Stephen Technologies Co.,Limited
www.stephen-tele.com
For each VLAN, the switch with the highest switch priority (the lowest numerical priority value)
is elected as the root switch. If all switches are configured with the default priority (32768), the
switch with the lowest MAC address in the VLAN becomes the root switch. The switch priority
value occupies the most significant bits of the bridge ID, as shown in Table 8-1.
A root port is selected for each switch (except the root switch). This port provides the best
path (lowest cost) when the switch forwards packets to the root switch.
When selecting the root port on a switch stack, spanning tree follows this sequence:
– Selects the lowest root bridge ID
– Selects the lowest path cost to the root switch
– Selects the lowest designated bridge ID
– Selects the lowest designated path cost
– Selects the lowest port ID
Only one outgoing port on the root switch is selected as the root port. The remaining switches
in the stack become its designated switches (Switch 2 and Switch 3) as shown in Figure 9-1.
The shortest distance to the root switch is calculated for each switch based on the path
cost.
A designated switch for each LAN segment is selected. The designated switch incurs the
lowest pathcost when forwarding packets from that LAN to the root switch. The port
through which the designated switch is attached to the LAN is called the designated port.
