STP Flashcards
What is the IEEE standard for:
STP
PVSTP
RSTP
802.1D
Cisco Proprietary
802.1w
What does RSTP stand for?
Rapid Spanning Tree Protocol
What STP mode do Cisco switches generally default to?
802.1D with PVST
Command to set a switch to use PVST
spanning-tree mode pvst
What 3 fields make up the BID?
- Priority
- System ID Extension
- System ID
What is the range for priority?
0-65535
What is unusual about the priority value?
It’s in multiples of 4096.
What is the value of an all 1’s priority field?
61140
What are the values of each bit in the priority field?
32768 16384 8192 4096
What is normally stored in the System ID extension field?
VLAN ID
How many bits in the System ID extension field?
12
How many bits in the priority field?
4
How many bits in the System ID field?
6 bytes
What’s in the System ID field?
The BIA of the switch
Is the System ID configurable?
No, that’s supplied automatically by the switch.
What is the decimal value of the first 2 bytes of the BID if the priority field is 0010 and the vlan is 12?
8204
Command to set the priority for a given VLAN
spanning-tree vlan vlan id priority priority
Command to set a port cost.
Conf t
interface gi1/0
spanning-tree [vlan vlan id] cost cost
What is the default BID priority?
32768
Is portfast or bpduguard enabled by default?
No
What are the 3 port types in STP?
- Root port
- Designated port
- Blocking port
What is a Designated port vs other port types?
It’s a port that is not the closest to the root switch (root port), but is still forwarding and not blocking (blocking port). This is also related to the fact that this port is where the root switch’s BPDU will be forwarded out of.
What state are all the ports on the root switch in?
They are all forwarding.
What types of ports (in STP language) does the root switch have: root, designated, blocking, disabled.
Designated