BROCADE IP PRIMER

Console49F 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 48IEEE 802.1d (Spanning Tree Protocol)STPXSTPNotice that one of the links is crossed out. This link is still alive. There's still anestablished Layer 1 link. The difference is that the switches know not to usethat link, unless the other link goes down. This means that we won't have anyLayer 2 loops, and we've still got redundant links between the switches. That'sa win-win.IEEE 802.1d (Spanning Tree Protocol)In review, the Spanning Tree Protocol is designed to prevent Layer 2 loops. Itdoes this by discovering where the loops are, and then making a decision as towhich single path will be used (and conversely, which paths will not be used).To discover loops (and to communicate in general with the rest of the switchesin the broadcast domain), it uses a special Layer 2 frame.Bridge Protocol Data Unit (BPDU)BPDUs are special Layer 2 frames that switches, configured with STP, can useto communicate with other switches in the broadcast domain. There are threetypes of BPDUs:• Configuration BPDU. These frames are used to elect a root switch, or toreceive messages from the root switch to control reconvergence after alink changes• Topology Change Notification (TCN) BPDU. These frames are used bynon-root switches to inform the root switch of a link change• Topology Change Notification Acknowledgment (TCA) BPDU. Theseframes are sent by the root switch to acknowledge receipt of a TCN BPDUfrom a non-root switchBPDUs use a multicast (not broadcast) MAC address to send their frames. Thisis how the communication flows just to switches. When BPDUs are sent, aswitch will use the unique MAC address of the port that is sending the messageas the source MAC address. It will use a special destination multicastMAC address-01:80:c2:00:00:00-as its destination. BPDUs are sent every twoseconds, by default.Brocade IP Primer 195