RSTP

Discarding

• On
• Blocking data traffic
• Might be receiving BPDUs
  • STP states: Disabled, Blocking and Listening

Learning

• On
• Building the CAM table
• Only forwarding BPDUs

Forwarding

• On
• Forwarding BPDUs and Data traffic

Root port

• AKA RP
• A switch has one RP

Designated port

• AKA DP
• Each link can have only one DP

Backup port

• Typically connected to a hub
• Used on shared segments
• Provides link redundancy

Edge ports

• Edge of the STP topology, meant for hosts
• Edge ports have portfast turned on

Non-Edge ports

• Have received a BPDUs
  • Could be attached to switches

Point-to-point ports

• Connect to other RSTP switches with full duplex

Propose bit

• Added in 802.1w
• Switches compare BPDUs
  • Switch with Superior BPDU
    • Sets the Propose and DP bits

Agree Bit

• Sent in response to a Propose Bit
  • “Yeah, I agree”
  • Transition immediately to forwarding

Packet Headers

Spanning Tree Protocol
    Protocol Identifier: Spanning Tree Protocol (0x0000)
    Protocol Version Identifier: Rapid Spanning Tree (2)
    BPDU Type: Rapid/Multiple Spanning Tree (0x02)
    BPDU flags: 0x0e, Port Role: Designated, Proposal
        0... .... = Topology Change Acknowledgment: No
        .0.. .... = Agreement: No
        ..0. .... = Forwarding: No
        ...0 .... = Learning: No
        .... 11.. = Port Role: Designated (3)
        .... ..1. = Proposal: Yes
        .... ...0 = Topology Change: No
    Root Identifier: 0 / 1 / 52:54:00:eb:eb:96
    Root Path Cost: 4
    Bridge Identifier: 32768 / 1 / 52:54:00:a9:d4:07
    Port identifier: 0x8001
    Message Age: 1
    Max Age: 20
    Hello Time: 2
    Forward Delay: 15
    Version 1 Length: 0

Proposal

Like 802.1D, we have BPDUs. To speed things up some logic has been added.

Is this port full-duplex? If so, it’s point-to-point, and if so, there is a RP and DP.

No BPDUs being received? Wait for the forward delay, transition port to forwarding.

Fastest Scenario, The Would-Be DP Is Already Transmitting

Captures

RSTP-initial-bringup-fastest.pcap

References

Cisco - Spanning Tree Protocol