Bridge Priority Value; Extended System Id; Stp Mac Address Allocation; Cisco Nexus 3000 Series Nx-Os Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1 - Cisco Nexus 3000 series Configuration Manual

Configuring Rapid PVST+

Bridge Priority Value

The bridge priority is a 4-bit value when the extended system ID is enabled.
In Cisco NX-OS, the extended system ID is always enabled; you cannot be disable the extended system
Note
ID.

Extended System ID

A 12-bit extended system ID field is part of the bridge ID.
Figure 7: Bridge ID with Extended System ID
The switches always use the 12-bit extended system ID.
Combined with the bridge ID, the system ID extension functions as the unique identifier for a VLAN.
Table 7: Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge Priority Value
Bit Bit
16 15
32768 16384

STP MAC Address Allocation

Note Extended system ID and MAC address reduction is always enabled on the software.
With MAC address reduction enabled on any switch, you should also enable MAC address reduction on all
other connected switches to avoid undesirable root bridge election and spanning tree topology issues.
When MAC address reduction is enabled, the root bridge priority becomes a multiple of 4096 plus the VLAN
ID. You can only specify a switch bridge ID (used by the spanning tree algorithm to determine the identity
of the root bridge, the lowest being preferred) as a multiple of 4096. Only the following values are possible:
• 0
• 4096
Extended System ID (Set Equal to the VLAN ID)
Bit Bit Bit Bit Bit
14 13 12 11 10
8192 4096 2048 1024 512

Cisco Nexus 3000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U1(1)

Bit 9 Bit 8 Bit 7 Bit 6 Bit 5
256 128 64 32 16
Understanding STP
Bit 4 Bit 3 Bit 2 Bit 1
8 4 2 1
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