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Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
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Release 8.1
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Chapter 7 Configuring Spanning Tree
Understanding How Bridge Identifiers Work
MISTP-PVST+ Mode
MISTP-PVST+ is a transition spanning tree mode that allows you to use the MISTP functionality on
Catalyst 4500 series switches while continuing to communicate with the older Catalyst 5000 family and
6500 series switches in your network that use PVST+. A switch using PVST+ mode and a switch using
MISTP mode connected together cannot see the BPDUs of the other switch, a condition that can cause
loops in the network. MISTP-PVST+ allows interoperability between PVST+ and pure MISTP, because
it detects the BPDUs of both modes. If you wish to convert your network to MISTP, you can use
MISTP-PVST+ to transition the network from PVST+ to MISTP in order to avoid problems.
MISTP-PVST+ conforms to the limits of PVST+; for example, you can only configure the amount of
VLAN ports on your MISTP-PVST+ switches that you configure on your PVST+ switches.
Understanding How Bridge Identifiers Work
The next two sections explain how MAC addresses are used in PVST+ and MISTP as unique bridge
identifiers.
MAC Address Allocation
Catalyst 4000 series switches have a pool of 1024 MAC addresses that can be used as bridge identifiers
for VLANs running under PVST+ or for MISTP instances. The Catalyst 4500 series switches have a pool
of only 64 MAC addresses. You can use the show module command to view the MAC address range.
MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1,
the second in the range assigned to VLAN 2, and so forth. The last MAC address in the range is assigned
to the supervisor engine in-band (sc0) management interface.
For example, if the MAC address range for the supervisor engine 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 forth. The in-band (sc0)
interface MAC address is 00-e0-1e-9b-31-ff.
MAC Address Reduction
The MAC address reduction feature is used on Catalyst 6500 series switches to enable extended-range
VLAN identification. If you have a Catalyst 6500 series -switch in your network and you have MAC
address reduction enabled on it, you should also enable MAC address reduction on all your Catalyst
4500 series switches to avoid problems in the spanning tree topology. When MAC address reduction is
enabled on Catalyst 4500 series switches, it disables the pool of MAC addresses used for the VLAN
spanning tree, leaving a single MAC address that identifies the switch. For detailed information on the
MAC address reduction feature, refer to the Catalyst 6500 Series Software Configuration Guide.
