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RSTP vs. RSTP-M


The RSTP-M protocol is a modified version of the RSTP protocol, optimized for use in security and automation systems.

RSTP-M complies with the requirements of security and automation systems to ensure a fast back-up route in the case of a failure and at the same time:

  • fully compatible with RSTP acc. to IEEE 802.1D-2004,
  • supports MESH topology,
  • reduces reconfiguration time to a minimum,
  • removes some flaws of RSTP. See examples "Failure of a line" and "Loss of a ROOT switch".

Picture 1

Failure of a Line
When the first failure occurs, the nearest switch (X) propagates information about route loss to the active side of the ring. If this information is received by any other switch knowing the alternative route (Switch A), it is its task to put it into operation.
RSTP: Switch A, after the reception of the information about the failure, is waiting for a periodically sent frame BPDU 
(by default every 2s) from the alternative route so that it can verify the activity of this back-up line. Only then can it unblock the alternative route.
RSTP-M: Switch A assumes that the alternative route is active and therefore unblocks the route immediately.

Picture 2

Example of Measured Values:

Table 1

Loss of a ROOT Switch

If switch 1 loses connectivity with ROOT switch (5), it declares itself as a ROOT switch (1) and propagates this information further to the active side. Switch 3 after reception of BPDU initiates a search for an alternative route to switch 5.
RSTP: As an alternative route can be considered a back-up line between switches 2-3 and this will result in the blocking of the so far functioning connection and opening of an alternative connection. This change does not benefit anyone; it only causes undesired data loss. Route 3-4 is thus unblocked later.
RSTP-M: Protocol actively monitors the state of its direct neighbors. Based on this information, switch 3 evaluates changes of route 2-3 as purposeless and there is no switchover. By contrast, it immediately reacts by the unblocking of route 3-4.

Picture 3

The resolution of these failures is mutually influencing. Some RSTP implementations have a well handled loss-of-a-ROOT-switch problem but lose because of that on handling of the line failure. RSTP-M limits the delays of both of the above mentioned problems and other failures to a minimum.
In security systems, we recommend using ring topology and LAN-RING protocol ensuring reconfiguration speed. That is its main advantage versus the RSTP. In systems with more complex topology, RSTP-M can be a suitable solution. It does not guarantee reconfiguration time but, in comparison with general RSTP protocol, it reduces the reconfiguration times to a minimum. Non-guaranteed network reconfiguration time can cause longer outages (tens of second and up to minutes) of the connection of VMS.