SPANNINIG TREE

WHAT IS SPANNING TREE ? 

The purpose of the protocol is to have bridges dynamically discover

 a subset of the topology that is loop-free (a tree) and yet has just

 enough connectivity so that where physically possible, there is a

 path between every switch.

Several flavors:

Traditional Spanning Tree (802.1d)

Rapid Spanning Tree or RSTP (802.1w)

Multiple Spanning Tree or MSTP (802.1s)

STP Variants:

On Cisco hardware, Spanning-Tree works in proprietary mode with

 either PVST+ (Per-VLAN Spanning Tree) or PVRST+ (Per-VLAN 

Rapid Spanning Tree).

Nom	Standard
Spanning-Tree (STP)	IEEE 802.1D
PVST+	STP Cisco
Rapid Spanning-Tree (RSTP)	IEEE 802.1w
PVRST+	RSTP Cisco
MIST	IEEE 802.1s

États Spanning-Tee (802.1d)	États Rapid STP (802.1w)
Blocking	Discarding
Listening	Discarding
Learning	Learning
Forwarding	Forwarding

Traditional Spanning Tree (802.1d)

Switches exchange messages that allow them to compute the Spanning Tree

These messages are called BPDUs (Bridge Protocol Data Units)

Two types of BPDUs:

Configuration

Topology Change Notification (TCN)

First Step: 

Decide on a point of reference: the Root Bridge

The election process is based on the Bridge ID, which is composed of:

The Bridge Priority: A two-byte value that is configurable

The MAC address: A unique, hardcoded address that cannot be changed.

Root Bridge Selection (802.1d)

Each switch starts by sending out BPDUs with a Root Bridge ID equal to its own Bridge ID

I am the root!

Received BPDUs are analyzed to see if a lower Root Bridge ID is being announced

If so, each switch replaces the value of the advertised Root Bridge ID with this new lower ID

Eventually, they all agree on who the Root Bridge is

Now each switch needs to figure out where it is in relation to the Root Bridge

Each switch needs to determine its Root Port

The key is to find the port with the lowest Root Path Cost

The cumulative cost of all the links leading to the Root Bridge

Each link on a switch has a Path Cost

Inversely proportional to the link speed

e.g. The faster the link, the lower the cost 

Root Path Cost is the accumulation of a link’s Path Cost and the Path Costs learned from neighboring Switches.

It answers the question: How much does it cost to reach the Root Bridge through this port?

1-Root Bridge sends out BPDUs with a Root Path Cost value of 0

2-Neighbor receives BPDU and adds port’s Path Cost to Root Path Cost received

3-Neighbor sends out BPDUs with new cumulative value as Root Path Cost

4-Other neighbor’s down the line keep adding in the same fashion

On each switch, the port where the lowest Root Path Cost was received becomes the Root Port

This is the port with the best path to the Root Bridge

 

Electing Designated Ports (802.1d)

OK, we now have selected root ports but we haven’t solved the loop problem yet, have we

The links are still active!

Each network segment needs to have only one switch forwarding traffic to and from that segment

Switches then need to identify one Designated Port per link

The one with the lowest cumulative Root Path Cost to the Root Bridge

Two or more ports in a segment having identical Root Path Costs is possible, which results in a tie condition

All STP decisions are based on the following sequence of conditions:

Lowest Root Bridge ID

Lowest Root Path Cost to Root Bridge

Lowest Sender Bridge ID

Lowest Sender Port ID

Blocking a port

Any port that is not elected as either a Root Port, nor a Designated Port is put into the Blocking State.

This step effectively breaks the loop and completes the Spanning Tree.

Spanning Tree Protocol States

Disabled

Port is shut down

Blocking

Not forwarding frames 

Receiving BPDUs

Listening

Not forwarding frames 

Sending and receiving BPDUs

Learning

Not forwarding frames

Sending and receiving BPDUs

Learning new MAC addresses

Forwarding

Forwarding frames

Sending and receiving BPDUs

Learning new MAC addresses

STP Topology Changes

Switches will recalculate if:

A new switch is introduced

It could be the new Root Bridge!

A switch fails

A link fails

Root Bridge Placement

Using default STP parameters might result in an undesired situation

Traffic will flow in non-optimal ways

An unstable or slow switch might become the root

You need to plan your assignment of bridge priorities carefully

Protecting the STP Topology

ome vendors have included features that protect the STP topology:

Root Guard

BPDU Guard

Loop Guard

UDLD

Etc.