Mod 5: STP Concepts

What can having redundant connections do to a L2 nework?

Cause loops

What is STP?

STP is a loop prevention network protocol

What is the protocol for STP?

802.1D

What is a broadcast storm?

When an extremely large amount of broadcasts flood the network

What does STP do?

It blocks redundant connections to prevent L2 loops

What kind of frames can cause loops in a network?

Unknown unicast, multicast, bradcast

What are the 4 steps in setting up STP?

Elect the root bridge, elect the root ports, elect designated ports, elect alternate ports

What do switches use to elect the root bridge, ports, designated ports, and alternate ports?

BPDU

What are the 4 fields in a BPDU?

Bridge priority, extended system ID, Mac address

What determines what switch becomes the root switch?

Having the lowest BID

What priority do all switches start with?

32768

How is the cost of a root path determined?

By the sum of all port costs on the way to the root

What are all the ports on the root switch?

Designated ports

What are the 3 types of ports in STP?

Root, designated, blocked

What 3 criteria does STP use when a switch has multiple equal cost paths to the root bridge?

Lowest sender BID, lowest sender priority, Lowest sender port ID

What is the default port priority?

128

What 3 timers does STP need?

Hello timer, forward delay timer, max age timer

What is the hello timer? What is the default?

The time interval between BPDUs, 2 secs

What is the forward delay timer? What is the default?

The time spent in listening and learning states, 15 secs

What is the max age timer? Whats the default?

The max amount of time a switch waits before attempting to change the STP topology, 20 secs

What are the 5 port states STP has?

Blocking, listening, learning, forwarding, disabled

What is PVST?

Per VLAN spanning tree has a root bridge for each VLAN

What is RSTP?

Rapid spanning tree protocol increases the speed of recalculation

What is the protocol for RSTP?

802.1W

What is Cisco’s version of RSPT?

Rapid PVST+

What are the 3 port states in RSTP?

Discarding, learning, forwarding

What are the 4 types of ports in RSTP?

Root, designated, backup, alternate

What is PortFast?

Transitions a port from listening/learning to forwarding instantly

What ports should PortFast be enabled on?

Access ports

What is BPDU guard?

Immediately shuts down the port in an errdisabled state on the receipt of any BPDU

What problem does PortFast solve?

Hosts having to wait 30 seconds for switches to discuss paths before being able to get an IP address