IEEE 802.1w: When IEEE 802.1dJust Isn't Fast EnoughIEEE 802.1w is also referred to as Rapid Spanning Tree Protocol (RSTP). It isan enhancement to 802.1d, largely dealing with convergence time. Where802.1d converges in at least 30 seconds, 802.1w may converge in literallyless than half a second.RSTP vs. STPRSTP is actually very similar to STP. It is, after all, an enhancement to STP.There is still a root switch. The main thing that makes it different is that it doesnot depend on timers to establish loops. Each port learns its state through initialhandshakes. Little time is wasted when roles need to change.The port states are a little different in RSTP:1. Discarding. This is state is analogous to STP's Blocking. It forwards notraffic, but it still receives BPDUs.2. Learning. This is corresponds to the Learning state in STP. The port isbuilding its MAC table, but not forwarding traffic yet.3. Forwarding. This corresponds to the Forwarding state in STP. The port isactive. It is now forwarding traffic.Also, like STP, a port may be in a Disabled state. This would indicate that theport itself is disabled (physically down or administratively disabled), or hasspanning-tree disabled.After initial handshakes, each participating interface will find itself in one ofthese roles:• Root. This port has been deemed the best path to the root switch.• Designated. All ports start in this role. On the root switch, all ports stay inthis role. It simply means that, of all the BPDUs you've received on thisport, yours are the best (highest priority, etc.).• Alternative. This is a non-root port. It will be in Discarding state, unlessthe switch's root port goes down.204 Brocade IP Primer