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Chapter 5

Using Spanning Tree with VLANs

However, there are some disadvantages of using a spanning-tree-per-

instance implementation:

The utilization on the switch is a factor because it needs to manage all
the STP instances.

You must take into consideration that the trunk links have to support
all the VLAN STP information as well.

Common Spanning Tree (CST)

The IEEE 802.1q is referred to as the

Common Spanning Tree (CST)

. It is

also called the Mono-Spanning Tree because it uses only one spanning tree
instance regardless of the size of the switched layer 2 network.

The CST runs on all VLANs by default, and all switches are involved in

the election process to find the root bridge. The switches then form an asso-
ciation with that root bridge. Typically, using CST does not allow for opti-
mization of the root bridge placement.

There are some advantages to CST. With one STP instance, there are

fewer BPDUs consuming bandwidth. Because there is only one instance of
STP in the network, there is less STP processing performed by the switches.

However, the disadvantages typically outweigh the advantages in a larger

network. With a single root bridge, the path that has been calculated as the
best cost to the root bridge might not be the most efficient for some users to
send their data. Another disadvantage with CST is that the STP topology
increases in size to make sure all ports in the network are found. This can
cause delays in the updates and convergence times if the network topology
is too large.

Per-VLAN Spanning Tree+ (PVST+)

Per-VLAN Spanning Tree+ (PVST+)

is an extension of the PVST standard.

Starting with the Catalyst software 4.1 or later, PVST+ is supported on Cisco
Catalyst switches. This allows Cisco switches to support the IEEE 802.1q
standard. Basically, the PVST+ extension of the PVST protocol provides sup-
port for links across an IEEE 802.1q CST region.

PVST+ also supports the Cisco default PVST and adds checking mecha-

nisms to make sure there are no configuration problems on trunked ports
and VLAN IDs across switches. PVST+ is plug-and-play compatible with
PVST with no configuration necessary. In order to provide support for the

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