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Rapid STP

Spanning Tree Versions

IEEE Standards Description
Spanning Tree Protocol (802.1D)

- The original STP
- All VLANs share one STP instance
- Therefore, cannot load balance
Rapid Spanning Tree Protocol (802.1w)

- Much faster at converging/adapating to network changes than 802.1D
- All VLANs share one STP instance
- Therefore, cannot load balance
Multiple Spanning Tree Protocol (802.1s)

- Uses modified RSTP mechanics
- Can group multiple VLANs into different instances (ie. VLANs 1-5 in instance 1, VLANs 6-10 in instance 2) to perform load balancing
Cisco Versions Description
Per-VLAN Spanning Tree Plus (PVST+)

- Cisco's upgrade to 802.1D
- Each VLAN has its own STP instance
- Can load balance by blocking different ports in each VLAN
Rapid Per-VLAN Spanning Tree Plus (Rapid PVST+)

- Cisco's upgrade to 802.1w
- Each VLAN has its own STP instance
- Can load balance by blocking different ports in each VLAN

Rapid Spanning Tree Protocol

{% hint style="info" %} RSTP is not a timer-based spanning tree algorithm like 802.1D. Therefore, RSTP offers an improvement over the 30 seconds or more that 802.1D takes to move a link to forwarding. The heart of the protocol is a new bridge-bridge handshake mechanism, which allows ports to move directly to forwarding. {% endhint %}

Similarities between STP and RSTP:

  • RSTP serves the same purpose as STP, blocking specific ports to prevent Layer 2 loops.
  • RSTP elects a root bridge with the same rules as STP.
  • RSTP elects root ports with the same rules as STP.
  • RSTP elects designated ports with the same rules as STP.
Speed STP Cost RSTP Cost
10 Mbps 100 2,000,000
100 Mbps 19 200,000
1 Gbps 4 20,000
10 Gbps 2 2,000
100 Gbps X 200
1 Tbps X 20

The Blocking, Listening and Disabled are now combined into a single state called DISCARDING.

STP Port State Send/Receive BPDUs Frame forwarding (regular traffic) MAC address learning Stable/ Transitional
Discarding NO/YES NO NO Stable
Learning YES/YES NO YES Transitional
Forwarding YES/YES YES YES Stable
  • If a port is administratively disabled (shutdown command) = discarding state.
  • If a port is enabled, but blocking traffic to prevent Layer 2 loops = discarding state.

Port Roles

  • The root port role remains unchanged in RSTP.
    • The port that is closest to the root bridge becomes the root port for the switch.
      • The root bridge is the only switch that does not have a root port.
  • The designated port role also remains unchanged in RSTP.
    • The port on a segment (collision domain) that sends the best BPDU is that segment's designated port (only one per segment).
  • The non-designated port is split into two separate roles in RSTP:
    • Alternate port role
    • Backup port role

RSTP: Alternate port role

  • Discarding port that receives a superior BPDU from another switch.
  • Functions as a backup to the root port.
  • If the root port fails, the switch can immediately move its best alternate port to forwarding.
    • This immediate move to forwarding state functions like a classic STP optional feature called UplinkFast. Beacuse it is build into RSTP, you do not need to activate UpLink when using RSTP/Rapid PVST+.

BackboneFast functionality

  • One more STP optional feature that was built into RSTP is BackboneFast.

  • BackboneFast allows SW3 to expire the max age timers on its interface and rapidly forward the superior BPDUs to SW2.

RSTP: Backup port role

  • Discarding port that receives a superior BPDU from another interface on the same switch.

  • This only happend when two interfaces are connected to the same collision domain (via a hub).
  • Function as a backup for a designated port.
  • The interface with the lowest port ID will be selected as the designated port, and the other will be the backup port.

Rapid STP is compatible with Classic STP. The interface(s) on the Rapid STP-enabled switch connected to the Classic STP-enabled switch will operate in Classic STP mode (timers, blocking > listening > learning > forwarding process, etc).

In classic STP, only the root bridge originated BPDUs are being forwarded. In rapid STP, ALL switches originate and send their own BPDUs from their designated ports.

  • All switches running Rapid STP send their own BPDUs every hello time (2 seconds).
  • Switches ‘age’ the BPDU information much more quickly. In classic STP, a switch waits 10 hello intervals (20 seconds). In rapid STP, a switch considers a neighbor lost if it misses 3 BPDUs (6 seconds). It will then ‘flush’ all MAC addresses learned on that interface.

RSTP Link Types

  • RSTP distinguishes between three different ‘link types’.
    • Edge: a port that is connected to an end host. Moves directly to forwarding, without negotiation.
    • Point-to-point: a direct connection between two switches.
    • Shared: a connection to a hub. Must operate in half-duplex mode.

Edge

  • Edge ports are connected to end hosts.
  • Because there is no risk of creating a loop, they can move straight to the forwarding state without the negotiation process.
  • They function like a classic STP port with PortFast enabled.
SW1(config-if)#spanning-tree porfast

Point to point

  • Point-to-point ports connect directly to another switch.
  • They function in full-duplex.
  • You don't need to configure the interface as point-to-point (it should be detected).
SW1(config-if)# spanning-tree link-type point-to-point

Shared

  • Shared ports connect to another switch (or switches) via a hub.
  • They function in half-duplex.
  • You don't need to configure the interface as shared (it should be detected).
SW1(config-if)# spanning-tree link-type shared