Cisco Nexus 5000 Series Nx-Os Layer 2 Switching Configuration Guide, Release 5.1(3)N1(1 - Cisco nexus 5000 series Configuration Manual

Configuring Rapid PVST+
Extended System ID
A 12-bit extended system ID field is part of the bridge ID.
Figure 14: 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 10: Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge Priority Value
Bit
16
32768
STP MAC Address Allocation
Extended system ID and MAC address reduction is always enabled on the software.
Note
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
• 8192
• 12288
• 16384
• 20480
• 24576
• 28672
• 32768
OL-25842-01
Extended System ID (Set Equal to the VLAN ID)
Bit Bit Bit Bit Bit
15 14 13 12 11
16384 8192 4096 2048 1024

Cisco Nexus 5000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.1(3)N1(1)

Bit Bit 9 Bit 8 Bit 7 Bit 6
10
512 256 128 64 32
Understanding STP
Bit 5 Bit 4 Bit 3 Bit 2 Bit 1
16 8 4 2 1
163