How is a bridging loop formed?
a. A loop formed between switches for redundancy.
b. A loop formed by STP
c. A loop formed between switches where frames circulate endlessly
d. The round-trip path a frame takes from source to
Which of these is one of the parameters used to elect a root
a. Root path cost
b. Path cost
c. Bridge priority
d. BPDU rev number
If all switches in a network are left at their default STP values, which one of the following is not true?
a. The root bridge will be the switch with the
lowest MAC address.
b. The root bridge will be the switch with the
highest MAC address.
c. One or more switches will have a bridge priority of 32,768.
d. A secondary root bridge will be present on the network.
Configuration BPDUs are originated by which of the following?
a. All switches in the STP domain
b. Only the root bridge switch
c. Only the switch that detects a topology change
d. Only the secondary root bridge when it takes
Which of these is the single most important design decision
to be make in a network running STP?
a. Removing redundant links
b. Making sure all switches run the same version of
c. Root bridge placement
d. Making sure all switches have redundant links
What happens to a port that is neither a root port now a
a. It is available for normal use
b. It can be used for load balancing
c. It is put into the Blocking state
d. It is disabled
What is the max number of root ports that a Catalyst switch
What mechanism is used to set STP timer values for all
switches in a network?
a. Configuring the timers on every switch in the
b. Configuring the timers on the root bridge switch
c. Configuring the timers on both primary and
secondary root bridge switches
d. The timers can’t be adjusted
MAC address can be placed into the CAM table, but no data can be sent or received if a switch port is in which of the following STP
What is the default hello time for IEEE 802.1D?
a. 1 sec
b. 2 sec
c. 30 sec
d. 60 sec
Which of the following is the STP defined in the IEEE 802.1Q
If a switch has 10 VLANS defined and active, how many instances of STP will run using PVST+ versus CST?
a. 1 for PVST+, 1 for CST
b. 1 for PVST+, 10 for CST
c. 10 for PVST+, 1 for CST
d. 10 for PVST+, 10 for CST
What is the IEEE standard for CST?
Define the following:
Common Spanning Tree. STP runs on the native VLAN. runs over 802.1Q
Per VLAN Spanning Tree Cisco Proprietary, runs STP for each VLAN. Works on ISL only (Inter Switch Link)
Per VLAN Spanning Tree +. Another Cisco proprietary, allows interaction with both CST (over 802.1Q, aka trunking) and PVST (over ISL).
what multicast MAC address is used by STP?
Bridge Protocol Data Unit.
What are the 2 types of BPDU's?
Configuration BPDU - used for STP computation
Topology Change Notification (TCN) BPDU - used to announce changes in NW topology
by default, hos often are BDDUs sent
every 2 seconds
How does the root bridge get elected?
Describe Bridge ID composition.
Lower Bridge ID wins.
- Bridge ID = Bridge Priority (2 bytes) def=32,768 (0 - 65,535)
- MAC address (6 bytes) can't be changed.
Note: every switch backplane has 1,024 pool addresses for use by the switch.
Define the components of STP.
Root bridge - get elected (lowest bridge ID wins).
Root port - gets elected, as the POC to talk to the root bridge. based on root path cost. cost equates to speed. faster path cost wins.
- Designated port - election is based on 4 things.
- 1. Lowest root bridge ID
- 2. Lowest root path cost to the root bridge
- 3. Lowest sender bridge ID
- 4. Lowest sender port ID
- All root bridge ports are designated ports.
- Root path cost is cumulative. cost field in BPDU is incremented as it passes through the switch.
If computing root path cost manually, increment the cost as the BPDU is received.
What are the 5 STP states, and their timing values?
Disabled - admin down. Not part of STP.
Blocking - every port starts in the Blocking state. can't tx or rx data. receives only BPDUs. Standby ports are also in the Blocking state.
Listening - Can't tx or rx data. can can TX and RX BPDUs. Uses STP to try and become root port or designated port. If port loses root port or designated port status, it returns to blocking. Uses forward delay timer value. (def 15 secs).
Learning - Still can tx and rx BPDUs (no data yet), plus can learn new MAC addresses. Uses forward delay timer value. (def 15 secs).
Forwarding - port can now tx and rx data. BPDUs still
What command allows you to view STP port states?
sh spanning int fa 0/1
What are the STP timer values?
Hello time - 2 secs
Forward delay - 15 secs - used by listening and learning states
Max age - 20 secs - the time interval the switch stores a BPDU before discarding it. This allows the network to re-calculate STP if the topology changes.
so, 15 *2 plus 2 plus 20 = 52 secs max that a network can have a topology change before it re-converges.
All timer values are based on a network diameter of 7 switches.
What happens when a topology change occurs?
A TCN BPDU is generated. Caused by sw moving port into fwd state or from fwd state to Learning or Blocking.
Continues to send TCNs until ack-ed from upstream neighbor.
root bridge sets TCN flag in its config BPDU to let all other sw's know topology has changed. Other sw's change bridge table ageing times from 300 secs (def) to FFWD delay time. This flushes the MAC addresses. duration is fwd delay & max age.
Note. If switchport is in portfast, TCN BPDUs will not be generated.
1. Direct Topology change
2. Indirect Topology change
3. Insignificant Topology change
1. One that can be detected on a switch interface. Link goes down.
2. Link is still up, but no traffic is received. traffic is either filtered or firewalled. no BPDUs are received.
3. A end user reboots his PC. TCN BPDU is generated. Root switch sends out Config BPDU, and all SWs flush idle mac-address entries from CAM tables. Portfast on the end user port keeps this from happening.