One of the biggest advantages of using
a switch instead of a bridge is the ability to implement VLANs.
However, the implementation of VLANs does impact the behavior of STP.
Cisco and the IEEE 802.1Q committee approach the Spanning-Tree and
VLAN issue in very different ways. Some of the major methods for
reconciling STP and VLANs are listed as follows:
- Common Spanning Tree (CST) is the
IEEE 802.1Q solution to VLANs and Spanning Tree. CST defines a
single instance of Spanning Tree for all VLANs. BPDU information
runs on VLAN 1.
- Per-VLAN Spanning Tree (PVST) is a
Cisco proprietary implementation. PVST requires Cisco
Inter-Switch Link (ISL) encapsulation in order to work. PVST
runs a separate instance of STP for every VLAN.
- PVST+ is a Cisco proprietary
implementation that allows CST information to be passed
correctly into PVST. A solution to the scaling and stability problems associated with
large Spanning-Tree networks is to create separate instances of
PVST.
Because each VLAN is a logical LAN
segment, one instance of STP maintains a loop-free topology in each
VLAN. Although many switches support a maximum of 1005 VLANs, you
can enable Spanning-Tree Protocol on a maximum of only 64 VLANs at
one time. If you configure more than 64 VLANs, you can still operate
the other VLANs with STP disabled. By default, STP is enabled on
VLANs 1 through 64.
Having a separate instance of
Spanning Tree per VLAN as shown in Figure
reduces the recovery
time for STP recalculation, and hence increases the reliability of
your network in the following ways:
- Reduces overall size of
Spanning-Tree topology
- Improves scalability and decreases
convergence time
- Provides faster recovery and
better reliability (see Figure
)
Disadvantages of a Spanning Tree per
VLAN include:
- Utilization of switch to support
Spanning-Tree maintenance for VLAN
- Utilization of bandwidth on trunk
links to support BPDUs for each VLAN
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