Describing the STP参考.ppt

Implementing Spanning Tree Describing the STP Transparent Bridging What Is a Bridge Loop? Preventing Bridge Loops 802.1D STP Configured root switch Redundant switch links Optimal path selection Bridge Protocol Data Unit The STP Root Bridge Reference point One root per VLAN Maintains topology Propagates timers Root Bridge Selection Criteria Extended System ID in Bridge ID Field 802.1D 16-bit Bridge Priority Field Using the Extended System ID Only four high-order bits of the 16-bit Bridge Priority field carry actual priority. Therefore, priority can be incremented only in steps of 4096, onto which will be added the VLAN number. Example: For VLAN 11: If the priority is left at default, the 16-bit Priority field will hold 32768 + 11 = 32779. Configuring the Root Bridge Root Bridge Selection Spanning Tree Operation One root bridge per network One root port per nonroot bridge One designated port per segment Nondesignated ports are blocking Spanning Tree Port States Local Switch Root Port Election Spanning Tree Path Cost Spanning Tree Protocol Root Port Selection STP Designated Port Selection Example: Layer 2 Topology Negotiation Enhancements to STP PortFast Per VLAN Spanning Tree+ (PVST+) Rapid Spanning Tree Protocol (RSTP) Multiple Spanning Tree Protocol (MSTP) MSTP is also known as Multi-Instance Spanning Tree Protocol (MISTP) on Cisco Catalyst 6500 switches and above Per VLAN Rapid Spanning Tree (PVRST) Describing PortFast Configuring PortFast Configuring spanning-tree portfast (interface command) or spanning-tree portfast default (global command) enables PortFast on all nontrunking ports Verifying show running-config interface fastethernet 1/1 IEEE Documents IEEE 802.1D - Media Access Control (MAC) bridges IEEE 802.1Q - Virtual Bridged Local Area Networks IEEE 802.1w - Rapid Reconfiguration (Supp. to 802.1D) IEEE 802.1s - Multiple Spanning Tree (Supp. to 802.1Q) Summary Transparent bridges require no client configuration. A bridge loop may occur when