In order to build a spanning tree for the bridge topology, the bridges must exchange control frames. These frames are called Bridge Protocol data units (BPDU).
BPDUs are sent to a Cisco-proprietary multicast MAC address 0100.0ccc.cccd or Extreme-proprietary multicast MAC address 0304.0800.0700.By default, the PVST+ BPDUs are sent to Extreme-proprietary multicast MAC address on Extreme switches. These are called SSTP (Single Spanning Tree Protocol) BPDUs. The format of the SSTP BPDU is nearly identical to the 802.1d BPDU after the SNAP header, except that a type-length-value (TLV) field is added at the end of the BPDU. The TLV has 2 bytes for type (0x0), 2 bytes for length, and 2 bytes for the VLAN ID. See Extreme R-PVST+ BPDU headers/fields and Cisco R-PVST+ BPDU headers/fields for an outline of the BPDU header content.
Topology Change Notification (TCN) BPDUs are used to inform other switches of port changes. TCNs are injected into the network by a non-root switch and propagated to the root. Upon receipt of the TCN, the root switch will set a Topology Change flag in its normal BPDUs. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries.