36
•
Backup
port
—Serves as the backup port of a designated port. When the designated port is
invalid, the backup port becomes the new designated port. A loop occurs when two ports of the
same spanning tree device are connected, so the device blocks one of the ports. The blocked
port acts as the backup.
•
Master
port
—Serves as a port on the shortest path from the local MST region to the common
root bridge. The master port is not always located on the regional root. It is a root port on the IST
or CIST and still a master port on the other MSTIs.
STP calculation involves root ports, designated ports, and alternate ports. RSTP calculation involves
root ports, designated ports, alternate ports, and backup ports. MSTP calculation involves all port
roles.
Port states
RSTP and MSTP define the following port states:
State Description
Forwarding
The port receives and sends BPDUs, and forwards user traffic.
Learning
The port receives and sends BPDUs, but does not forward user traffic. Learning is an
intermediate port state.
Discarding
The port receives and sends BPDUs, but does not forward user traffic.
STP defines the following port states: Disabled, Blocking, Listening, Learning, and Forwarding. The
Disabled, Blocking, and Listening states correspond to the Discarding state in RSTP and MSTP.
LLDP
The Link Layer Discovery Protocol (LLDP) operates on the data link layer to exchange device
information between directly connected devices. With LLDP, a device sends local device information
as TLV (type, length, and value) triplets in LLDP Data Units (LLDPDUs) to the directly connected
devices. Local device information includes its system capabilities, management IP address, device
ID, and port ID. The device stores the device information in LLDPDUs from the LLDP neighbors in a
standard MIB. LLDP enables a network management system to quickly detect and identify Layer 2
network topology changes.
LLDP agent
An LLDP agent is a mapping of an entity where LLDP runs. Multiple LLDP agents can run on the
same interface.
LLDP agents are divided into the following types:
•
Nearest bridge agent.
•
Nearest customer bridge agent.
•
Nearest non-TPMR bridge agent.
LLDP exchanges packets between neighbor agents and creates and maintains neighbor information
for them.
Transmitting LLDP frames
An LLDP agent operating in TxRx mode or Tx mode sends LLDP frames to its directly connected
devices both periodically and when the local configuration changes. To prevent LLDP frames from
