3Com 7600 manual Designated Bridge on a LAN Spanning Tree port selection

11-10CHAPTER 11: ETHERNET LANS AND BRIDGES IN THE 7600 CARD

Designated Bridge on a LAN

Spanning Tree port selection

Among all bridges on a LAN, the bridge with the shortest path to the root bridge (through its root port) is selected as the designated-bridgeon that LAN.

Each bridge selects its root port and any of its ports (LANs) on which it is the designated bridge to be in the spanning tree. All other ports are blocked to data traffic (but not to CBPDUs). Thus, traffic on a LAN flows only through its designated bridge.

Re-configuring the When a change occurs in the network, such as a new bridge or link or a Spanning Tree device failure, the STP acts to establish a new Spanning Tree

configuration. Until the new Spanning Tree configuration stabilizes, it is necessary to restrict data packet transmissions in order to avoid temporary loops. For this purpose the 801.2 standard calls for two intermediate bridge port states: the listening state followed by the learning state. In the listening state is a wait state in which a port neither learns station addresses nor forwards data packets. In the learning state a port learns station addresses but still does not forward data packets. The management parameter forward-delaycontrols the amount of time that a bridge port remains in the listening or the learning state.

Port States Summing up, a bridge port can be in one of the following port states:

QListening - When the Spanning Tree begins re-configuring after a network change, all ports are placed in the listening state. Data packets received on a port are discarded. After the forward-delay time, the bridge port enters the learning state, unless set to the blocking state by the STP.

QLearning - Data packets are received for the purpose of learning station addresses only. After the forward-delay time, the bridge port enters the forwarding state, unless set to the blocking state by the STP.

QBlocking - The bridge continues to send and receive CBPDUs on that port but does not receive data packets.

QForwarding - The bridge performs standard bridging functions.

Disabling the STP The STP can be disabled and enabled by management either for the entire bridge or for specific ports. When disabled, the port or bridge does not participate in the Spanning Tree algorithm but continues to receive and transmit data packets.