Stp Overview; Spanning-Tree Topology And Bpdus - Cisco NCS 4200 Series Configuration Manual
Lan switching
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STP Overview
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STP Overview
STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the
network. For a Layer 2 Ethernet network to function properly, only one active path can exist between any
two stations. Multiple active paths among end stations cause loops in the network. If a loop exists in the
network, end stations might receive duplicate messages. Switches might also learn end-station MAC addresses
on multiple Layer 2 interfaces. These conditions result in an unstable network. Spanning-tree 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.
The STP uses a spanning-tree algorithm to select one switch of a redundantly connected network as the root
of the spanning tree. The algorithm calculates the best loop-free path through a switched Layer 2 network by
assigning a role to each port based on the role of the port in the active topology:
• Root—A forwarding port elected for the spanning-tree topology
• Designated—A forwarding port elected for every switched LAN segment
• Alternate—A blocked port providing an alternate path to the root bridge in the spanning tree
• Backup—A blocked port in a loopback configuration
The switch that has all of its ports as the designated role or the backup role is the root switch. The switch that
has at least one of its ports in the designated role is called the designated switch.
Spanning tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning
tree fails and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology and
activates the standby path. Switches send and receive spanning-tree frames, called bridge protocol data units
(BPDUs), at regular intervals. The switches do not forward these frames but use them to construct a loop-free
path. BPDUs contain information about the sending switch and its ports, including switch and MAC addresses,
switch priority, port priority, and path cost. Spanning tree uses this information to elect the root switch and
root port for the switched network and the root port and designated port for each switched segment.
When two ports on a switch are part of a loop, the spanning-tree port priority and path cost settings control
which port is put in the forwarding state and which is put in the blocking state. The spanning-tree port priority
value represents the location of a port in the network topology and how well it is located to pass traffic. The
path cost value represents the media speed.
Spanning-Tree Topology and BPDUs
The stable, active spanning-tree topology of a switched network is controlled by these elements:
• The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch.
LAN Switching Configuration Guide, Cisco IOS XE Everest 16.6.1 (Cisco NCS 4200 Series)
38
Configuring Port Priority, page 53
Configuring Path Cost, page 55
Configuring the Switch Priority of a VLAN, page 56
Configuring Spanning-Tree Timers, page 57
Displaying the Spanning-Tree Status, page 60
Configuring PVST+ and RPVST+
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