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Varieties of Spanning Tree Protocols

STP has been improved multiple times since its introduction in the original IEEE 802.1D specification. A network administrator should know which type to implement based on the equipment and topology needs.

Comparing the STP Varieties

Identify each of the STP varieties described in the following list:

  • ___________________: This is an IEEE that maps multiple VLANs into the same spanning tree instance.
  • ___________________: This is an evolution of STP that provides faster convergence than STP.
  • ___________________: This is an updated version of the STP standard, incorporating IEEE 802.1w.
  • ___________________: This is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network.
  • ___________________: This is a Cisco enhancement that provides a separate instance of 802.1w per VLAN.
  • ___________________: This is the original IEEE 802.1D version (802.1D-1998 and earlier) that provides a loop-free topology in a network with redundant links.

Complete the cells in Table 2-2 to identify each the characteristics of each STP variety.

Table 2-2 STP Characteristics - Exercise 1

Protocol

Standard

Resources Needed

Convergence

Tree Calculation

STP

Low

Cisco

802.1w

Rapid PVST+

802.1s, Cisco

Medium or high

In Table 2-3, indicate which varieties of STP are best described by the characteristic. Some characteristics apply to more than one STP variety.

Table 2-3 STP Characteristics - Exercise 2

Characteristic

STP

PVST+

RSTP

Rapid PVST+

MSTP

MST

A Cisco implementation of 802.1s that provides up to 16 instances of RSTP.

Cisco enhancement of RSTP.

The default STP mode for Cisco Catalyst switches.

Has the highest CPU and memory requirements.

Can lead to suboptimal traffic flows.

Cisco proprietary versions of STP.

Cisco enhancement of STP. Provides a separate 802.1D spanning-tree instance for each VLAN.

There is only 1 root bridge and 1 tree.

Uses 1 IEEE 802.1D spanning-tree instance for the entire bridged network, regardless of the number of VLANs.

Supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.

An evolution of STP that provides faster STP convergence.

Maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance.

First version of STP to address convergence issues, but still provided only one STP instance.

PVST+ Operation

After a switch boots, the spanning tree is immediately determined as ports transition through five possible states and three BPDU timers on the way to convergence. Briefly describe each state:

  • Blocking:

     

     

  • Listening:

     

     

  • Learning:

     

     

  • Forwarding:

     

     

  • Disabled:

     

     

Once stable, every active port in the switched network is either in the _______________ state or the _______________ state.

List and briefly describe the four steps PVST+ performs for each VLAN to provide a loop-free logical topology.

In Table 2-4, answer the “Operation Allowed” question with “yes” or “no” for each port state.

Table 2-4 Operations Allowed at Each Port State

Operation Allowed

Port State

Blocking

Listening

Learning

Forwarding

Disabled

Can receive and process BPDUs

Can forward data frames received on interface

Can forward data frames switched from another interface

Can learn MAC addresses

Rapid PVST+ Operation

RSTP (IEEE _______________) is an evolution of the original _______________standard and is incorporated into the IEEE _______________-2004 standard. Rapid PVST+ is the Cisco implementation of RSTP on a per-VLAN basis. What is the primary difference between Rapid PVST+ and RSTP?

 

Briefly describe the RSTP concept that corresponds to the PVST+ PortFast feature.

 

What command implements Cisco’s version of an edge port?

 

 

In Table 2-5, indicate whether the characteristic describes PVST+, Rapid PVST+, or both.

Table 2-5 Comparing PVST+ and Rapid PVST+

Characteristic

PVST+

Rapid PVST+

Both

Cisco proprietary protocol.

Port roles: root, designated, alternate, edge, backup.

CPU processing and trunk bandwidth usage is greater than with STP.

Ports can transition to forwarding state without relying on a timer.

The root bridge is determined by the lowest BID + VLAN ID + MAC.

Runs a separate IEEE 802.1D STP instance for each VLAN.

Possible to have load sharing with some VLANS forwarding on each trunk.

Sends a BPDU “hello message” every 2 seconds.

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