Difference between Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP)
- 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.
- 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.
- 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)
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Rapid STP (802.1w)
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In stable topology only the
root sends BPDU and relayed by others.
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In stable topology all bridges generate
BPDU every Hello (2 sec) : used as“keepalives” mechanism.
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Port states
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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.
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Port roles
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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).
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- 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).
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Topology changes and convergence
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Use timers for convergence
(advertised by the root):
Hello(2 sec)
Max
Age(20
sec = 10 missed hellos)
Forward delay timer (15 sec)
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- Introduce proposal and
agreement process for synchronization (< 1 sec).-
Hello, Max Age and Forward delay timer used only for backward compatibility
with standard STP
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Only RSTP port receiving STP
(802.1d) messages will behaves as standard STP.
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Slow transition (50sec):
Blocking (20s) =>Listening (15s) =>Learning (15s) =>Forwarding
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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.
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Use only 2 bits from the flag
octet:Bit 7 : Topology Change Acknowledgment.Bit 0 : Topology Change
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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.
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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.
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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.
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Wait for 3*Hello on a root port
(advertised from the root) before deciding to act.
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Wait until TC reach the root +
short timer (~Forward delay) expires, then flash all root DB entries
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Delete immediately local DB except
MAC of the port receiving the topology changes (proposal)
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Difference between PVST+ (Per VLan Spanning Tree) & MST (Multiple Spanning Tree)
- 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.
- MSTP (Multiple Spanning Tree Protocol)- 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).
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