ZyXEL Communications 2002 Series Guia do Utilizador descarregar pdf (Página 42)

VES-1000 Series Ethernet Switch

4-4 General, Switch and IP Setup

The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value (MAC address).

Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of

the link to which a port is attached. The slower the media, the higher the cost - see the following table.

Table 4-3 Path Cost

LINK SPEED RECOMMENDED VALUE RECOMMENDED RANGE ALLOWED RANGE

Path Cost 4Mbps 250 100 to 1000 1 to 65535

Path Cost 10Mbps 100 50 to 600 1 to 65535

Path Cost 16Mbps 62 40 to 400 1 to 65535

Path Cost 100Mbps 19 10 to 60 1 to 65535

Path Cost 1Gbps 4 3 to 10 1 to 65535

Path Cost 10Gbps 2 1 to 5 1 to 65535

On each bridge, the root port is the port through which this bridge communicates with the root. It is the port on

this switch with the lowest path cost to the root (the root path cost). If there is no root port, then this switch has

been accepted as the root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root among the

bridges connected to the LAN.

After a bridge determines the lowest cost-spanning tree with STP, it enables the root port and the ports that are the

designated ports for the connected LANs, and disables all other ports that participate in STP. Network packets are

therefore only forwarded between enabled ports, eliminating any possible network loops.

Figure 4-3 Root Ports and Designated Ports

STP-aware devices exchange Bridge Protocol Data Units (BPDUs) periodically. When the bridged LAN topology

changes, a new spanning tree is constructed.

Once a stable network topology has been established, all devices listen for Hello BPDUs transmitted from the root

bridge. If a device does not get a Hello BPDU after a predefined interval (Max Age), the device assumes that the

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ZyXEL Communications 2002 Series Guia do Utilizador Página 42