HP Cisco Nexus 5000 Cisco Nexus 5000 Series Switch CLI Software Configuration - Page 196

Understanding STP Information About Rapid PVST+ Understanding STP STP Overview For an Ethernet network to function properly, only one active path can exist between any two stations. STP operation is transparent to end stations, which cannot detect whether they are connected to a single LAN segment or a switched LAN of multiple segments. When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a network. The STP algorithm calculates the best loop-free path throughout a switched network. LAN ports send and receive STP frames, which are called Bridge Protocol Data Units (BPDUs), at regular intervals. Switches do not forward these frames, but use the frames to construct a loop-free path. 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. Cisco Nexus 5000 Series Switch CLI Software Configuration Guide 150 OL-16597-01