Overview: Rstp/Mstp - Avaya 8600 Engineering

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1. Overview: RSTP/MSTP

The original IEEE 802.1D Spanning Tree Protocol (hereby referred to as STP) standard (802.1D-1998
clause 8) provides loop protection and recovery, but, it is slow to respond to a topology change in the
network (for example, a dysfunctional link in a network).
The Rapid Spanning Tree protocol (RSTP or IEEE 802.1w / 802.1D-2004 clause 17) reduces the
recovery time after a network break down. It also maintains a backward compatibility with the original
IEEE 802.1D STP standard which was the Spanning Tree implementation prior to RSTP. Typically, the
recovery time of RSTP is within a second compared to STP which can take upwards of 30-50 seconds to
recover.
RSTP also reduces the amount of flooding in the network by enhancing the way the Topology Change
Notification (TCN) packet is generated.
The Multiple Spanning Tree Protocol (MSTP or IEEE 802.1s / 802.1Q-2005 clause 13) is an extension to
RSTP allowing for multiple Spanning Tree instances on the same switch. Both 802.1D and 802.1w
spanning tree protocols operate without any regards to a network's VLAN configuration whereas 802.1s
maps VLANs to multiple spanning tree instances. This allows the switch to use different paths in the
network to effectively load balance or distribute VLANs evenly where each Spanning Tree instance will
block the appropriate port(s) within its own instance.
When configuring MSTP, one or more VLANs are assigned to an MST instance (MSTI) and each switch
is assigned to an MSTP MST region. Hence, each MST region consists of one or more MSTP switches
where each switch must be configured with the same VLANs, at least one MST instance, and the same
MST region name. A Common Spanning Tree (CST), base instance 0, is used to interconnect individual
MST regions or MST regions with RSTP or STP LANs. MSTP connects all switches and LANs together
with a single common and internal spanning tree (CIST) where one single CIST root bridge is elected and
one CIST regional root bridge is elected per MST region. In summary, MSTI instances provide loop free
switching within a region for VLANs whereas CIST provides loop free switching between regions with no
regards to VLANs.
As noted above, MSTP is backwards compatible with both RSTP and STP likewise RSTP with STP.
MSTP effectively uses the RSTP BPDUs extended to include region information and MSTI instance
messages. These constitute the MSTP BPDU which, like both RSTP and STP BPDUs, are always
untagged.
If an MSTP bridge detects that a neighboring bridge is operating in RSTP mode, the interconnecting
interface on the MSTP bridge will downgrade to RSTP operation, whereby only RSTP BPDUs are
generated on that interface. Likewise if an RSTP or MSTP bridge detects that a neighboring bridge is
operating in STP mode, the interconnecting interface on the MSTP/RSTP bridge will downgrade to STP
operation, whereby only STP BPDUs are generated on that interface. If a number of MSTP bridges
forming an MST Region are interconnected to RSTP/STP switches, the RSTP/STP domain will see the
MSTP region as one hop.
RSTP and MSTP achieve rapid convergence by enhancing the Port Role Transition state machine with
regards to Forwarding transitions. Whenever a failure in the network occurs and a link or node in the
active topology is broken/lost, one or more Forward transitions on some other links, previously
Discarding, are required to restore connectivity. In the original STP protocol, a Forwarding transition was
only possible after expiry of the Listening and Learning timers (30 seconds in total) which would
guarantee enough delay for every bridge in the STP domain to receive and converge on the new topology
information. With RSTP/MSTP Forward transition delays can be dramatically reduced via the Proposal &
Agreement exchanges whereby every bridge wanting to perform a Forward transition on a Designated
port can propose such a change to it's downstream peer bridge and can do so as soon as agreed by it's
RSTP/MSTP Technical Configuration Guide
7
July 2010