BPDU
A bridge sends a BPDU frame using the unique MAC address of the port itself as a source address, and a destination address of the STP multicast address 01:80:C2:00:00:00.
aa:bb:cc:00:01:00 is MAC address of SW1 interface E0/0:
Every 2 second, the root bridge SW1 sends out BPDU
SW1#sh spanning-tree
VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 32769
Address aabb.cc00.0100
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 32769 (priority 32768 sys-id-ext 1)
Address aabb.cc00.0100
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
Et0/0 Desg FWD 100 128.1 Shr
Et0/1 Desg FWD 100 128.2 Shr
Et0/2 Desg FWD 100 128.3 Shr
Et0/3 Desg FWD 100 128.4 Shr
SW1#
The bridge ID (BID) is a critical element for the creation of the spanning-tree, loop-free topology.
The bridge ID consists of a 2-byte bridge priority and a 6-byte MAC address. The default priority is 32,768 and can only be configured in multiples of 4096. In the following screenshot, we can see the Bridge ID is HEX format is 80 01 + aa bb cc 00 01 00
In the value "80 01", the 4 most-significant bits of the 802.1d two-octet priority field is priority, and the least-significant 12 bits of that field as the extended system ID, in above screenshot, "8" is the priority, "0 01" is VLAN#. Hex 8000 is 32768 is Dec. Since only 4 bits as priority, so total there is 16 different priority values.
Let's take a look the value of VLAN 150, the Hex value is 80 96. Hex 96 is Dec 150
There are two types of BPDUs in the original STP specification (the Rapid Spanning Tree (RSTP) extension uses a specific RSTP BPDU):
- Configuration BPDU (CBPDU), used for Spanning Tree computation
- Topology Change Notification (TCN) BPDU, used to announce changes in the network topology
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