Understanding Bridge Protocol Data Units - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Chapter 17 Configuring STP and MST
Extended System ID
A 12-bit extended system ID field is part of the bridge ID (see
series switches have 64 MAC addresses and always use the 12-bit extended system ID.
Table 17-1 Bridge Priority Value and Extended System ID with the Extended System ID Enabled
Bridge Priority Value
Bit 16 Bit 15 Bit 14
32768 16384 8192
STP MAC Address Allocation
Catalyst 6500 series switches have 64 addresses available to support software features such as STP. To
view the MAC address range, enter the show catalyst6000 chassis-mac-address command.
STP uses the extended system ID plus a MAC address to make the bridge ID unique for each VLAN.
With MAC address reduction enabled on any device, you should also enable MAC address reduction on
all other Layer 2 connected network devices 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. With MAC address reduction enabled, a switch bridge ID (used by the spanning tree
algorithm to determine the identity of the root bridge, the lowest being preferred) can only be specified
as a multiple of 4096. Only the following values are possible: 0, 4096, 8192, 12288, 16384, 20480,
24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440.
If another bridge in the same spanning tree domain does not run the MAC address reduction feature, it
could win root bridge ownership because of the finer granularity in the selection of its bridge ID.

Understanding Bridge Protocol Data Units

Bridge protocol data units (BPDUs) are transmitted in one direction from the root bridge. Each network
device sends configuration BPDUs to communicate and compute the spanning tree topology. Each
configuration BPDU contains the following minimal information:
•
•
•
•
•
•
When a network device transmits a BPDU frame, all network devices connected to the LAN on which
the frame is transmitted receive the BPDU. When a network device receives a BPDU, it does not forward
the frame but instead uses the information in the frame to calculate a BPDU, and, if the topology
changes, initiate a BPDU transmission.
A BPDU exchange results in the following:
•
OL-11439-03
Extended System ID (Set Equal to the VLAN ID)
Bit 13 Bit 12 Bit 11
4096 2048 1024
The unique bridge ID of the network device that the transmitting network device believes to be the
root bridge
The STP path cost to the root
The bridge ID of the transmitting bridge
Message age
The identifier of the transmitting port
Values for the hello, forward delay, and max-age protocol timers
One network device is elected as the root bridge.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
Bit 10 Bit 9 Bit 8 Bit 7
512 256 128 64
Understanding How STP Works
Table 17-1 on page 17-3). Catalyst 6500
Bit 6 Bit 5 Bit 4 Bit 3
32 16 8 4
Bit 2 Bit 1
2 1
17-3