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STP Spanning Tree Protocol step by step

in blog, Networking Notes on July 5, 2014

STP is enabled by default for all VLANs in a switch. To change the priority to a lower value for root switch elections, use one of the following commands:

Switch(config)#spanning-treevlan 1 priority 4096


Switch(config)#spanning-treevlan 1 root

STP Topology Changes and Enhancements

In the event of a topology change, formerly blocked ports might transition to a forwarding state. It might take up to 50 seconds to transition from a blocking state to a forwarding state.

An exception to these 50 seconds is if the following Cisco enhancements are in place to speed up convergence:

PortFast skips the listening and learning states on end-devices such as servers, PCs, and printers. PortFast can cause switching loops if a hub or switch is connected. BPDU Guard adds protection by disabling a port if the interface receives a BPDU.

UplinkFast skips the listening and learning transitions when a direct failure occurs on its root port on a switch with redundant uplinks to a distribution switch.

BackboneFast speeds up convergence by skipping the max age time when switches learn of a failure indirectly.




EtherChannel is a Cisco method of bundling redundant links between switches to act as a sin­gle aggregated link. This allows utilization of all the link’s bandwidth, because STP treats the link as a single interface (no blocking/discarding ports). In the case of a link failure, EtherChannel automatically distributes the traffic load over the remaining links in milliseconds.

To add an interface to an EtherChannel bundle (up to eight), use the channel-group chan­nel# mode on command in the interface configuration.

 Rapid Spanning Tree Protocol

Incorporates several of Cisco’s STP enhancements and ensures a safe and quick transition to a forwarding state and topology convergence by removing the overdepen­dence on STP timers.

RSTP Port States
State Function STP Equivalent
Disabled The interface is administratively shut down or disabledfrom port violation. Disabled
Discarding Does not forward any user data. All ports start out in this state.Does not send, but still can receive BPDUs to react totopology changes. Blocking and listening
Learning Begins to build MAC addresses learned on the interface.No user data is sent. Learning
Forwarding User data is forwarded. Forwarding


RSTP Link Types

State                                                            Function

Link type point-to-point                     Full-duplex links between switches

Link type shared                                     Half-duplex links between switches or hubs

Edge type                                                    Connections to end devices such as PCs, printers, and servers


RSTP Convergence

Edge ports immediately transition to a forwarding state when connected to RSTP ports. For point-to-point link types, transitioning to a forwarding state involves a synchronization process:

  1. After switches are connected to a point-to-point link, they exchange BPDUs.
  2. If a switch determines its port will become a designated port, it sends a proposal to start forwarding to its neighbor.
  3. The neighboring switch receives the proposal. If its port is a root port, it synchronizes the change by putting all nonedge ports into a discarding state and sending an agree­ment back to the original switch. If its port is a discarding port, it does not respond to the proposal.
  4. The original switch immediately transitions to a forwarding state if it receives an agreement or eventually transitions to a forwarding state after a forward delay occurs.

RSTP uses BPDUs as keepalives to detect if a neighboring switch goes down. When the topol­ogy change is detected, RSTP immediately starts aging out the affected MAC address and tells its neighbors to do the same.

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