Mac Address Allocation - Cisco Catalyst 4500 Series Configuration Manual

About STP
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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. Use 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.
STP and IEEE 802.1Q Trunks
802.1Q VLAN trunks impose some limitations on the spanning tree strategy for a network. In a network
of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree
for each VLAN allowed on the trunks. However, non-Cisco 802.1Q switches maintain only one instance
of spanning tree for all VLANs allowed on the trunks.
When you connect a Cisco switch to a non-Cisco device (that supports 802.1Q) through an 802.1Q trunk,
the Cisco switch combines the spanning tree instance of the 802.1Q native VLAN of the trunk with the
spanning tree instance of the non-Cisco 802.1Q switch. However, all per-VLAN spanning tree
information is maintained by Cisco switches separated by a network of non-Cisco 802.1Q switches. The
non-Cisco 802.1Q network separating the Cisco switches is treated as a single trunk link between the
switches.
Note For more information on 802.1Q trunks, see
Per-VLAN Rapid Spanning Tree
Per-VLAN Rapid Spanning Tree (PVRST+) is the same as PVST+, although PVRST+ utilizes a rapid
STP based on IEEE 802.1w rather than 802.1D to provide faster convergence. PVRST+ uses roughly the
same configuration as PVST+ and needs only minimal configuration. In PVRST+, dynamic CAM entries
are flushed immediately on a per-port basis when any topology change is made. UplinkFast and
BackboneFast are enabled but not active in this mode, because the functionality is built into the Rapid
STP. PVRST+ provides for rapid recovery of connectivity following the failure of a bridge, bridge port,
or LAN.
When you interconnect PVST and Rapid-PVST bridges, if the PVST bridge has a higher priority than
Note
the Rapid PVST bridge, then the Rapid PVST bridge transitions directly to the forwarding state,
bypassing the intermediate transition states.
Like per-VLAN Spanning Tree (PVST+), per-VLAN Rapid Spanning Tree (PVRST+) instances are
equal to the number of vlans configured on the switch and can go up to a maximum of 4094 instances.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
20-6
Disabled—In this state, the Layer 2 interface does not participate in spanning tree and does not
forward frames.
Chapter 20
Chapter 17, "Configuring Layer 2 Ethernet Interfaces."
Configuring STP and MST
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