Advanced STP features

Question 1

Which STP feature essentially disables STP on a port-by-port basis?

Answer 1

BPDU filter

Question 2

What command enables BPDU guard on all portfast ports?

Answer 2

spanning portfast bpduguard default

Question 3

Because of RLQs, backbone fast must be enabled on ___.

Answer 3

All switches in the network.

Question 4

What multicast address does uplink fast use?

Answer 4

0100.0ccd.cdcd

Question 5

When the root port goes down on a switch with uplink fast enabled, what will the switch do besides unblocking a port from its uplink group?

Answer 5

It will update neighboring switches of the change by sending multicast frames from every MAC address in its CAM table.

Question 6

Why does uplink fast increase switch and port cost when it’s enabled?

Answer 6

To prevent the switch from becoming the root, and to make it unlikely to be used by downstream switches. This is because uplink fast is meant for access layer

Question 7

Can you enable uplink fast on a per-VLAN basis?

Answer 7

No, it can only be enabled globally.

Question 8

Which campus layers should uplink fast be used on?

Answer 8

Access only, not on core or distribution.

Question 9

Describe an uplink group.

Answer 9

Any ports that lead to the root bridge, regardless of whether they’re blocked or not.

Question 10

Portfast can be configured in which two ways?

Answer 10

Globally or port-by-port.

Question 11

Can portfast be enabled on a trunk port?

Answer 11

Yes, but the “trunk” option must be added to the end of the command or it will not work.

Question 12

What does PortFast do?

Answer 12

Allows an access port to go directly from blocking to forwarding.

Question 13

Briefly describe BPDU filtering.

Answer 13

Configured globally or per-port, BPDU filtering stops BPDUs from being received or forwarded out any portfast-enabled ports.

Question 14

Briefly describe BPDU guard.

Answer 14

Configured per-port, BPDU guard helps to protect against rogue network devices by setting a port to err-disable when a BPDU of any type is received on it.

Question 15

Describe how root guard works.

Answer 15

A per-port configuration that tells a switch to ignore incoming superior BPDUs, thus making sure the root bridge does not change if a rogue switch is added to the network. If a superior BPDU is received, it’s discarded and the port is put into root-inconsistent state until the superior BPDUs cease.

Question 16

Describe how RLQ works.

Answer 16

When a switch detects an indirect link failure, a RLQ BPDU will be sent out any port actively recieving BPDUs. The receiving switch will validate the root bridge specified in the RLQ against its own database. If the roots match, the RLQ is forwarded to the root bridge and finally returned. If the root bridges don’t match, the RLQ will be returned to the switch generating it and the STP election process will begin again.

Question 17

Briefly define RLQ.

Answer 17

Root link query. Used by a switch detecting an indirect link to ensure that it still has a path to the root.

Question 18

In backbone fast, why does eliminating the max age time help a switch to deal with indirect link failures?

Answer 18

This allows a blocked port (leading to switch generating inferior BPDUs) begin the STP port process 20 seconds faster, allowing the superior BPDU to be forwarded out that port.

Question 19

Briefly define backbone fast.

Answer 19

Helps switches to recover from indirect link failures by eliminating the max age time. Also sends RLQ BPDUs to ensure that the switch detecting the failure has a good path to the root.

Question 20

When uplink fast is enabled, what two actions immediately take place?

Answer 20

1. Switch priority is set to 49152. 2. STP port cost is increased by 3000.

Question 21

In uplinkfast, how long will it take for the primary path to be used once it’s come online again?

Answer 21

2 X forward delay; plus five seconds.

Question 22

Briefly explain uplinkfast.

Answer 22

Like portfast for closets. Adds potential paths to the root into an “uplink group”. If the main path to the root goes down, a port from the uplink group is immediately moved to the forwarding stage.

Question 23

How would you enable portfast globally?

Answer 23

Spanning portfast default.

Question 24

Briefly define portfast.

Answer 24

Allows an access port to go immediately from blocking to forwarding.

Question 25

Briefly describe loop guard.

Answer 25

Enabled globally or per-port, loop guard protects against damaged cables that may cause a switching loop.

Question 26

Describe how loop guard works.

Answer 26

Meant to be enabled on any port that should be receiving BPDUs, loop guard will cause a port that stops receiving BPDUs to enter loop-inconsistent mode.

Question 27

Loop guard operates on a ___ basis.

Answer 27

Per-VLAN.

Question 28

Describe loop-inconsistent mode.

Answer 28

Like blocking mode, loop-inconsistent mode keeps a port from sending or receiving frames. Once BPDUs begin to be received on the port again, it will work it’s way back towards forwarding mode automatically.

Question 29

Describe UDLD aggressive mode.

Answer 29

UDLD frames are sent every second. If eight frames are not returned, the port will be put into err-disabled mode.

Question 30

Describe UDLD normal mode.

Answer 30

UDLD frames are sent every 15 seconds. If a frame is not returned, a syslog message is generated.

Question 31

Describe how UDLD works.

Answer 31

Configured per-port or globally, a UDLD enabled port will send out a UDLD frame with information about itself. The receiving port will return the frame, ensuring a bi-directional link. Must be configured on both ends. Comes in two flavors, normal and aggressive.

Question 32

Briefly describe UDLD.

Answer 32

Unidirectional link detection. Used to ensure that bidirectional links (usually fiber) are fully functional.

Question 33

Briefly describe the err-disabled state.

Answer 33

Usually caused by some kind of a security violation, a port in err-disabled state will not forward or receive packets. You may clear an err-disabled port by shut/no shut on the port.

Question 34

Describe the root-inconsistent state.

Answer 34

Similar to a listening state, root-inconsistent will receive and forward BPDUs (except for superior ones) and not pass frames.