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Dell Configuration Guide for the S4048–ON System 9.14.2.5

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Enabling PVST+ Extend System ID

In the following example, ports P1 and P2 are untagged members of different VLANs. These ports are untagged because the hub is VLAN unaware. There is no data loop in this scenario; however, you can employ PVST+ to avoid potential misconfigurations.

If you enable PVST+ on the Dell EMC Networking switch in this network, P1 and P2 receive BPDUs from each other. Ordinarily, the Bridge ID in the frame matches the Root ID, a loop is detected, and the rules of convergence require that P2 move to blocking state because it has the lowest port ID.

To keep both ports in a Forwarding state, use extend system ID. Extend system ID augments the bridge ID with a VLAN ID to differentiate BPDUs on each VLAN so that PVST+ does not detect a loop and both ports can remain in a Forwarding state.

Figure 1. PVST+ with Extend System ID
Illustration of PVST+ with Extend System ID
Augment the bridge ID with the VLAN ID.
PROTOCOL PVST mode
extend system-id

Example of Viewing the Extend System ID in a PVST+ Configuration

DellEMC(conf-pvst)#do show spanning-tree pvst vlan 5 brief

VLAN 5
Executing IEEE compatible Spanning Tree Protocol
Root ID Priority 32773, Address 0001.e832.73f7
Root Bridge hello time 2, max age 20, forward delay 15
Bridge ID Priority 32773 (priority 32768 sys-id-ext 5), Address 0001.e832.73f7
We are the root of Vlan 5
Configured hello time 2, max age 20, forward delay 15

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