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  • Table of Contents
  • Layer 2 Technologies
    • Ethernet Switching
      • L2 Switch Operations
      • Spanning Tree
        • 802.1d – STP
        • 802.1w – RSTP
        • 802.1s – MSTP
      • VTP 101
      • Private VLANs
      • VLANs
      • EtherChannel 101
    • Layer 2 WAN Protocols
      • HDLC
        • HDLC 101
      • PPP
        • PPP 101
        • PPP Authentication - PAP
        • PPP Authentication – CHAP
        • PPP Authentication – EAP
        • PPP Multilink
        • PPPoFR – PPP over Frame Relay
        • PPPoE – PPP over Ethernet
      • Frame Relay
        • Frame Relay 101
        • Frame Relay 102
        • Frame Relay Encapsulations – IETF vs Cisco
        • Multilink Frame Relay
        • Frame Relay Switching
        • Routing over Frame Relay
      • Bridging
        • Bridging on a router
        • MTU 101
    • Wireless
      • Wireless Principles
      • Wireless Implementations
      • Wireless Roaming
      • Wireless Authentication
        • WPA2 PSK
        • WPA2 802.1X
  • IPv4
    • IPv4 Addressing
      • Backup Interfaces
      • FHRP 101
      • DHCP 101
      • DNS 101
      • ARP 101
      • IPv4 101
      • Tunnel Interfaces
        • GRE Tunnels
      • BFD – Bidirectional Forwarding Detection
    • IPv4 Routing
      • How the routing table is built
        • How CEF works
        • Routing Order of Operations
        • NSF – Non Stop Forwarding
      • RIP
        • RIP 101
      • EIGRP
        • EIGRP 101
        • EIGRP Metric
        • More EIGRP Features
      • OSPF
        • OSPF 101
        • OSPF Areas
        • OSPF LSAs
        • OSPF Mechanics
      • IS-IS
        • IS-IS 101
        • IS-IS Mechanics – CLNP
      • BGP
        • BGP 101
        • BGP Attributes
        • More BGP
      • Route Redistribution
      • Policy based Routing
      • PfR 101 – Perfromance Routing
      • ODR
  • IPv6
    • IPv6-101
    • IPv6 Routing
    • Interconnecting IPv6 and IPv4
  • MPLS
    • MPLS 101
    • MPLS L3 VPN
  • Multicast
    • Multicast 101
    • PIM 101
    • IGMP 101
    • Inter Domain Multicast
    • IPv6 Multicast
    • Multicast features on switches
  • Security
    • NAT 101
    • NAT for Overlapping Networks
    • ACLs 101
    • ACLs 102
    • Cisco IOS Firewall
    • Zone Based Firewall
    • AAA 101
    • Controlling CLI Access
    • Control Plane
    • Switch Security
      • Switchport Traffic Control
      • Switchport Port Security
      • DHCP Snooping and DAI
      • 802.1x
      • Switch ACLs
    • IPSec VPN 101
      • IKE / ISAKMP 101
      • IPSEC Crypto Maps 101
      • IPSEC VTI 101
      • DMVPN 101
    • EAP 101
  • Network Services
    • NTP 101
    • HTTP 101
    • File Transfer 101 – TFTP & FTP
    • WCCP 101
  • QoS
    • QoS 101
    • Classification and Marking
    • Congestion Management
      • Legacy Congestion Management
      • SPD – Selective Packet Discard
      • CBWFQ
      • IP RTP Priority
    • Congestion Avoidance – WRED
    • Policing and Shaping
      • CAR 101
    • Compression and LFI
      • Header and Payload Compression
      • LFI for MultiLink PPP
    • Frame Relay QoS
      • Per VC Frame Relay QoS
    • RSVP 101
    • Switching QoS
  • Network Optimization
    • NetFlow 101 – TNF – Traditional NetFlow
    • NetFlow 102 – FNF – Flexible NetFlow
    • IP SLA 101
    • IP Accounting 101
    • Logging 101
    • SNMP and RMON 101
    • Cisco CLI Tips and Tricks
    • AutoInstall
    • Enhanced Object Tracking
    • Troubleshooting 101
    • SPAN, RSPAN, ERSPAN
  • Network Architecture
    • Hierarchical Network Architecture
    • SD Access
    • SD WAN
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On this page
  • New features in RSTP
  • RSTP Port States
  • RSTP Port Roles
  • RSTP BPDU Format
  • New BPDU Handling
  • Uses a Backbone Fast mechanism by default
  • Rapid Transitions to Forwarding State
  • New TCN Mechanism
  • Proposal/Handshake Mechanism
  • Compatibility with 802.1d STP

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  1. Layer 2 Technologies
  2. Ethernet Switching
  3. Spanning Tree

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:

Sw(config)#spanning-tree mode rapid-pvst

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

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+:

Sw(config-if)# spanning-tree portfast

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:

Sw(config-if)#spanning-tree link-type {shared|point-to-point}

To verify the link type, use:

Sw# show spanning-tree [vlan VLAN-ID]
Fa0/1               Desg FWD 19        128.1    P2p
Fa0/2               Desg FWD 19        128.2    Shr
Fa0/3               Desg FWD 19        128.3    P2p Edge

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.

Previous802.1d – STPNext802.1s – MSTP

Last updated 2 years ago

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RSTP includes a mechanism similar to the that will work by default without any additional configuration.

Backbone Fast enhancement in STP