1 - Cisco Icnd2 - Stp

Front 1

Three classes of problems solved by STP

Back 1

1. Broadcast storms
2. Mac table instability
3. Multiple frame transmission

Front 2

IEEE designation for STP

Back 2

802.1d

Front 3

STP - Three criteria to put a port in forwarding state

Back 3

1. Port is on root switch
2. Port is root-port (lowest cost path to root)
3. Port is designated port (port on switch with lowest cost back to root)

Front 4

STP - The bridge ID is made up of…

Back 4

2-byte priority followed by 6-byte MAC address

Front 5

STP - Which switch is elected as root?

Back 5

The switch with lowest bridge id

Front 6

STP - What is a BPDU?

Back 6

Bridge Protocol Data Unit
(name for STP messages)

Front 7

STP - What are the fields in a Hello BPDU?

Back 7

1. Root bridge id
2. Sender's bridge id
3. Cost to reach root.
4. Timers: Hello timer, MaxAge timer, Forward Delay timer

Front 8

STP - What is a root port?

Back 8

For non-root switches, one and only one port has the lowest-cost path back to root.

Front 9

STP - What is a designated port?

Back 9

For each LAN segment, the switch port that advertises the lowest-cost Hello BPDU onto that segment. In case of tie, lower bridge ID wins.

Front 10

STP - What are the default port costs? (original/revised)

Back 10

10 Mbps - 100/100
100 Mbps - 10/19
1 Gbps - 1/4
10 Gbps - 1/2

Front 11

STP - How often does root switch send Hello BPDU

Back 11

Every 2 seconds by default
Note: only the root switch sends Hello BPDUs

Front 12

STP - What is the default MaxAge timer and how is it used?

Back 12

10 times Hello, normally 20 seconds
Defines how long a switch should wait before trying to change STP topology

Front 13

STP - What is the default forward delay timer?

Back 13

15 seconds. Number of seconds that a port stays in the listening state, then learning state.

Front 14

STP - What are the four possible port states and which are stable?

Back 14

Blocking (stable)
Listening
Learning
Forwarding (stable)

Front 15

STP - What does the listening state do?

Back 15

Does not forward frames. Old, incorrect MAC table entries are timed-out.

Front 16

STP - What does the learning state do?

Back 16

Does not forward frames, but begins to fill in the MAC address table

Front 17

STP - By default, what is the total time for convergence?

Back 17

50 seconds
MaxAge (20 seconds) + Listening (15 seconds) + Learning (15 seconds)

Front 18

STP - in what state(s) will a port forward frames?

Back 18

Only in the forwarding state.

Front 19

STP - What is PortFast?

Back 19

Allows a switch to place a port immediately into the forwarding state. Appropriate for connection to end-user devices.

Front 20

STP - What is BPDUGuard?

Back 20

Disables a port if BPDUs are received. Appropriate for access ports.

Front 21

STP - How does Cisco modify the bridge ID for per VLAN STP?

Back 21

First 4 bits are the priority (multiple of 4096)
Next 12 bits are the System ID extension (typically VLAN ID)
6 byte MAC address

Front 22

STP - CLI commands to set bridge priority

Back 22

#spanning-tree vlan <id> root {primary | secondary}
#spanning-tree vlan <id> priority <value>

Front 23

STP - CLI command to set interface cost

Back 23

#spanning-tree vlan <id> cost <value>

Front 24

STP - CLI command to set PortFast

Back 24

#spanning-tree portfast

Front 25

STP - CLI command to set BPDUGuard

Back 25

#spanning-tree bpduguard

Front 26

STP - CLI command to show spanning-tree info

Back 26

#show spanning-tree vlan <vlanid>

Front 27

STP - CLI command to show spanning-tree roots

Back 27

#show spanning-tree root

Front 28

STP - CLI command to debug spanning tree

Back 28

#debug spanning-tree events

Front 29

STP - CLI command to configure Rapid PVST

Back 29

#spanning-tree mode rapid-pvst

Front 30

STP - What is UplinkFast?

Back 30

For access-layer switches, use uplinkfast on ports that uplink to distribution routers for faster reconvergence (2-4 seconds)

Front 31

STP - (802.1D): Which switches will send Hellos once the network is stable?

Back 31

Only the root

Front 32

STP: Two commands that will show the STP status (forwarding or blocking) for an interface?

Back 32

#show spanning-tree
#show spanning-tree interface <fa0/1>

Front 33

STP: How is the cost of a forwarded HELLO message calcuated?

Back 33

Cost of received hello plus cost of port on which it was RECEIVED (the return path).

Front 34

STP: Will a switch forward HELLO messages out its root port if they were received on some other port?

Back 34

No.