Now that we’ve got 802.1D STP covered, let’s take a look at its improvement 802.1w STP. Rapid Spanning Tree Protocol, or RSTP, is basically STP with some tweaks, and most of which, I’ve already talked about in the previous post. In addition to tweaks, there are also new features (items or I should say) and compatibility issues and features. All in all, here is a brief summary of the new or different features available in 802.1w STP (compared to 802.1D STP):

  • Integration of BackboneFast and UplinkFast
  • PortFast
  • New Port Roles, Port Types and Port States
  • 802.1D compatibility
  • BPDU Flags
  • Synchronization

Port Roles and Port States

RSTP uses one of the following Port States to identify a switch port in its STP process:

  • Discarding: replaces the original Discarded, Blocked, and Listening state where the switch port can send and receive BPDU, but can not forwarding user traffic or BPDUs.
  • Learning: same as the original Learning state where the switch port can learn MAC address, send and receive BPDU, but not forwarding user traffic.
  • Forwarding: same as the original Forwarding state where the switch port can learn MAC address, send and receive BPDU, and forwarding user traffic.

RSTP uses one of the following Port Roles to identify the purpose of the switch port in the STP process:

  • Root Port: Port closest to the Root Bridge in forwarding state
  • Designated Port: Port closer to the Root Bridge on a network segment in forwarding state
  • Alternate Port: standby port that becomes the Root Port if it fails; in discarding state
  • Backup Port: standby port that becomes the Designated Port (for this network segment) if it fails; in discarding state

And finally, RSTP also uses different description for different Port Types:

  • Edge Port: links connected to non-STP ports, these ports have PortFast enabled.
  • Point to point Port: Full Duplex link that actively participate in STP

RSTP BPDU

In RSTP, every single switch generates its own configuration BPDU to let the other switches know its current status. Whereas in STP, only the Root Bridge generates its configuration BPDU. The only difference between a RSTP configuration BPDU and STP configuration BPDU is the Flags field. In RSTP, the Flags field is composed of the following bits:

  • TC
  • Proposal
  • Port Role (2 bits)
  • Learning
  • Forwarding
  • Agreement
  • TC acknowledgement

STP do not accept RSTP configuration BPDU; they are dropped. When it comes to compatibility, RSTP has a special compatibility mode. When RSTP send out configuration BPDU, it starts a migration delay timer, which is by default 3 seconds. During this period, if the switch receives a 802.1D configuration BPDU, the switch will change that port’s mode of operation to 802.1D.

 

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