difference b/w stp rstp pvst & mstp

Difference b/w STP, RSTP, PVST & MSTP

Difference between Spanning Tree Protocol (STP) and Rapid Spanning Tree

Protocol (RSTP)

1. The main difference between Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) and Spanning

Tree Protocol (STP IEEE 802.1D) is that Rapid Spanning Tree Protocol (RSTP IEEE 802.1W)

assumes the three Spanning Tree Protocol (STP) ports states Listening, Blocking, and Disabled are

same (these states do not forward Ethernet frames and they do not learn MAC addresses).

Hence Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) places them all into a new called

Discarding state. Learning and forwarding ports remain more or less the same.

2. In Spanning Tree Protocol (STP IEEE 802.1D), bridges would only send out a BPDU when they

received one on their Root Port. They only forward BPDUs that are generated by the Root Switch

(Root Bridge). Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) enabled switches send out

BPDUs every hello time, containing current information.

3. Spanning Tree Protocol (STP IEEE 802.1D) includes two port types; STP Root Port and Designated

Port. Rapid Spanning Tree Protocol (RSTP IEEE 802.1W) includes two additional port types called

as alternate ports and backup ports.

An alternate port is a port that has an alternative path or paths to the Root Switch (Root Bridge) but is

currently in a discarding state (can be considered as an additional unused Root Port). A backup port is a

port on a network segment that could be used to reach the root switch, but there is already an active

STP Designated Port for the segment (can be considered as an additional unused designated port).

Table View

STP (802.1d) Rapid STP (802.1w)

In stable topology only the

root sends BPDU and relayed by

others.

In stable topology all bridges generate BPDU every Hello (2 sec) : used

as“keepalives” mechanism.

Port states

Disabled

Blocking

Listening

Learning

Forwarding

Discarding (replaces disabled, blocking and listening)

Learning

Forwarding


To avoid flapping, it takes 3 seconds for a port to migrate from one protocol to another (STP / RSTP) in a

mixed segment.

Port roles

Root (Forwarding)

Designated (Forwarding)

Non-Designated (Blocking)

Root (Forwarding)

Designated (Forwarding)

Alternate(Discarding)Backup (Discarding)

Additional configuration to make

an end node port aport fast (in

case a BPDU is received).

- An edge port (end node port) is an integrated Link type which

depends on the duplex : Point-to-point for full duplex & shared for

half duplex).

Topology changes and convergence

Use timers for convergence

(advertised by the root):

Hello(2 sec)

Max Age(20 sec = 10 missed

hellos)

Forward delay timer (15 sec)

- Introduce proposal and agreement process for synchronization (< 1

sec).- Hello, Max Age and Forward delay timer used only for backward

compatibility with standard STP

Only RSTP port receiving STP (802.1d) messages will behaves as

standard STP.

Slow transition (50sec): Blocking

(20s) =>Listening (15s) =>Learning

(15s) =>Forwarding

Faster transition on point-to-point and edge ports only:Less states –

No learning state, doesn’t wait to be informed by others,

instead, actively looks for possible failure by RLQ (Request Link Query)

a feedback mechanism.

Use only 2 bits from the flag

octet:Bit 7 : Topology Change

Acknowledgment.Bit 0 : Topology

Change

Use other 6 bits of the flag octet (BPDU type 2/version 2):

Bit 1 : ProposalBit 2, 3 : Port roleBit 4 : LearningBit 5 : ForwardingBit 6

: AgreementBit 0, 7 : TCA & TCN for backward compatibility

The bridge that discover a change

in the network inform the root,

that in turns informs all others by

sending BPDU with TCA bit set and

instruct them to clear their DB

entries after “short timer”

(~Forward delay) expire.

TC is flooded through the network, every bridge generate TC

(Topology change) and inform its neighbors when it is aware of a

topology change andimmediately delete old DB entries.


If a non-root bridge doesn’t receive

Hello for 10*Hello (advertised from

the root), start claiming the root

role by generating its own Hello.

Wait for 3*Hello on a root port (advertised from the root) before

deciding to act.

Wait until TC reach the root + short

timer (~Forward delay) expires,

then flash all root DB entries

Delete immediately local DB except MAC of the port receiving the

topology changes (proposal)

Difference between PVST+ (Per VLan Spanning Tree) & MST (Multiple

Spanning Tree)

1. PVST+ (Per VLan Spanning Tree)- It's difference from 802.1Q CST because instead of having

a single root switch for all VLAN, The PVST+ mean STP is run in each VLAN.

2. MST (Multiple Spanning Tree)- MST maps multiple VLANs into a spanning tree instance,

with each instance having a spanning tree topology independent of other spanning tree

instances.This architecture provides multiple forwarding paths for data traffic, enables load

balancing, and reduces the number of STP instances required to support a large number of

VLANs. MST improves the fault tolerance of the network because a failure in one instance

(forwarding path) does not affect other instances (forwarding paths).

difference b/w stp rstp pvst & mstp

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