Advanced STP tuning

Question 1

Root Guard

Answer 1

Root Guard is a STP feature that is enabled on a port-by-port basis. It prevents a configured port from becoming a root port.

It prevents a downstream switch from becoming a root bridge in a topology.

Question 2

STP Portfast

Answer 2

The STP portfast feature disabled TCN generation for access ports.

It also bypasses the earlier 802.1D STP states learning and listening.

Question 3

BPDU guard

Answer 3

BPDU guard is safety mechanism that places ports configured with STP portfast into an ErrDisabled state upon receipt of a BPDU.

BPDU guard is typically configured with all hist-facing ports that are enabled with portfast.

Question 4

Error Recovery Service

Answer 4

The Error Recovery service can be used to reactivate ports that are shut down for a specific problem, thereby reducing the administrative overhead.

Question 5

BPDU filter

Answer 5

BPDU filter simply blocks BPDU’s from being transmitted out a port.

Question 6

STP loop guard

Answer 6

STP loop guard prevents any alternative or root ports from becoming designated ports (ports toward downstream switches) due to loss of BPDU’s on the root port.

Question 7

Unidirectional Link Detection (UDLD)

Answer 7

UDLD allows for the bidirectional monitoring of fiber-optic cables. UDLD operates by transmitting UDLF packets to a neighbor device that includes the system ID and port ID.

• Normal mode
  If a frame is not acknowledged, the link is considered undetermined and the port remains active.
• Aggressive mode
  When a frame is now acknowledged, the switch sends another eight packets in 1-second intervals. If those packets are not acknowledged, the port is places into an error state.