BROCADE IP PRIMER

IEEE 802.1d (Spanning Tree Protocol)The link between Switch 5 and Switch 7 is still blocked (as it doesn't reflect theshortest path to the root switch for Switch 5 or 7), but the frame takes a muchshorter path. It leaves Switch 7, goes to Switch 6, goes to Switch 4 (the rootswitch), arriving at Switch 5, and finally, to the host itself. Dealing with sevenswitches, I'm sure you see that we wouldn't be able to maintain the redundancyand provide the shortest possible path for everyone in the broadcastdomain. But you need to understand that there are times when leaving theelection untampered will yield some undesirable results. You can shape thiselection in a way that best suits your infrastructure.ConvergenceConvergence is the process any switch interface goes through when participatingin the Spanning Tree Protocol. Any interface participating in STP will gothrough these port states:1. Blocking. Layers 1 and 2 are still active in this state. It's just not passingany normal traffic. The interface in Blocking state can still send andreceive BPDUs, but it will neither send nor receive any other traffic. As faras the switch infrastructure is concerned, that port is dead (unusable). Aninterface starts life in this state as part of a “guilty until proven innocent”method used by STP.2. Listening. This state is also not allowing any traffic other than BPDUs.This state begins by the switch sending out a series of BPDUs and listeningto see if they arrive back on another interface. If they do, we have aswitching loop, and it will be noted for the next port state. By default, aninterface will remain in Listening state for 15 seconds.3. Learning. This state also passes no traffic other than BPDUs. It takes thedata we've learned from the Listening state, and it now makes its decisionon which ports should be designated and which ports should not. Thedesignated ports are the ports which have the shortest path to the rootswitch. MAC addresses of the switches in the infrastructure are added tothe switch's MAC table. The switch learns how its port fits into the SpanningTree architecture. By default, an interface will remain in Learningstate for 15 seconds, as well.4. Forwarding/Blocking. Some books will tell you that Forwarding is the laststate. In reality, it depends on what the results of the two previous statesare. If the interface in question has been deemed a designated port (onewith the shortest path to the root switch), it will go from Learning to Forwarding.At this point, it will forward and receive all traffic. If the interfacein question is deemed not a designated port, it will go into Blocking (forwardingno traffic other than BPDUs). It will remain in blocking until achange in topology happens, and a switch may need to decide againwhich of its ports are designated and which are not.With this information in mind, you may have come to the conclusion that ittakes a surprisingly long time for a switch port to be added to the infrastructure,using Spanning Tree. You'd be right. It takes a little over 30 seconds for aBrocade IP Primer 199