Stp Port States; Mac Address Allocation - Cisco 4500M Software Manual

Chapter 14 Understanding and Configuring STP
Figure 14-1 Spanning Tree Topology
DP
RP = Root Port
DP = Designated Port
For example, assume that one port on Switch B is a fiber-optic link, and another 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 higher
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 network. 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 frame
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:
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•
•
•
•

MAC Address Allocation

The supervisor engine has a pool of 1024 MAC addresses that are used as the bridge IDs for the 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 range 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-e0-1e-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.
OL-6696-01
DP
DP
A
RP
DP
RP DP
RP
B C
Blocking—In this state, the Layer 2 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 Layer 2 interface should participate in frame forwarding.
Learning—In this state, the Layer 2 interface prepares to participate in frame forwarding.
Forwarding—In this state, the Layer 2 interface forwards frames.
Disabled—In this state, the Layer 2 interface does not participate in spanning tree and does not
forward frames.
D
DP DP
Software Configuration Guide—Release 12.2(25)EW
Overview of STP
14-5