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Appendix C Switch Port Functionality
Spanning Tree Protocol (STP)
Spanning Tree Protocol (STP) Spanning Tree Protocol (STP) is a link management protocol that provides path redundancy while
preventing undesirable loops in the network. For an Ethernet network to function properly, only one
active path can exist between any two stations. When two ports on a switch are in a loop, the spanning
tree port priority and port path cost setting determine which port to put in the forwarding state and which
port to put in the blocking state.
The 802.1Q standard defines the method for running multiple VLANs over single or multiple physical
LAN segments and defines a unique spanning tree instance to be created on each of the VLAN instances
for all the VLANs in a network.
A mono spanning tree (MST) network lacks some flexibility, compared to a per VLAN spanning tree
(PVST) network, which runs one instance of STP per VLAN. One spanning tree is created for every new
VLAN created on a FESMIC interface. STP is enabled by default on VLAN 1 and on all newly created
VLANs.
Cisco developed PVST+ to allow running several STP instances (even over an 802.1Q network) by using
a tunneling mechanism. Although beyond the scope of this document, PVST+ can be briefly described
as utilizing a Cisco device to connect a MST zone (typically another vendor’s 802.1Q-based network) to
a PVST zone (typically a Cisco 802.1Q-based network). There is no specific configuration to enter in
order to achieve this. PVST+ is a spanning tree that allows the coexistence of both PVST and Shared
Spanning Tree Protocol (SSTP) in a mixed vendor environment.
The STP described in IEEE 802.1D standard takes a substantial amount of time to converge to a loop
free topology. It fails to take advantage of the point-to-point wiring found in modern networks. PVST is
enabled on all switch platforms. Rapid Spanning Tree Protocol (RSTP), specified in IEEE 802.1w[9],
improves the operation of STP, while maintaining compatibility with equipment based on the (original)
802.1d Spanning Tree standard.
Note The Cisco Shared Spanning Tree Architecture documents use the terms MST and SST to mean “Mono
Spanning Tree” and “Shared Spanning Tree” respectively. The IEEE 802.1s[10] uses the same terms but
with exactly opposite meanings, i.e. MST is “Multiple Spanning Trees” and SST is” Single Spanning
Tree.”
When you connect two Cisco switches through 802.1Q trunks, the switches exchange spanning-tree
bridge packet data units (BPDUs) on each VLAN allowed on the trunks. The BPDUs on the native
VLAN of the trunk are sent untagged to the reserved IEEE 802.1d spanning-tree multicast MAC address
(01-80-C2-00-00-00). The BPDUs on all other VLANs on the trunk are sent tagged to the reserved
Shared Spanning Tree Protocol (SSTP).
One spanning tree is created for every new VLAN that is created on the FESMIC. STP is enabled by
default on VLAN 1 and on all the newly created VLANs.
PVST and PVST+ are enabled by default on the FESMIC.
For detailed information on how STP works, go to http://www.cisco.com.