Editing Spanning Tree Protocol (section)

=== Multiple Spanning Tree Protocol ===
{{Main|Multiple Spanning Tree Protocol}}

The Multiple Spanning Tree Protocol (MSTP), originally defined in [[IEEE 802.1s]]-2002 and later merged into [[IEEE 802.1Q]]-2005, defines an extension to RSTP to further develop the usefulness of VLANs.

In the standard, a spanning tree that maps one or more VLANs is called a ''multiple spanning tree'' (MST). Under MSTP, a spanning tree can be defined for individual VLANs or for groups of VLANs. Furthermore, the administrator can define alternate paths within a spanning tree. Switches are first assigned to an MST region, then VLANs are mapped against or assigned to this MST. A ''common spanning tree'' (CST) is an MST to which several VLANs are mapped, this group of VLANs is called ''MST instance'' (MSTI). CSTs are backward compatible with the STP and RSTP standard. A MST that has only one VLAN assigned to it is an ''internal spanning tree'' (IST).<ref name="Solomon"/>

Unlike some proprietary per-VLAN spanning tree implementations,<ref>{{cite web|url = https://www.cisco.com/en/US/prod/collateral/netmgtsw/ps6504/ps6528/ps2425/white_paper_c07-552114.html#wp9003215 |title = CiscoWorks LAN Management Solution 3.2 Deployment Guide |access-date = 2010-01-25 |date = August 2009}}</ref> MSTP includes all of its spanning tree information in a single BPDU format. Not only does this reduce the number of BPDUs required to communicate spanning tree information for each VLAN, but it also ensures backward compatibility with RSTP and, in effect, classic STP too. MSTP does this by encoding an additional region of information after the standard RSTP BPDU as well as a number of MSTI messages (from 0 to 64 instances, although in practice many bridges support fewer). Each of these MSTI configuration messages conveys the spanning tree information for each instance. Each instance can be assigned a number of configured VLANs and frames assigned to these VLANs operate in this spanning tree instance whenever they are inside the MST region. In order to avoid conveying their entire VLAN to spanning tree mapping in each BPDU, bridges encode an MD5 digest of their VLAN to instance table in the MSTP BPDU. This digest is then used by other MSTP bridges, along with other administratively configured values, to determine if the neighboring bridge is in the same MST region as itself.

MSTP is fully compatible with RSTP bridges in that an MSTP BPDU can be interpreted by an RSTP bridge as an RSTP BPDU. This not only allows compatibility with RSTP bridges without configuration changes but also causes any RSTP bridges outside of an MSTP region to see the region as a single RSTP bridge regardless of the number of MSTP bridges inside the region itself. In order to further facilitate this view of an MSTP region as a single RSTP bridge, the MSTP protocol uses a variable known as remaining hops as a time to live counter instead of the message age timer used by RSTP. The message age time is only incremented once when spanning-tree information enters an MST region, and therefore RSTP bridges will see a region as only one ''hop'' in the spanning tree. Ports at the edge of an MSTP region connected to either an RSTP or STP bridge or an endpoint are known as boundary ports. As in RSTP, these ports can be configured as edge ports to facilitate rapid changes to the forwarding state when connected to endpoints.