User's Manual

213
Users Manual of CS-6306R
Chapter 19. STP Configuration
Configuring STP 19.1
19.1.1 STP Introduction
The standard Spanning Tree Protocol (STP) is based on the IEEE 802.1D standard. A switch stack appears
as a single spanning-tree node to the rest of the network, and all stack members use the same bridge ID.
Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack.
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.
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:
The standard Spanning-Tree Protocol (STP) is defined in IEEE 802.1D. It simplifies the LAN topology
comprising several bridges to a sole spinning tree, preventing network loop from occurring and ensuring
stable work of the network.
The algorithm of STP and its protocol configure the random bridging LAN to an active topology with simple
connections. In the active topology, some bridging ports can forward frames; some ports are in the congestion
state and cannot transmit frames. Ports in the congestion state may be concluded in the active topology.
When the device is ineffective, added to or removed from the network, the ports may be changed to the
transmitting state.
In the STP topology, a bridge can be viewed as root. For every LAN section, a bridging port will forward data
from the network section to the root. The port is viewed as the designated port of the network section. The
bridge where the port is located is viewed as the designated bridge of the LAN. The root is the designated
bridge of all network sections that the root connects. In ports of each bridge, the port which is nearest to the
root is the root port of the bridge. Only the root port and the designated port (if available) is in the transmitting
state. Ports of another type are not shut down but they are not the root port or the designated port. We call
these ports are standby ports.
The following parameters decide the structure of the stabilized active topology:
(1) Identifier of each bridge
(2) Path cost of each port
(3) Port identifier for each port of the bridge