spanning tree Flashcards
(29 cards)
What is the purpose of spanning tree protocol?
(IEEE 802.1D) provide redundancy and prevent broadcast storms (layer 2 loops)
Designated Port
port in a forwarding state
Blocked ports
Ports that have been blocked in order to prevent loops. Can be switched to a forwarding state if designated port goes down. Can send or receive BPDUs only
How does a switch know that another switch is connected to it?
STP switches sends out “hello” BPDUs every 2 secs and switches receiving it know
How is it determined which switch is the root bridge?
In order of precedence
1. Bridge ID
2. MAC address
What is special bout the root bridge?
all ports on it are in a forwarding state and all other switches must have a path to it
PVST
Per VLAN Spanning Tree. this is Cisco’s version that allows for STP for each VLAN
Root port
non root switches have one of their interfaces as a root port and it is the one with the lowest cost
How do you determine which port should be the designated port?
- lowest root cost
- lowest bridge ID
What are transitional states in STP and what are the two of them?
States the interface passes through when the interface is turned on or moved from blocking to forwarding.
1. Listening
2. Learning
What is listening state and how long does it last?
15 secs default, the interface will forward BPDU traffic only
What is learning state and how long does it last?
15 secs default, The interface will forward BPDU traffic and also learn MAC address
Forward Delay
How long the interface goes through the listening and learning state before getting into forwarding
How does the switch react to not receiving BPDUs on an interface?
If an interface does not get a hello BPDU in 20s (default), it will start changing its topology (blocking -> designated or vice versa). Resets timer if recieves BPDU
PortFast
A special feature that allows an interface to transition immediately into a forwarding state without the transitional states. recommended for end hosts only
BPDUguard
a feature that will shut down the interface if an unauthorized switch tries to send a BPDU into the network
RootGuard
The designated root bridge will not change even if a switch with a lower bridge ID is added
Loopguard
Even if a interface stops receiving BPDUs, it will not start forwarding (goes to broken/root inconsistent state)
BPDU filter
port will not send, recieve, or forward BPDUs
how do you manually change the root bridge?
spanning-tree vlan num root primary
How do you create a secondary root bridge (if first fails)
spanning-tree vlan num root secondary
What’s special about Multiple Spanning Tree Protocol?
can group multiple vlans into different instances
What’s special about Per VLan Spanning Tree Plus Protocol?
load balance by blocking different ports for different vlans
Discarding state
port is disabled or blocking traffic in RSTP
