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Monday, May 27, 2013

Differences between Rapid STP (802.1w) and the legacy STP(802.1d)

The following table outlines the main differences between Rapid STP (802.1w) and the legacy STP(802.1d):

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
DisabledBlockingListeningLearningForwardingDiscarding (replaces disabled, blocking and listening)
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)
Root (Forwarding)
(Discarding)Backup (Discarding)
Additional configuration to make an end node port a port 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):
(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 ChangeUse 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 and immediately 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 entriesDelete immediately local DB except MAC of the port receiving the topology changes (proposal)


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