User's Manual

Multiple active paths between end stations cause loops in the network. If a loop exists in the network, end
stations might receive duplicate messages and switches might learn end station MAC addresses on multiple
LAN ports. These conditions result in a broadcast storm, which creates an unstable network.
STP defines a tree with a root bridge and a loop-free path from the root to all switches in the network. STP
forces redundant data paths into a blocked state. If a network segment in the spanning tree fails and a redundant
path exists, the STP algorithm recalculates the spanning tree topology and activates the blocked path.
When two LAN ports on a switch are part of a loop, the STP port priority and port path cost setting determine
which port on the switch is put in the forwarding state and which port is put in the blocking state.
Understanding How a Topology is Created
All switches in an extended LAN that participate in a spanning tree gather information about other switches
in the network by exchanging of BPDUs. This exchange of BPDUs results in the following actions:
•
The system elects a unique root switch for the spanning tree network topology.
•
The system elects a designated switch for each LAN segment.
•
The system eliminates any loops in the switched network by placing redundant interfaces in a backup
state; all paths that are not needed to reach the root switch from anywhere in the switched network are
placed in an STP-blocked state.
The topology on an active switched network is determined by the following:
•
The unique switch identifier Media Access Control (MAC) address of the switch that is associated with
each switch
•
The path cost to the root that is associated with each interface
•
The port identifier that is associated with each interface
In a switched network, the root switch is the logical center of the spanning tree topology. STP uses BPDUs
to elect the root switch and root port for the switched network, as well as the root port and designated port
for each switched segment.
Understanding the Bridge ID
Each VLAN on each switch has a unique 64-bit bridge ID consisting of a bridge priority value, an extended
system ID (IEEE 802.1t), and an STP MAC address allocation.
Bridge Priority Value
The bridge priority is a 4-bit value when the extended system ID is enabled.
In Cisco NX-OS, the extended system ID is always enabled; you cannot be disable the extended system
ID.
Note
Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)
64 OL-26590-01
Configuring Rapid PVST+
Understanding STP