Types of Spanning Tree Protocol (STP)
This
section reviews the three traditional types of Spanning Tree Protocol (STP)
that are encountered in switched networks and how they relate to one another.
No specific configuration commands are associated with the various types of STP
here. Instead, you need a basic understanding of how they interoperate in a
network.
Common Spanning Tree
The IEEE 802.1Q
standard specifies how VLANs are to be trunked between switches. It also
specifies only a single instance of STP that encompasses all VLANs. This
instance is referred to as the Common Spanning Tree (CST). All CST BPDUs
are transmitted over trunk links using the native VLAN with untagged frames.
Having a single
STP for many VLANs simplifies switch configuration and reduces switch CPU load
during STP calculations. However, having only one STP instance can cause limitations,
too. Redundant links between switches will be blocked with no capability for
load balancing. Conditions also can occur that would cause CST to mistakenly
enable forwarding on a link that does not carry a specific VLAN, whereas other
links would be blocked.
Per-VLAN Spanning Tree
Cisco has a
proprietary version of STP that offers more flexibility than the CST version. Per-VLAN
Spanning Tree (PVST) operates a separate instance of STP for each individual VLAN.
This allows the STP on each VLAN to be configured independently, offering
better performance and tuning for specific conditions. Multiple spanning trees
also make load balancing possible over redundant links when the links are
assigned to different VLANs. One link might forward one set of VLANs, while
another redundant link might forward a different set.
Because of its
proprietary nature, PVST requires the use of Cisco Inter-Switch Link (ISL) trunking
encapsulation between switches. In networks where PVST and CST coexist, interoperability
problems occur. Each requires a different trunking method, so BPDUs are never
exchanged between STP types.
Per-VLAN Spanning Tree Plus
Cisco has a second
proprietary version of STP that allows devices to interoperate with both PVST
and CST. Per-VLAN Spanning Tree Plus (PVST+) effectively supports three groups
of STP operating in the same campus network:
■ Catalyst
switches running PVST
■ Catalyst
switches running PVST+
■ Switches
running CST over 802.1Q
Table 2-14 summarizes the three STP types and their
basic functions.
Type of STP
|
Function
|
CST
|
1 instance of STP, over the native VLAN;
802.1Q based
|
PVST
|
1 instance of STP per VLAN; Cisco ISL based
|
PVST+
|
Provides interoperability between CST and PVST;
operates over both 802.1Q and ISL
|
To
do this, PVST+ acts as a translator between groups of CST switches and groups
of PVST switches. PVST+ can communicate directly with PVST by using ISL trunks.
To communicate with CST, however, PVST+ exchanges BPDUs with CST as untagged
frames over the native VLAN. BPDUs from other instances of STP (other VLANs)
are propagated across the CST portions of the network by tunneling. PVST+ sends
these BPDUs by using a unique multicast address so that the CST switches
forward them on to downstream neighbors without interpreting them first.
Eventually, the tunneled BPDUs reach other PVST+ switches where they are
understood.
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