Cisco – Migrating from STP to RSTP

February 11, 2018 at 8:05 pm

CCIE

A frequent CCIE-level written exam topic is STP (PVST+) to RSTP (Rapid-PVST) migrations. This post covers the high points you should be aware of.

Some Quick Notes

  • BackboneFast and UplinkFast are no longer necessary in RSTP environments as the equivalent advantages are built-in to RSTP
  • BPDUs are version 2 in RSTP which is compatible with version 0
  • All ports now send BPDUs and they act as a keepalive mechanism
  • The new port roles are designated, root, alternate, and backup
  • The new port states are forwarding, discarding, and learning
  • The new port types are shared, edge (PortFast), and P2P
  • When an RSTP port receives a legacy BPDU, it reverts back to compatible behavior and performance enhancement benefits are lost
  • BackboneFast is configured on all switches in order to function in STP and UplinkFast is configured on access layer devices

Migrating

  • You are not forced into removing the BackboneFast and UplinkFast commands – if they are left in place, they do not impact the operation of RSTP
  • Removing the Uplinkfast command impacts the bridge priority and port cost values, so be sure you consider this
  • CatOS (FLASHBACK!) does not permit the BackboneFast command within RSTP so you must remove it in that legacy case
  • Plan a phased migration and use a maintenance window as traffic flows can be impacted
  • The configuration of the features such as PortFast, BPDUguard, BPDUfilter, root guard, and loopguard are applicable in rapid-PVST+ mode also