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Chapter 7
Configuring Spanning Tree
MAC Address Reduction
For Catalyst 6500 series 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 that are required on the switch. MAC address reduction reduces the
number of MAC addresses that are 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 that is 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.
Figure 7-8
identifier consists of the bridge priority and the MAC address.
Figure 7-8
Figure 7-9
consists of the bridge priority, the system ID extension, and the MAC address. The bridge priority and
the system ID extension combined are known as the bridge ID priority. The bridge ID priority is the
unique identifier for the VLAN or the MISTP instance.
Figure 7-9
Bridge Priority
When you enter the show spantree command, you can see the bridge ID priority for a VLAN in PVST+
or for an MISTP instance in MISTP or MISTP-PVST+ mode.
This example shows the bridge ID priority for VLAN 1 when you enable MAC address reduction in
PVST+ mode. The unique identifier for this VLAN is 32769.
Console> (enable) show spantree 1
VLAN 1
Spanning tree mode
Spanning tree type
.
.
.
Bridge ID MAC ADDR
Bridge ID Priority
Bridge Max Age 20 sec
OL-8978-04
shows the bridge identifier when you do not enable MAC address reduction. The bridge
Bridge Identifier without MAC Address Reduction
Bridge Priority
2 bytes
shows the bridge identifier when you enable MAC address reduction. The bridge identifier
Bridge Identifier with MAC Address Reduction Enabled
Bridge ID Priority
System ID Ext.
4 bits
12 bits
Hello Time 2
MAC Address
6 bytes
MAC Address
6 bytes
PVST+
ieee
00-d0-00-4c-18-00
32769
(bridge priority: 32768, sys ID ext: 1)
sec
Forward Delay 15 sec
Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
Understanding How Bridge Identifiers Work
7-15
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