ZyXEL Communications VES-1616 Support Notes page 94

ZyXEL
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".
One 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.
The 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.
On 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 non-root switch/bridge, there will be one root port. The root port is the port
through which this non-root switch / bridge communicates with the root bridge (the
"leaf" side of the "tree").
The root port is the port on the non-root bridge with the lowest path cost to
the root bridge. The root port is normally in forwarding state.
Path 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.
All contents copyright 2008 ZyXEL Communications Corporation.
VES-1616/24FA-5x Series Support Notes
93