NEO Training Center | Học CCNA tốt nhất – Rapid STP
Rapid Spanning Tree Protocol
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RSTP (Rapid Spanning- Tree Protocol)

  • RSTP was originally defined in the IEEE 1w standard to provide significant faster convergence while still backwards compatible.
  • RSTP was later incorporated as section 17 in IEEE 802.1D-2004 standard for
  • The remainder of this section will reflect on the 802.1w



> RSTP uses type 2 (a.k.a. version 2) BPDUs, which makes it easy for a 802.1w bridge to detect legacy STP bridges.

> With RSTP BPDUs are generated by every bridge every hello interval (default 2 seconds) and are used as keepalives between bridges.

> The most recent BPDU received on a port is stored for up to the max-age timer.

> With RSTP the stored BPDUs are generated by the neighboring bridges and not by the root bridge, meaning they are not validated.

> An inferior BPDU, contains information about the root bridge that is worse than the BPDU currently stored for the port it was received on.

> A superior BPDU, contains information about the root bridge that is better than the BPDU currently stored for the port it was received on.

> A superior BPDU received on a port, results in the previous BPDU being overwritten and the port promoted to root/designated port.



– BPDU Fast Aging

> When three consecutive hellos from a neighbor are not received, the max-age timer expires.

> This results in the stored BPDU previously received from that bridge to be aged out immediately.



– RSTP Port Roles

> Root port

>> Remains the same as the 802.D root port.

> Designated port

>> Remains the same as the 802.D designated port.

> Alternate port

>> Is a discarding port with an alternate (inferior) path to the root bridge which can be used in the event that the root port fails.

> Backup port

>> Is a discarding port on the same segment as the designated port, since it has an inferior port path cost.

>> Think of it as a backup designated port.


RSTP Port States

> Discarding

>> Incoming data frames are simply dropped; no MAC addresses are learned.

>> Combines the 802.1D (STP) disabled, blocking and listening states.

> Learning

>> Incoming data frames are dropped, but MAC addresses are learned.

> Forwarding

>> Incoming data frames are forwarded according to MAC addresses that have been (and are being) learned.



– RSTP Port Types

> Edge Ports

>> Incorporates the Cisco Portfast extension.

>> Edge ports are intended to connect to hosts or end stations only.

>> Edge ports immediately transitions to the forwarding state, skipping the forwarding delay.

>> If a BPDU is received on an edge port, the port will be treated a normal spanning-tree port.

> Point-to-Point Ports

>> Ports in full-duplex state are assumed to be point-to-point ports.

>> Immediately transitions to the forwarding state, skipping the forwarding delay.

>> Can be configured with “spanning-tree link-type”.

> Shared Ports

>> Is a port connecting to a shared network with multiple bridges.

>> A half-duplex port is considered to be a shared port.

>> Will use the traditional STP listening and learning mechanism.


– RSTP Rapid Transitioning to Forwarding State

> Rapid state transitioning is one of benefits of RSTP, since a port transitioning does not have to wait for the forwarding delay timers.

> Only edge ports and point-to-point port types are candidates for rapid transitioning.

> Let’s take a look at how rapid transitioning works:

>> When a port comes up, the port is placed in the designated block state.

>> In this state the bridge waits to receive BPDUs.

>> When a superior BDPU is learned (with the Proposal flag) on the current non-root port, a new root port is discovered.

>> This initiates an operation called Sync.

>> The local port then becomes the new root port, while the upstream port is in designated blocking.

>> During the Sync operation non-edge designated ports are placed in a blocking state.

>> Once the non-edge designated ports are blocked, the local bridge authorizes the upstream bridge to immediate transition its port to designated forwarding state.

>> This is done by setting the Agreement bit in the outgoing BPDU.

>> The local bridge generates new BPDUs, sets the Proposal flag and sends them out its designated blocked ports.

>> If the receiving downstream bridges considers the new BPDU as superior, the above process is repeated.

>> If not the port between the bridges is blocked on one end.

>> Edge ports remain in the forwarding state.

RSTP Sync Process (Proposal and Agreement)

1. Switch will elect a Root Port
2. All of the non-edge ports on this switch are marked as designated but they are all in discarding state.
3. Next Proposals are sent out of these designated ports, the proposal sets the port role to designated and contains the root bridge information
4. Downstream switches receive the proposal and they either agree to this information or they disagree to the proposal and send back the better information they have ( that is when they have better path to the Root switch)
4a. If the downstream switches agree to the proposal, they send an agreement back, upon receiving the agreement the switch will unblock the designated port and transitions it from discarding to forwarding state.
4b. If the downstream switch denies the proposal and sends a better Root path information then the local switch will change its Root Port.

The Sync Process in RSTP prevents the temporary loops by blocking the designated ports initially until the proposal and agreement process completes and it also ensures that all switches in the topology agree to the same Root Switch.

In RSTP, if the Root Port fails then the Alternate Port takes over and synchronizes this information with downstream switches. But if there is no Alternate port then the local switch will declare itself as the root switch and then syncs with the downstream switches and will adapt to the better Root  information sent by downstream switches, if downstream switches have a better Root Path information else this local switch becomes the Root Switch and downstream the topology undergoes the sync process again until all switches in the topology learn the new Root information.

Note: Also when the downstream switch agrees to the proposal then it goes through the same process again
1. Select the Root Port
2. Put all non-edge ports in designated but discarding state
3. start the Sync Process on all designated Ports






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