JUNOS Enterprise Switching

Rapid Spanning Tree Protocol

In order to fix some of the deficiencies in the original STP, the IEEE defined 802.1w, Rapid Spanning Tree Protocol, later described in the 802.1D-2004 specification.

STP converges very slowly, after a topological change. When using default values for the max age (20 sec) and the forward delay (15 sec) parameters, it takes 50 seconds for the bridge to finally converge:

convergence delay = 2 × forward delay + max age

This setup was OK for early networks, but like the Commodore 64, it became unacceptable as technology advanced. RSTP vastly improves that time, often providing subsecond transitions.

Essentially, the major difference is a centralized versus distributed model. In STP, the root bridge has most of the responsibility for spanning tree maintenance and change notifications and is the “central” point. In RSTP, all bridges take an active role in the network’s connectivity. This and other new protocol specifications lead to increased reconvergence times.

New BPDU Definition and Function

RSTP slightly modifies the BPDUs’ format and function and sends them out at a quicker rate (two-second default). They are now used as “hellos,” as opposed to in original STP, where BPDUs were relayed from the root bridge. Every bridge will send out the BPDU with current information every hello time, even if it does not receive a BPDU from the root bridge. This method is quicker and more distributed.

The first change is that the BPDU and type field are now version 2, which ...