802.1w – RSTP
Rapid Spanning Tree is an updated version of the original Spanning Tree protocol, standardized as 802.1w. It includes many of the Cisco proprietary features of PVST+. RSTP is backwards compatible with 802.1d STP but it has a few new features. To enable RSTP, use:
New features in RSTP
RSTP Port States
RSTP uses only 3 states:
Discarding – Port is not forwarding data so it breaks the loops.
Thi state is seen in a stable active topology and during topology synchronization
Learning – Port is in a transition state from DIS to FWD. The learning state accepts data frames to populate the MAC table to limit flooding of unknown unicast frames.
This state is seen in a stable actice topology an during topology synchronization
Forwarding – Port is forwarding data.
This state is seen only in a stable active topology
RSTP Port Roles
Root Port – Same as in 802.1d – The port that receives the best BPDU on a switch is the Root Port. It is considered the closest port to the Root Bridge
Designated Port – Same as in 802.1d – On any segment, the port that sends the best BPDU is considered the Desingated Port.
Alternate Port – Alternate to a Root Port – BPDUs from the Root Bridge can be received on other ports than the Root Port, but they are normally disabled. Still, they can offer an alternate path to the Root Bridge, in case the Root Port fails. This is similar to the UplinkFast feature of PVST+. When the path through the Root Port fails, the switch can quickly chose one of the Alternate Ports as the new Root Port.
Backup Port – Backup of a Designated Port – On a segment, ports on the same switch as the Designated Bridge can be used as Backup ports of the Designated Port. This means that they can offer a backup path for that segment, but they cannot guarantee a different path towards the Root Bridge
RSTP BPDU Format
802.1D BPDUs are sent with Version set to 1 and Type set to 1. RSTP BPDUs are sent with Version set to 2 and Type set to 2. Legacy bridges will drop these frames. Alos, new flags are implemented in the Flags Byte, that encode the Role and State of the sending port:
| Bit | Function |
|---|---|
0 | Topology Change |
1 – NEW | Proposal |
2,3 – NEW | 00 – Unknown 01 – Alternate / Backup 10 – Root 11 – Designated |
4 – NEW | Learning |
5 – NEW | Forwarding |
6 – NEW | Agreement |
7 | Topology Change ACK |
New BPDU Handling
In 802.1d, a non-root bridge would only generate BPDUs when it received one on its root port. In 802.1w, a bridge sends BPDUs every Hello Time (default: 2sec), regardless of the BPDUs it receives from the root bridge. In 802.1d, BPDUs had to go missing for Max Age(default: 20 sec) before the BPDU information was aged out for that port and new information would be processed. In 802.1w, the BPDUs act as a keepalive mechanism, so if 3 BPDUs are missed, the bridge considers the connection to its neighbor down and it ages out BPDU information for that port.
Uses a Backbone Fast mechanism by default
RSTP includes a mechanism similar to the Backbone Fast enhancement in STP that will work by default without any additional configuration.
Rapid Transitions to Forwarding State
Rapid Transition is achieved on Edge Ports and on Point to Point links.
Edge Ports
An Edge Port is similar to a PVST+ PortFast port.
An Edge Port does not generate TCN when its link flaps.
An Edge Port that receives a BPDU loses its Edge status
To set an Edge Port, use the same command as in PVST+:
Link Type
All Links fall in 2 categories in RSTP. By default, all full duplex links are considered Point to Point, while all Half Duplex links are considered Shared Links. They can also be manually set, using:
To verify the link type, use:
New TCN Mechanism
In RSTP, only non-Edge ports that are moving to a FWD state will cause a TCN. When a bridge sends a TCN, it first starts a TC While timer, equal to 2xHello Time. Over this period, it sends TCN BPDUs out all its non-Edge Designated and Root Ports. It also flushes the MAC addresses associated with these ports. When a bridge receives a BPDU with TC bit set, it starts a TC While timer, equal to 2xHello Time. Over this period, it sends TCN BPDUs out all its non-Edge Designated and Root Ports, except the port where it received the TCN. It also flushes the MAC addresses associated with these ports. This way, the TCN is quickly flood and there is no need for the TCN to travel upstream to the Root and then downstream, as in 802.1D STP.
Proposal/Handshake Mechanism
Convergence in RSTP is achieved using a Link-by-link handshake mechanism. On each link, when it comes up, both ports are set to a Designated Role and to a BLK State. The 2 bridges send each other BPDUs with the Proposal bit set. The superior bridge (the Root Bridge or the Designated Bridge on the path to the Root) will transition its designated port to a FWD state only after the inferior bridge will be “in sync”. Being “in sync” means the inferior bridge will put all its non-Edge Designated Ports into a BLK state and will start a similar Handshake mechanism Downstream. When all non-Edge Designated Ports are blocked, the first port can transition to a Root Port Role and a FWD State and will send an Agreement BPDU to the superior bridge. The superior bridge can now transition its port into a FWD State. This way the loops are prevented and the “sync” boundary moves downstream to the other bridges. As you can see, before agreeing to the RSTP proposal, a bridge must have all other ports “in sync”. A port is considered “in sync” if it is an Edge Port or if it is in a BLK State. If ports are not in BLK, they are transitioned to BLK in order to agree to the proposal.
This kind of mechanism is faster because it doesn’t use timers like in 802.1d STP, but it can only take place over P2P links. On Shared links, the protocol falls back to 802.1d timers mechanism. This also happens if no agreement is received.
Compatibility with 802.1d STP
802.1d Bridges do not understand RSTP BPDUs, but RSTP bridges understand STP BPDUs. When a RSTP bridge detects an 802.d BPDU, it falls back to using 802.1d mechanism on that port.
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