Asante Technologies IC36240 Spanning Tree Port Configuration, Rapid Spanning Tree Protocol Rstp

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