BROCADE IP PRIMER

49FLinkPower49FLinkPower50F50FConsole1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Console1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Console49F Link 50F1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 47Power1F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 4849FLinkPower50FConsole1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 4849FLinkPower50FConsole1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 4849FLinkPower49FLinkPower50F50FConsole1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Console1 3 5 7 9 1113 15 17 19 21 2325 27 29 31 33 3537 39 41 43 45 471F 2F 3F 4F2 4 6 8 10 1214 16 18 20 22 2426 28 30 32 34 3638 40 42 44 46 48Chapter 9: Spanning Tree ProtocolThe Root SwitchPurists will tell you that the correct term here is root bridge. I would like to submitthat the term root switch be considered. Bridges are very rarely used thesedays, while switches are extremely popular. You will notice that the term “rootbridge” and “root switch” are both found in this book. They are synonymous.What is a root switch? When the STP process begins, the first item of businessis to decide who is right. We have several different paths through the broadcastdomain (which, unchecked, would be considered Layer 2 loops). We cantemporarily shut down the redundant paths, but how do we decide whichpaths to keep up and which to shut down? To start, we elect one switch thatwill always be right. This is the root switch.The root switch's links will never be shut down. The root switch is king. Whenevera decision is to be made, the root switch's way is the right way.How do we decide who is the root switch? BPDUs are sent out. Contained inthe BPDUs is the switch's Bridge ID. The one with the lowest Bridge IDbecomes the root switch. The Bridge ID is a combination of the switch's priority,followed by the MAC address of the switch. The priority is a 16-bit number(0 to 65,535). By default, all Brocade switches have a priority of 32,768. If weleave it to the defaults, all of the switches will have the same bridge priority(32,768), so the election will be determined by who has the lowest MACaddress.After the root switch is elected, each switch calculates the shortest path to theroot switch. This path is what will remain active. The switch port involved inthat path is referred to as the designated port. All other paths will be temporarilydisabled. On the root switch itself, all ports are listed as designated. Afterall, the root switch is king. It's always right.Remember our seven-switch Layer 2 loop example earlier in the chapter? Let'stake a look at that again:Switch 1Switch 2Switch 3Switch 4Switch 5Switch 6Switch 7196 Brocade IP Primer