35
•
PVST
—
PVST
allows every VLAN to have its own spanning tree, which increases usage of links
and bandwidth.
•
MSTP
—Defined in IEEE 802.1s. MSTP overcomes the limitations of STP and RSTP. It supports
rapid network convergence and allows data flows of different VLANs to be forwarded along
separate paths. This provides a better load sharing mechanism for redundant links.
Spanning tree modes
The spanning tree modes include:
•
STP
mode
—All ports of the device send STP BPDUs. Select this mode when the peer device
of a port supports only STP.
•
RSTP
mode
—All ports of the device send RSTP BPDUs. A port in this mode automatically
transits to the STP mode when it receives STP BPDUs from a peer device. The port does not
transit to the MSTP mode when it receives MSTP BPDUs from a peer device.
•
PVST
mode—All ports of the device send PVST BPDUs. Each VLAN maintains a spanning tree.
In a network, the number of spanning trees maintained by all devices equals the number of
PVST-enabled VLANs multiplied by the number of PVST-enabled ports. If the number of
spanning trees exceeds the capacity of the network, device CPUs become overloaded, packet
forwarding is interrupted, and the network becomes unstable. The number of spanning trees
that a device can maintain varies by device model.
•
MSTP
mode
—All ports of the device send MSTP BPDUs. A port in this mode automatically
transits to the STP mode when it receives STP BPDUs from a peer device. The port does not
transit to the RSTP mode when it receives RSTP BPDUs from a peer device.
MSTP basic concepts
MSTP divides a switched network into multiple spanning tree regions (MST regions). MSTP
maintains multiple independent spanning trees in an MST region, and each spanning tree is mapped
to specific VLANs. Such a spanning tree is referred to as a multiple spanning tree instance (MSTI).
The common spanning tree (CST) is a single spanning tree that connects all MST regions in the
switched network. An internal spanning tree (IST) is a spanning tree that runs in an MST region. It is
also called MSTI 0, a special MSTI to which all VLANs are mapped by default. The common and
internal spanning tree (CIST) is a single spanning tree that connects all devices in the switched
network. It consists of the ISTs in all MST regions and the CST.
Devices in an MST region have the following characteristics:
•
A spanning tree protocol enabled.
•
Same region name.
•
Same VLAN-to-instance mapping configuration.
•
Same MSTP revision level.
•
Physically linked together.
Port roles
Spanning tree calculation involves the following port roles:
•
Root
port
—Forwards data for a non-root bridge to the root bridge. The root bridge does not
have any root port.
•
Designated
port
—Forwards data to the downstream network segment or device.
•
Alternate
port
—Serves as the backup port for a root port or master port. When the root port or
master port is blocked, the alternate port takes over.
