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 |
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Cisco |
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802.1w |
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Rapid PVST+ |
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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. |
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Cisco enhancement of RSTP. |
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The default STP mode for Cisco Catalyst switches. |
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Has the highest CPU and memory requirements. |
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Can lead to suboptimal traffic flows. |
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Cisco proprietary versions of STP. |
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Cisco enhancement of STP. Provides a separate 802.1D spanning-tree instance for each VLAN. |
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There is only 1 root bridge and 1 tree. |
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Uses 1 IEEE 802.1D spanning-tree instance for the entire bridged network, regardless of the number of VLANs. |
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Supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard. |
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An evolution of STP that provides faster STP convergence. |
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Maps multiple VLANs that have the same traffic flow requirements into the same spanning-tree instance. |
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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 |
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Blocking |
Listening |
Learning |
Forwarding |
Disabled |
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Can receive and process BPDUs |
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Can forward data frames received on interface |
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Can forward data frames switched from another interface |
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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. |
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Port roles: root, designated, alternate, edge, backup. |
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CPU processing and trunk bandwidth usage is greater than with STP. |
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Ports can transition to forwarding state without relying on a timer. |
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The root bridge is determined by the lowest BID + VLAN ID + MAC. |
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Runs a separate IEEE 802.1D STP instance for each VLAN. |
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Possible to have load sharing with some VLANS forwarding on each trunk. |
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Sends a BPDU “hello message” every 2 seconds. |