Interoperability With Legacy Stp - Avaya 8600 Engineering

peer (without having to wait for timers to expire). This process ripples throughout the RSTP/MSTP
instance domain, away from the root bridge, until all switches in the domain have converged on the new
topology.
This fast Forward transition mechanism allows RSTP/MSTP to reconverge in about 1 or 2 seconds,
depending on the number of bridges participating in the RSTP/MSTP instance and on the type of failure.
However, fast Forwarding transitions are only possible under certain conditions. The port which needs to
transition into Forwarding state must (a) be operating in Full Duplex mode and (b) there must be an
MSTP/RSTP bridge at the other end of that link.
Hence, in the presence of legacy half-duplex links (where a hub segment could result in more than one
bridge neighbor) RSTP/MSTP will fall back on the legacy STP timers. Also in the case of switch ports
where end stations are directly connected condition (b) is not satisfied and again legacy STP timers are
used (in this case, to obtain fast Forwarding transitions, the port needs to be configured as an Edge Port
as discussed later).
Failure of the Root bridge for the RSTP/MSTP instance, while still an improvement on STP, can also
result in higher convergence times. Typically a root bridge failure where its immediate neighbors detect a
link loss on their interfaces will result in a 2 seconds reconverge time. However if the root bridge fails and
its immediate neighbors do not detect this via loss of link (for instance, a software failure on the root
device), then the reconverge time occurs after 2 consecutive BPDUs are missed i.e. 6 seconds
Overall, RSTP and MSTP enable the switch to achieve the following:

Reduce convergence time from 30-50 seconds to less than 1 or 2 seconds under most (but not
all) failure conditions

Eliminate unnecessary flushing of the MAC database and flooding of traffic to the network, with a
new enhanced Topology Change mechanism whereby only a fraction of the MAC tables need to
be flushed and re-learnt

Backward compatibility with other switches that are running legacy 802.1D STP or Avaya MSTG
(STP group 1 only).
The user can configure an Avaya switch to run Avaya STP, RSTP (802.1w), or MSTP (802.1s)
configuration.

1.1 Interoperability with Legacy STP

RSTP provides a new parameter ForceVersion for backward compatibility with legacy STP. You can
configure a port in either STP compatible mode or RSTP mode.

An STP compatible port transmits and receives only STP BPDUs. Any RSTP BPDU that the port
receives in this mode will be discarded.

An RSTP port transmits and receives only RSTP BPDUs. If an RSTP port receives an STP BPDU
it becomes an STP port. User intervention is required to bring this port back to RSTP mode. This
process is called Port Protocol Migration.
Normally an RSTP or MSTP bridge will automatically detect the STP version type of a neighboring bridge
and automatically adapt to that STP version without having to use the ForceVersion parameter.
July 2010
RSTP/MSTP Technical Configuration Guide
avaya.com
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