ATL Telecom R1-SW Ethernet Switch - Manual (Page 261)

Configuring STP/RSTP
13-3
If the path 3 is blocked in the network configuration mentioned previously, you can have a
loop-free path between Switch A and Switch C as follows:
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 the frames to construct a
loop-free path.
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.
BDPU(Bridge Data Protocol Unit)
Spanning tree consists of a root switch, designated switches, root port, and designated ports.
The root switch is the logical center of the spanning-tree topology in a switched network. A
designated switch is a switch used to forward packets from that LAN to the root switch. A root
port is a forwarding port elected for the spanning-tree topology. A designated port is a
forwarding port elected for every switched LAN segment.
Switch A
Switch C
Path 1
(Forwarding)
Path 2
(Forwarding)
Path 3
(Blocking) Switch B
Root Switch
Root Port
Designated
Switch
Designated
Switch
Designated
Switch
Designated Port
Root Port