Rapid (Per-VLAN) Spanning-Tree (RSTP/RPVST+) - IEEE 802.1w

ccnp-switching
How Rapid Spanning Tree (802.1w) replaces timers with a proposal and agreement handshake, its port states and link types, and RPVST+ configuration.
Published

Nov 19, 2017

RSTP is not timer-based unlike 802.1D which is timer-based. RSTP uses a proposal and agreement process to build up the spanning tree faster.

Do you remember the port states in PVST?

In RSTP the port states are:

We still have 2 types of BPDUs:

In PVST the root bridge makes BPDUs and they are flooded by other switches. In RSTP each switch makes its own BPDU.

Upgrading to RSTP

  • In a live environment, you may upgrade to RSTP in phases.
  • RSTP is backward compatible with legacy STP 802.1D. If an RSTP enabled port receives a (legacy) 802.1d BPDU, it will automatically configure itself to behave like a legacy port. It sends and receives 802.1d BPDUs only.
    • You can run 802.1W RSTP with 802.1D STP in the same network, but not on the same switch (for example per VLANs).

Link/Port Types

  • Edge Port: A port at the “edge” of the network, where only a single host connects. Traditionally, this has been identified by enabling the STP PortFast feature. RSTP keeps the PortFast concept for familiarity. By definition, the port cannot form a loop as it connects to one host, so it can be placed immediately in the Forwarding state. However, if a BPDU ever is received on an edge port, the port immediately loses its edge port status.
To configure a port as an RSTP edge port, use the following interface configuration command: 
Switch(config-if)# spanning-tree portfast

  • Root port: The port that has the best cost to the root of the STP instance. Only one root port can be selected and active at any time, although alternative paths to the root can exist through other ports. If alternative paths are detected, those ports are identified as alternative root ports and immediately can be placed in the Forwarding state when the existing root port fails.

  • Point-to-point (P2P) port: Any port that connects to another switch and becomes a designated port. A quick handshake with the neighboring switch, rather than a timer expiration, decides the port state. BPDUs are exchanged back and forth in the form of a proposal and an agreement. One switch proposes that its port becomes a designated port; if the other switch agrees, it replies with an agreement message.

  • Shared port: in the same segment (when a hub is in between)

Configuration

Remember that we change the spanning-tree mode in switches in phases. 
Distribution_1(config)#spanning-tree mode rapid-pvst

Distribution_2(config)#spanning-tree mode rapid-pvst

Switch(config)#spanning-tree mode rapid-pvst
See the verification command: 
Access#show spanning-tree

VLAN0001
  Spanning tree enabled protocol rstp
  Root ID    Priority    32769
             Address     001d.707b.4200
             Cost        19
             Port        25 (FastEthernet1/0/23)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0022.be5a.0680
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa1/0/21         Altn BLK 19        128.23   P2p Peer(STP)
Fa1/0/23         Root FWD 19        128.25   P2p

BLK in RSTP means in discarding state

Now, I am going to disconnect Fa1/0/23 and see the output of debug spanning-tree events command on Access switch: 
00:15:21: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet1/0/23, changed state to down
00:15:21: RSTP(1): updt roles, root port Fa1/0/23 going down
00:15:21: RSTP(1): Fa1/0/21 is now root port
Now, I am going to connect back the port Fa1/0/23: 
00:15:40: %LINK-3-UPDOWN: Interface FastEthernet1/0/23, changed state to up
00:15:40: RSTP(1): initializing port Fa1/0/23
00:15:40: RSTP(1): Fa1/0/23 is now designated
00:15:40: RSTP(1): transmitting a proposal on Fa1/0/23
00:15:40: RSTP(1): updt roles, received superior bpdu on Fa1/0/23
00:15:40: RSTP(1): Fa1/0/23 is now root port
00:15:40: RSTP(1): Fa1/0/21 blocked by re-root
00:15:40: RSTP(1): synced Fa1/0/23
00:15:40: RSTP(1): Fa1/0/21 is now alternate
00:15:40: RSTP(1): transmitting an agreement on Fa1/0/23 as a response to a proposal
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