SmartBridges sB3021 manual Configuring Spanning Tree Protocol STP, STP Settings Configuration

i n t e l l i g e n t w i r e l e s s p l a t f o r m

2.7.3 Configuring Spanning Tree Protocol (STP)

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 redundant links.

To create a fault-tolerant network, there needs to be a loop-free path between all nodes in the network. The Spanning Tree Algorithm calculates the best loop-free path throughout a Layer 2 network. Infrastructure devices such as wireless bridges and switches send and receive spanning tree frames, called Bridge Protocol Data Units (BPDUs), at regular intervals. The devices do not forward these frames but use them to construct the loop-free path.

Multiple active paths among end stations cause loops in the network. If a loop exists in the network, end stations might receive duplicate messages. Infrastructure devices might also learn end-station MAC addresses on multiple Layer 2 interfaces. Such conditions result in an unstable network.

STP defines a tree with a root bridge and a loop-free path from the root to all infrastructure devices in the Layer 2 network.

STP 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.

When two interfaces on a device are part of a loop, the spanning tree port priority and path cost settings determine which interface is put in the forwarding state and which is put in the blocking state. The port priority value represents the location of an interface in the network topology and how well it is located to pass traffic. The path cost value represents the media speed.

2.7.4 STP Settings Configuration

STP is disabled by default. The table below lists the default STP settings when STP is enabled.

Table 2-4 Default STP Values

Setting	Default Value	Range	Purpose
Bridge priority	32768	0-65535	A parameter used to identify the
			root bridge in a spanning tree
			(instance of STP). The bridge
			with the lowest value has the
			highest priority and is the root.
Bridge max age	20	6-40	The interval a bridge will wait for
			a hello packet from the root
			bridge before initiating a topology
			change.
Bridge hello time	2	1-10	The interval of time between
			each configuration BPDU sent by
			the root bridge.

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