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Reducing the values of these timers makes the spanning tree react faster when the topology changes, but may cause
temporary loops as the tree stabilizes in its new configuration. Increasing the values of these timers makes the
spanning tree react more slowly to changes in topology, but will make an unintended reconfiguration less likely. All of
the bridges on the network will use the values set by the root bridge. It is only necessary to reconfigure that bridge if
changing the parameters.
5.4.2 Spanning Tree Port Configuration
You can configure the following parameters from interface configuration mode:
Switch(config)# interface eth1
Switch(config-if-eth1)# spanning-tree ?
disable Disable spanning tree protocol in this interface
edge-port Enable port admin edge
link-type Configure the link type
path-cost Set interface path cost
port-priority Set interface priority
Switch(config-if-eth1)#
Port Priority
The port priority is a spanning tree parameter that ranks each port, so that if two or more ports have the same path
cost, the STP selects the path with the highest priority (the lowest numerical value). By changing the priority of a port,
it can be more, or less, likely to become the root port. The default value is 128, and the value range is 0–255.
Port Path Cost
Port path cost is the spanning tree parameter that assigns a cost factor to each port. The lower the assigned port
path cost is, the more likely that port will be accessed. The default port path cost for a 10 Mbps or 100 Mbps port is
the result of the equation:
Path cost = 1000/LAN speed (in Mbps)
Therefore, for 10 Mbps ports, the default port path cost is 100. For 100 Mbps ports, it is 10. To allow for faster
networks, the port path cost for a 1000 Mbps port is set by the standard at 4.
5.4.3 Rapid Spanning Tree Protocol (RSTP)
Rapid Spanning Tree Protocol makes use of point-to-point link type and expedites into a rapid convergence of the
spanning tree. Re-configuration of the spanning tree can occur in less than 1 second (as opposed to 50 seconds with
the default settings in the legacy spanning tree), which is critical for networks carrying delay-sensitive traffic, such as
voice and video.
Port Roles and the Active Topology
RSTP provides rapid convergence of the spanning tree by assigning port roles and by determining the active
topology. RSTP uses the same underlying spanning tree calculation and algorithm as legacy STP to select the bridge
with the highest bridge priority (lowest numerical priority value) as the root bridge. Then RSTP assigns one of these
port roles to bridge ports:
• Root port—provides the best path (lowest cost) when the bridge forwards packets to the root switch.
• Designated port—connects to the designated switch, which has the lowest path cost when forwarding packets
from that LAN to the root bridge. The port through which the designated switch is attached to the LAN is called
the designated port.
48 Asante IntraCore IC36240 User’s Manual