IOS releases are pre-standard. Additional commands are needed for the standard and pre-standard implementations to work together, which are discussed later in this lesson.
Content 3.3 Implementing MSTP 3.3.2 Describing MST Regions MSTP differs from other spanning tree implementations in that it combines some, but not necessarily all, VLANs into logical spanning tree instances. This raises the problem of determining which VLAN to associate with which instance, which involves tagging BPDUs so that receiving devices can identify the instances and the VLANs to which they apply. This issue is irrelevant with the 802.1D standard, in which all instances are mapped to a unique and common instance, CST. In the PVST+ implementation, different VLANs carry the BPDUs for their respective instances (one BPDU per VLAN), based on the VLAN tagging information. To provide this logical assignment of VLANs to spanning trees, each switch running MSTP in the network has a single MSTP configuration that consists of three attributes: To be part of a common MSTP region, a group of switches must share the same configuration attributes. It is up to the network administrator to properly propagate the configuration throughout the region. Currently, this step is only possible by using the command-line interface (CLI) or Simple Network Management Protocol (SNMP). Other methods can be implemented in the future because the IEEE specification does not explicitly mention how to accomplish this step. To ensure a consistent VLAN-to-instance mapping, it is necessary for the protocol to be able to exactly identify the boundaries of the regions. For that purpose, the characteristics of the region are included in BPDUs. The exact VLAN-to-instance mapping is not propagated in the BPDU, because the switches only need to know whether they are in the same region as a neighbor. Therefore, only a digest of the VLANs-to-instance mapping table is sent, along with the revision number and the name. Once a switch receives a BPDU, it extracts the digest (a numerical value derived from the VLAN-to-instance mapping table through a mathematical function) and compares it with its own computed digest. If the digests differ, the mapping must be different, so the port on which the BPDU was received is at the boundary of a region. A port is at the boundary of a region if the designated bridge on its segment is in a different region or if it receives legacy 802.1D BPDUs. In Figure , the port on B1 is at the boundary of region A, whereas the ports on B2 and B3 are internal to region B.
Content 3.3 Implementing MSTP 3.3.3 Describing the Extended System ID As with PVST, the 12-bit Extended System ID field is used in MSTP. In MSTP, this field carries the MSTP instance number. The 802.1D protocol states that each bridge must have a unique bridge identifier. In PVST, each VLAN is considered a different logical bridge. Therefore, each VLAN needs a unique bridge ID. Prior to supporting 4000 VLANs, Cisco supported a maximum of 1024 VLANs, which required 1024 bridge IDs. MAC address reduction is a feature that ensures bridge ID uniqueness for all 4000 VLANs, even when there are only 1024 or 64 MAC addresses available on the switch. It accomplishes this uniqueness by making the 16-bit Bridge Priority field in the BPDU unique for each VLAN. Prior to this feature, the Bridge Priority field was fully configurable and did not have to be unique, because the appending 48-bit MAC address was unique for each VLAN. MAC address reduction splits the 16-bit field into two fields: a configurable 4-bit field and a nonconfigurable 12-bit field. The 12-bit field carries the VLAN ID (VID) or, with MSTP, the MSTP instance number. The two fields are merged to create the unique Bridge Priority field for a particular VLAN or MSTP instance. The appending MAC address remains the same for all instances.
Content 3.3 Implementing MSTP 3.3.4 Interacting Between MST Regions and 802.1D Networks One issue that arises from MSTP design is interoperability with the CST implementation in 802.1D. According to the IEEE 802.1s specification, an MSTP switch must be able to handle at least one internal spanning tree (IST). The MSTP region consists of one IST and an arbitrary number of MSTP instances. Figure shows two functionally equivalent diagrams. Notice the location of the different blocked ports. In a typical bridged network, you expect to see a blocked port between switches M and B. Instead of blocking on switch D, you expect to have the second loop broken by a blocked port somewhere in the middle of the MSTP region. However, due to the IST, the entire region appears as one virtual bridge that runs a single spanning tree (CST). This approach makes it possible to understand that the virtual bridge blocks an alternate port on switch B and that the virtual bridge causes Switch D to block its port connecting to switch C. The MSTP instances are simple RSTP instances that exist only inside a region. The MSTP instances run RSTP automatically by default, without any extra configuration. Unlike IST instances, MSTP instances never interact with devices outside the region. (MSTP runs only one spanning tree outside the region.) Therefore, except for the IST instance, regular instances inside the region have no outside counterpart. Additionally, MSTP instances do not send BPDUs outside a region, only the IST does. MSTP instances do not send independent individual BPDUs. Inside the MSTP region, bridges exchange MSTP BPDUs that can be seen as normal RSTP BPDUs for the IST while containing additional information for each MSTP instance. The IST (instance 0) runs on all bridges within an MSTP region. An important characteristic of the IST is that it provides interaction at the boundary of the MSTP region with other MSTP regions. More importantly, it is responsible for providing compatibility between the MSTP regions and the spanning tree of 802.1D (CST) and PVST+ networks connected to the region. The IST receives and sends BPDUs to the CST for compatibility with 802.1D. The IST is capable of representing the entire MSTP region as a CST virtual bridge to switched networks outside the MSTP region. The following highlights key characteristics of MSTP interaction with a CST or PVST environment: