Preventing Bridge Loops

Station A

Segment A_f

Reference Point

Segment B

Station B

Bridge loops can be prevented by disabling the redundant path.

©2006 Cisco Systems, Inc. All rights reserved.

A loop-free network is one in which no Layer 2 loops exist; therefore, the network cannot create Layer 2 broadcast storms or flooded unicast storms. A loop-free network can be achieved manually by shutting down or disconnecting all redundant links between bridges. However, this leaves no redundancy in the network and requires manual intervention in the event of a link failure.

STP resolves this problem. Where there are alternative links to a destination on a switch, only one link will be used to forward data, unless there is a failure on that link. The switch ports associated with alternative paths remain aware of the topology of the network and can be enabled if a failure occurs on a primary link. In the case of primary link failure, the switch will begin forwarding frames over an alternative link.

The spanning tree algorithm (STA) runs on each switch to activate or block redundant links. To find the redundant links, the STA chooses a reference point in the network and determines if there are redundant paths to that reference point. If the STA finds a redundant path, it chooses which path will forward frames and which redundant path or paths will be blocked. This effectively severs the redundant links within the network until they are needed when the primary link toward the reference point fails.

Spanning tree standards often refers to a "bridge," but it is likely that all the devices exchanging spanning tree information will be Layer 2 switches.

© 2006 Cisco Systems, Inc.

Implementing Spanning Tree

Describing the 802.1D STP

This topic describes the 802.1D STP.

802.1D STP

• Configured root switch

Root

Root

The 802.1D STP provides a mechanism for switches to reconfigure the paths over which they forward frames, making possible a loop-free path when there are redundant switch paths through the network. This is accomplished by forwarding traffic over specific ports and by disabling other ports to prevent frames from being sent repeatedly or in a loop. STP prevents loops by using these mechanisms:

■ STP is implemented through the exchange of bridge protocol data unit (BPDU) messages between adjacent switches.

■ A single "root bridge" is elected to serve as the reference point from which a loop-free topology is built for all switches exchanging BPDUs.

■ Each switch, except for the root bridge, determines a "root port" that provides the best path to the root bridge.

■ In a triangular design similar to the one in the figure, on the link between the two nonroot switch ports, a port on one switch will become a designated port, and the port on the other switch will be in a blocking state, not forwarding frames. This effectively breaks any loop. Typically, the designated port will be on the switch with the best path to the root bridge.

■ Any port state change on any switch is considered a network topology change (for example, if a port goes up or down and the STA must be run on all switches to adapt to the new topology).

3-8 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Continue reading here: The STP Root Bridge

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