Local-Area Networks and LAN Switching - CCIE Routing and Switching Exam Certification Guide - page 134

BPDU Types
The two types of a BPDU follow:
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Configuration BPDU

•

Topology Change Notification (TCN) BPDU

Configuration BPDUs are sent from the root bridge with the root BID. The configuration
BPDUs flow through all active paths, which provides the root BID and path cost information.
The TCN BPDUs flow upstream to the root bridge to alert it of a topology change. The spanning-
tree algorithm is recalculated by the bridges to determine any necessary changes in the path.
After the network converges, no TCN BPDUs are present in the network.

Bridge Port States
Each port of a transparent bridge exists in the following states:
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Disabled —The port is inactive and does not participate in STP.

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Blocking —When a port is enabled, it first moves to the blocked state before listening to
the network. In this state, it does not participate in frame forwarding. It receives bridge
PDUs and sends them to the STP algorithm for processing.

•

Listening —When the bridge determines that the port should participate in frame
forwarding, it changes to the listening state. In this state, the bridge does not forward
frames and does not learn of network MAC addresses. The bridge does receive and
process BPDUs and network management frames, but it does not send BPDUs.

•

Learning —The bridge port discards incoming frames. The bridge begins to add MAC
addresses associated with this port into the table. BPDU and network management
messages are processed. The bridge processes, generates, and sends BPDUs in this state.

•

Forwarding —The full functional state for a bridged port. In this state, the bridge does not
discard incoming frames. The bridge forwards frames to other ports; the bridge also
forwards frames out this port. BPDUs and network management frames are processed.

Designated Ports
If connected with more than one port to the same segment, bridges select a designated port. The
designated port is the port that sends and receives frames on the segment; other ports are placed
in the blocking state (nondesignated ports).

Multi-Instance Spanning-Tree Protocol (MISTP)
Each VLAN configured in a switch runs an independent instance of the STP. MISTP is a
proprietary spanning-tree mode in Cisco switches, which allows the grouping of multiple
VLANs under a single instance of the STP. The MISTP instance has its own root switch and