Understanding Bpdus - Cisco nexus 5000 series Cli Configuration Manual

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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:
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STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
If another bridge in the same spanning tree domain does not run the MAC address reduction feature, it
Note
could achieve root bridge ownership because its bridge ID may fall between the values specified by the
MAC address reduction feature.

Understanding BPDUs

Switches transmit bridge protocol data units (BPDUs) throughout the STP instance. Each switch sends
configuration BPDUs to communicate and compute the spanning tree topology. Each configuration
BPDU contains the following minimal information:
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Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
1-4
0
4096
8192
12288
16384
20480
24576
28672
32768
36864
40960
45056
49152
53248
57344
61440
The unique bridge ID of the switch that the transmitting switch determines is the root bridge
The STP path cost to the root
The bridge ID of the transmitting bridge
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Chapter 1 Configuring Rapid PVST+
OL-16597-01