© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Implementing Spanning Tree Describing the STP

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Transparent Bridging A switch has the same characteristics as a transparent bridge.

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Bridge loops can occur any time there is a redundant path or loop in the bridge network. What Is a Bridge Loop?

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Bridge loops can be prevented by disabling the redundant path. Preventing Bridge Loops

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v D STP Configured root switch Redundant switch links Optimal path selection

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Bridge Protocol Data Unit BPDUs provide for the exchange of information between switches.

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v The STP Root Bridge Reference point One root per VLAN Maintains topology Propagates timers

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Root Bridge Selection Criteria

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Extended System ID in Bridge ID Field Bridge ID Without the Extended System ID Bridge ID with the Extended System ID

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v D 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 = Priority Values (Hex) Priority Values (Dec) (default) F bits12 bits PriorityVLAN Number

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Configuring the Root Bridge Switch(config)#spanning-tree vlan 1 root primary This command forces this switch to be the root. Switch(config)#spanning-tree vlan 1 root secondary This command configures this switch to be the secondary root. Or Switch(config)#spanning-tree vlan 1 priority priority This command statically configures the priority (in increments of 4096).

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Root Bridge Selection Which switch has the lowest bridge ID?

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Spanning Tree Operation One root bridge per network One root port per nonroot bridge One designated port per segment Nondesignated ports are blocking

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Spanning Tree Port States Spanning tree transitions each port through several different states.

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Local Switch Root Port Election

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Spanning Tree Path Cost Link SpeedCost (Revised IEEE Spec)Cost (Previous IEEE Spec) 10 Gbps21 1 Gbps Mbps Mbps100

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v SW X is the root bridge. SW Y needs to elect a root port. Which port is the root port on SW Y? Fast Ethernet total cost = Ethernet total cost = Spanning Tree Protocol Root Port Selection

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v STP Designated Port Selection Switch X is the root bridge. All ports on the root bridge are designated ports because they have a path cost of 0. Because the Ethernet segment has a path cost of 100, switch Y will block on that port. Do all segments have a designated port?

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Example: Layer 2 Topology Negotiation

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v 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)

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Describing PortFast

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v 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

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v 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) IEEE 802.1t- Local and Metropolitan Area Network: Common Specifications

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v Summary Transparent bridges require no client configuration. A bridge loop may occur when there are redundant paths between switches. A loop free network eliminates redundant paths between switches. The 802.1D protocol establishes a loop-free network. The root bridge is a reference point for STP. Each STP port will host a specific port role. Enhancements now enable STP to converge more quickly and run more efficiently.

© 2006 Cisco Systems, Inc. All rights reserved.BCMSN v