Cisco Systems N3KC3064TFAL3, N3KC3048TP1GE Bit extended system ID field is part of the bridge ID

Configuring Rapid PVST+

Understanding STP

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 6: Bridge Priority Value and Extended System ID with the Extended System ID Enabled

Bridge Priority Value				Extended System ID (Set Equal to the VLAN ID)
Bit	Bit	Bit	Bit	Bit	Bit	Bit	Bit 9	Bit 8	Bit 7	Bit 6	Bit 5	Bit 4	Bit 3	Bit 2	Bit 1
16	15	14	13	12	11	10
32768	16384	8192	4096	2048	1024	512	256	128	64	32	16	8	4	2	1

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

•8192

•12288

•16384

•20480

•24576

•28672

•32768

Cisco Nexus 3000 NX-OS Layer 2 Switching Configuration Guide, Release 5.0(3)U3(1)

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