Cisco Extended System ID

The original 802.1D Spanning Tree standard defined by the IEEE only supported one unique Bridge ID per switch. This worked fine in the early days, but as VLANs were introduced Cisco needed to assign new MAC addresses for each VLAN Bridge ID as a way of staying compliant with the 802.1D standard. This quickly burned up valuable MAC addresses from the switch’s MAC table for organizations using large number of VLANs.

In response, the IEEE introduced the Extended System ID feature as a way to expand the Spanning Tree VLAN scaling support on a an individual switch. Defined by IEEE 802.1t, the Extended System ID feature allows a switch to support up to 4096 STP instances without the need to consume a separate MAC address per STP instance. This allows a single switch to use one MAC address to create all its Bridge IDs, adding the VLAN number to make each instance value unique.

To check if the Extended System ID feature is enabled on a Cisco switch, look for the presence of the spanning-tree extend system-id command in the running configuration. Most new switches use the Extended System ID feature by default and do not allow it to be removed from the configuration due to MAC address depletion concerns.

The show spanning-tree command output displays the Bridge ID information for each VLAN. Notice the Bridge ID Priority line from SW2.

SW2#show run | i spanning-tree extend system-id
spanning-tree extend system-id
SW2#show spanning-tree vlan 10

Spanning tree enabled protocol rstp
Root ID Priority 24586
Address 0026.0b8e.a500
Cost 4
Port 23 (GigabitEthernet1/0/23)
Hello Time 5 sec Max Age 40 sec Forward Delay 20 sec

Bridge ID Priority 32778 (priority 32768 sys-id-ext 10)
Address 0025.b463.1d80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300 sec

Interface Role Sts Cost Prio.Nbr Type
——————- —- — ——— ——– ——————————–
Gi1/0/23 Root FWD 4 128.23 P2p Peer(STP)
Gi1/0/24 Altn BLK 4 128.24 P2p Peer(STP)

SW2 is using the Extended System ID feature in the output above. If we look closer at the Bridge ID, SW2 is using the default STP priority value of 32768 but the VLAN ID of 10 is added to produce a Bridge ID priority of 32778.

When the Extended System ID feature is enabled, the Bridge ID priority is the sum of the priority (default is 32768) + the System ID Extension value (sys-id-ext). The System ID Extension is just the VLAN number.

Author Aaron

Aaron knows networks. He's been involved in building and supporting world-class data networks for the past 10 years - from international cloud service providers to Fortune 50 data centers. Aaron consults independently and is focused on building the best training platform available.

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