RSTP Operation

RSTP optimizes convergence by significantly reducing the time to reconfigure the network‘s active topology when physical topology or configuration parameter changes occur. RSTP provides rapid connectivity following the failure of a switching device, switch port, or the addition of a switch into the network.

A new root port may forward as soon as any recent root ports are put into blocking.

A designated port may forward with the exchange of two BPDUs in rapid succession. The designated port presents new BPDU information with a proposal request. The attached port processes the BPDU and may respond immediately with an agreement. Upon reception of that agreement BPDU, the designated port may move to forwarding. Another feature of RSTP is that designated ports transmit periodic BPDUs regardless of reception of BPDUs at the root port. This insulates the network from jitter in receiving BPDUs, particularly at the edge.

Important STP timers are max age, hello time, and forward delay. The default values for the timers are:

• Hello time – 2 seconds
• Forward delay – 15 seconds
• Max age – 20 seconds

The operational values from these timers are derived from the root bridge. The current IEEE standard for Spanning Tree fixes hello time at 2 seconds. The Extreme Network switches covered in this document do not enforce this restriction to allow existing configurations to remain compatible. It is not recommended that a value other than 2 seconds be used. Other values may not interact well with other non-variable protocol times such as edgeDelayWhile or mDelayWhile. The max age timer may be adjusted to change the network diameter. Take care to consider that failure in the network may cause the topology to “unravel” causing the diameter to become larger than anticipated. An insufficient value could cause devices near or at the edge of the network to become unreachable. For example, in a ring topology of 10 bridges, no bridge is more than 5 hops from the root. A max age that accounts for 6 hops would be sufficient. A failure of ports immediately interconnecting a bridge with the root would break the ring topology and change the furthest hop from the root from 5 to 9. Any bridges beyond the configured network diameter of 6 would cause the Spanning Tree topology not to converge.