Dell PowerConnect J-8216 Software Guide for JUNOS version 10.3 - Page 1348

Dell PowerConnect J-Series Ethernet Switch Complete Software Guide for Junos OS • Configuration BPDUs: Contain configuration information about the transmitting switch and its ports, including switch and port MAC addresses, switch priority, port priority, and port cost. • Topology Change Notification (TCN) BPDUs: When a bridge needs to signal a topology change, it starts to send TCNs on its root port. The designated bridge receives the TCN, acknowledges it, and generates another one for its own root port. The process continues until the TCN reaches the root bridge. STP uses the information provided by the BPDUs to elect a root bridge, identify root ports for each switch, identify designated ports for each physical LAN segment, and prune specific redundant links to create a loop-free tree topology. All leaf devices calculate the best path to the root device and place their ports in blocking or forwarding states based on the best path to the root. The resulting tree topology provides a single active Layer 2 data path between any two end stations. Related • Understanding MSTP for J-EX Series Switches on page 1277 Documentation • Understanding RSTP for J-EX Series Switches on page 1276 • Understanding VSTP for J-EX Series Switches on page 1281 • Understanding Layer 2 Protocol Tunneling on J-EX Series Switches on page 1056 Understanding RSTP for J-EX Series Switches J-EX Series Switches use Rapid Spanning Tree Protocol (RSTP) to provide better reconvergence time than that provided by the base Spanning Tree Protocol (STP). RSTP identifies certain links as point to point. When a point-to-point link fails, the alternate link can transition to the forwarding state. Although STP provides basic loop prevention functionality, it does not provide fast network convergence when there are topology changes. STP's process to determine network state transitions is slower than RSTP's because it is timer-based. A device must reinitialize every time a topology change occurs. The device must start in the listening state and transition to the learning state and eventually to a forwarding or blocking state. When default values are used for the maximum age (20 seconds) and forward delay (15 seconds), it takes 50 seconds for the device to converge. RSTP converges faster because it uses a handshake mechanism based on point-to-point links instead of the timer-based process used by STP. For networks with VLANs, you can use VLAN Spanning Tree Protocol (VSTP) to run one or more STP or RSTP instances for each VLAN on which VSTP is enabled. VSTP takes the paths of each VLAN into account when calculating routes.. VSTP uses RSTP instances by default. An RSTP domain running on a switch has the following components: • A root port, which is the "best path" to the root device. • A designated port, which indicates that the switch is the designated bridge for the other switch connecting to this port. 1276