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Spanning Tree Protocol continually explores the network, so when the network topology changes, STP automatically reconfigures the switch ports in order to avoid the failure by blocking certain port.
Spanning tree algorithm aware switches (bridges) exchange configuration messages periodically. The configuration message is a multicast frame called BPDU (Bridge Protocol Data Unit) or Hello message. According to BPDU, these STP aware switches (bridges) will construct a loop free network with a "tree" architecture.
STP operation is described below:
1. Select a root bridge
Only one switch/ bridge can be selected as the root bridge in a given network. All other decisions in the network, such as which port is blocked and which port is put in forwarding mode, are made regarding this root bridge. The root bridge is the "root" of the constructed "tree".
zOne of the important fields included in the BPDU is the bridge ID.
Each bridge has unique bridge ID. The root bridge is the bridge with the lowest bridge ID in the spanning tree network.
zThe bridge ID includes two parts, bridge priority (2 bytes) and bridge MAC address (6 bytes). The 802.1d default bridge priority is 32768. E.g. for a switch with default priority 32768 (8000 hex), MAC address is 00:A0:C5:12:34:56, its bridge ID is 8000:00A0:C512:3456.
zOn the root bridge, all its ports are designated ports. Designated ports are always in the forwarding state. While in forwarding state, port can receive and send traffic.
2. Select a root port for the non-root bridge
For the
zThe root port is the port on the
zPath cost is the total cost of transmitting a frame on to a LAN through that port to bridge root. It is assigned according to the bandwidth of the link. The slower the media, the higher the cost.
Some of the path costs specified in the IEEE 802.1d specification are listed below.
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