Spanning Tree Flashcards
How is the root bridge elected in STP?
Lowest bridge ID. Bridge ID is made up of lowest priority and MAC address.
What are the 3 port types of STP?
Designated Port = FWD Root Port = FWD Non Designated (Alternate) Port = Blocking
What ports are found on a root bridge?
All ports on the Root Bridge are Designated Ports
What ports are closest (facing) the Root Bridge?
Root Ports
What determines the closest path to a Root Bridge?
COST of the links it take to get back to the Root. 1G=4 100M = 19 10M= 100
What is the special frame used to negotiate STP?
BPDU (Bridge Protocol Data Unit)
What is the default STP Priority?
32768
What are the 4 STP Modes?
Blocking Listening Learning Forwarding
How long do each of the modes take?
Blocking 20 Sec Listening 15 Sec Learning 15 Sec Forwarding
What is happening during the listening mode?
The switch will only be sending and receiving BPDU’s
What is happening during the learning mode?
The switch will only be sending and receiving BPDU’s along with learning MAC address. No data transmission until in forward mode.
What are default Hello Times for STP?
2 seconds
What is the default Max age timers for STP?
20 Seconds
What is CST & PVST+ Reconvergence time?
50 Seconds Total
- 20 Sec Max Age Timer
- 15 Sec Listening
- 15 Sec Learning
What is the default STP mode for cisco switches?
PVST
What does PortFast do.
It skips the Listening and Learning stage of STP.
A port with Portfast will never create a topology change noticication (TCN).
When STP has to make a decesion, what 4 tie breakers will it use?
1. Lowest bridge ID: the switch with the lowest bridge ID becomes the root bridge.
2. Lowest path cost to root bridge: when the switch receives multiple BPDUs it will select the interface that has the lowest cost to reach the root bridge as the root port.
3. Lowest sender bridge ID: when a switch is connected to two switches that it can use to reach the root bridge and the cost to reach the root bridge is the same, it will select the interface connecting to the switch with the lowest bridge ID as the root port.
4. Lowest sender port ID: when the switch has two interfaces connecting to the same switch, and the cost to reach the root bridge is the same it will use the interface with the lowest number as the root port.
What is RSTP and PVRSTP average converge times?
2-3 seconds.
How do you enable PortFast?
under the interface
#spanning-tree portfast
or
Global#spanning-tree portfast Default (enable portfast on all access mode interfaces)
What are the steps of RSTP?
1, Discarding
- Learning
- Forwarding
Who send BPDU after the network has been converged in CST?
The root bridge generates BPDU’s and they are relayed by none root bridges.
Who generates BPDU’s in RSTP?
All Switches generate BPDU every two seconds (Hello Time)
How are BPDU’s used as keepalive mechanism in RSTP?
If a switch misses 3 BPDU from a neighbor it will assume connectivity to ths switch has been lost and it will remove all MAC address.
Name the two types of BPDU’s?
Type 1 : configuration BPDU
Type 2: TCN BPDU
