CCNA3-4冗余拓扑、STP.ppt

Medium-Sized Switched Network Construction Improving Performance with Spanning Tree Interconnection Technologies Determining Equipment and Cabling Needs Advantages of EtherChannel Redundant Topology Broadcast Frames Broadcast Storms Multiple Frame Copies MAC Database Instability Loop Resolution with STP Spanning-Tree Operation STP Root Bridge Selection Spanning-Tree Port States Describing PortFast Configuring and Verifying PortFast Spanning-Tree Operation Example Spanning-Tree Path Cost Spanning-Tree Recalculation Per VLAN Spanning Tree Plus PVST+ Extended Bridge ID Rapid Spanning Tree Protocol Default Spanning-Tree Configuration Cisco Catalyst switches support three types of STPs: PVST+ PVRST+ MSTP The default STP for Cisco Catalyst switches is PVST+ : A separate STP instance for each VLAN One root bridge for all VLANs No load sharing PVRST+ Configuration Guidelines Enable PVRST+. Designate and configure a switch to be the root bridge. Designate and configure a switch to be the secondary root bridge. Verify the configuration. PVRST+ Implementation Commands Verifying PVRST+ Configuring the Root and Secondary Bridges Configuring the Root and Secondary Bridges: SwitchA Configuring the Root and Secondary Bridges: SwitchB Summary A redundant switched topology includes multihomed switches and EtherChannel. A redundant switched topology causes looping issues such as broadcast storms. The 802.1D STP establishes a loop-free network. The original STP has been enhanced by PVST+ and RSTP. Emphasize: Layer 2 has no mechanism (like a TTL) to stop loops. Layer 3 of 3 Emphasize: Layer 2 has no TTL mechanism to stop looping frames. Layer 2 of 2 Layer 2 of 2 Emphasize: The three general rules when dealing with STP are as follows: 1. One root bridge per network. The root is the bridge with the lowest bridge ID. All the ports on the root bridge are designated ports (forwarding). 2. For every non-root bridge, there is a root port (forwarding). The root port is the port with