Cisco Systems WSC4500X16SFP STP Port States, MAC Address Allocation

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Software Configuration Guide—Release 12.2(25)SG

OL-7659-03

Chapter13 Understanding and Conf iguring STP

Overview of STP

Figure13-1 Spanning Tree Topology

For example, assume that one port on Switch B is a fiber-optic link, and ano ther port on Switch B (an

unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the

high-speed fiber-optic link. By changing the spanning tree port priority on the fiber-optic port to a highe r

priority (lower numerical value) than the priority set for the root port, the fiber-optic port becomes the

new root port.

STP Port States

Propagation delays can occur when protocol information passes through a switched LAN. As a result,

topology changes can take place at different times and at different places in a switched net work. When

a Layer 2 interface transitions directly from nonparticipation in the spanning tree topology to the

forwarding state, it can create temporary data loops. Ports must wait for new topology information to

propagate through the switched LAN before starting to forward frames. They must allow the fram e

lifetime to expire for frames that have been forwarded under the old topology.

Each Layer 2 interface on a switch that uses spanning tree exists in one of the following five states:

•Blocking—In this state, the Layer2 interface does not participate in frame forwarding.

•Listening—This state is the first transitional state after the blocking state when spanning tree

determines that the Layer2 interface should participate in frame forwarding.

•Learning—In this state, the Layer2 interface prepares to participate in frame forwarding.

•Forwarding—In this state, the Layer2 interface forwards frames.

•Disabled—In this state, the Layer 2 interface does not part icipate in spanning tree and does not

forward frames.

MAC Address Allocation

The supervisor engine has a pool of 1024 MAC addresses that are used as the bridge IDs for t he VLAN

spanning trees. You can use the show module command to view the MAC address range (allocation

range for the supervisor) that the spanning tree uses for the algorithm.

MAC addresses for the Catalyst 4506 switch are allocated sequentially, with the first MAC address in

the range assigned to VLAN 1, the second MAC address in the rang e assigned to VLAN 2, and so forth.

For example, if the MAC address range is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge

ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e 0-1e-9b-2e-01, the VLAN 3 bridge ID is

00-e0-1e-9b-2e-02, and so on. On other Catalyst 4500 series platforms, all VLANS map to the same

MAC address rather than mapping to separate MAC addresses.

S5688

RP DP

RP = Root Port

DP = Designated Port