Mistp-Pvst+ Mode; Bridge Identifiers; Mac Address Allocation; Mac Address Reduction - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Chapter 8 Configuring Spanning Tree

MISTP-PVST+ Mode

MISTP-PVST+ is a transition spanning tree mode that allows you to use the MISTP functionality on
Catalyst 6000 family switches while continuing to communicate with Catalyst 5000 and 6000 switches
in your network that use PVST+. A switch using PVST+ mode that is connected to a switch using MISTP
mode 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 sees the BPDUs of
both modes. To convert your network to MISTP, use MISTP-PVST+ to transition the network from
PVST+ to MISTP.
Because MISTP-PVST+ conforms to the limits of PVST+, you cannot configure more VLAN ports on
your MISTP-PVST+ switches than on your PVST+ switches.

Bridge Identifiers

These sections explain how MAC addresses are used in PVST+ and MISTP as unique bridge identifiers:
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MAC Address Allocation

Catalyst 6000 family switches have a pool of 1024 MAC addresses that can be used as bridge identifiers
for VLANs running under PVST+ or for MISTP instances. 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 MAC address in the range assigned to VLAN 2, and so on. 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 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

For Catalyst family switches that support 4096 VLANs, MAC address reduction allows up to
4096 VLANs running under PVST+ or 16 MISTP instances to have unique identifiers without increasing
the number of MAC addresses required on the switch. MAC address reduction reduces the number of
MAC addresses required by the STP from one per VLAN or MISTP instance to one per switch. However,
because VLANs running under PVST+ and MISTP instances running under MISTP-PVST+ or MISTP
are considered logical bridges, each bridge must have its own unique identifier in the network.
When you enable MAC address reduction, the bridge identifier stored in the spanning tree BPDU
contains an additional field called the system ID extension. Combined with the bridge priority, the system
ID extension functions as the unique identifier for a VLAN or an MISTP instance. The system ID
extension is always the number of the VLAN or the MISTP instance; for example, the system ID
extension for VLAN 100 is 100, and the system ID extension for MISTP instance 2 is 2.
78-13315-02
MAC Address Allocation, page 8-13
MAC Address Reduction, page 8-13
Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4

Bridge Identifiers

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