Cisco Multiple Spanning-Tree Protocol (MSTP) Configuration Guide: User Manual PDF (EN)

Restrictions for Multiple Spanning-Tree Protocol

Configuring Multiple Spanning-Tree Protocol Prerequisites for Multiple Spanning Tree Protocol, on page 1 Restrictions for Multiple Spanning-Tree Protocol, on page 1 Information About Multiple Spanning Tree Protocol, on page 2 How to Configure Multiple Spanning Tree Protocol and Parameters, on page 16 Feature History for Multiple Spanning Tree Protocol, on page 28 Prerequisites for Multiple Spanning Tree Protocol • For two or more devices to be in the same multiple spanning tree (MST) region, they must have the same VLAN-to-instance map, the same configuration revision number, and the same name.

• For load-balancing across redundant paths in the network to work, all VLAN-to-instance mapping assignments must match; otherwise, all traffic flows on a single link. • For load-balancing between a per-VLAN spanning tree plus (PVST+) and an MST cloud or between a rapid-PVST+ and an MST cloud to work, all MST boundary ports must be forwarding. MST boundary ports are forwarding when the root of the internal spanning tree (IST) of the MST cloud is the root of the common spanning tree (CST).

If the MST cloud consists of multiple MST regions, one of the MST regions must contain the CST root, and all of the other MST regions must have a better path to the root contained within the MST cloud than a path through the PVST+ or rapid-PVST+ cloud. You might have to manually configure the devices in the clouds. Restrictions for Multiple Spanning-Tree Protocol • The switch stack supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance is the maximum active VLAN supported by a given switch. • PVST+, Rapid PVST+, and MSTP are supported, but only one version can be active at any time.

(For example, all VLANs run PVST+, all VLANs run Rapid PVST+, or all VLANs run MSTP.) • VLAN Trunking Protocol (VTP) propagation of the MST configuration is not supported. However, you can manually configure the MST configuration (region name, revision number, and VLAN-to-instance mapping) on each device within the MST region by using the command-line interface (CLI) or through the Simple Network Management Protocol (SNMP) support. Configuring Multiple Spanning-Tree Protocol 1

Стр. 1

Multiple Spanning Tree Protocol Configuration Guidelines

• Partitioning the network into a large number of regions is not recommended. However, if this situation is unavoidable, we recommend that you partition the switched LAN into smaller LANs interconnected by routers or non-Layer 2 devices. • A region can have one member or multiple members with the same MST configuration; each member must be capable of processing rapid spanning tree protocol (RSTP) Bridge Protocol Data Units (BPDUs). There is no limit to the number of MST regions in a network, but each region can only support up to 65spanning-tree instances. You can assign a VLAN to only one spanning-tree instance at a time.

Information About Multiple Spanning Tree Protocol The following sections provide information about Multiple Spanning-Tree Protocol (MSTP): Multiple Spanning Tree Protocol Configuration Multiple Spanning-Tree Protocol (MSTP), which uses Rapid Spanning-Tree Protocol (RSTP) for rapid convergence, enables multiple VLANs to be grouped into and mapped to the same spanning-tree instance, reducing the number of spanning-tree instances that are needed to support many VLANs. The MSTP provides for multiple forwarding paths for data traffic, enables load balancing, and reduces the number of spanning-tree instances that are required to support many VLANs.

It improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). The multiple spanning-tree (MST) implementation is based on the IEEE 802.1s standard. Note The most common initial deployment of MSTP is in the backbone and distribution layers of a Layer 2 switched network. This deployment provides the highly available network that is required in a service-provider environment. When the device is in the MST mode, the RSTP, which is based on IEEE 802.1w, is automatically enabled.

The RSTP provides rapid convergence of the spanning tree through explicit handshaking that eliminates the IEEE 802.1D forwarding delay and quickly transitions root ports and designated ports to the forwarding state. Both MSTP and RSTP improve the spanning-tree operation and maintain backward compatibility with equipment that is based on the (original) IEEE 802.1D spanning tree, with existing Cisco-proprietary Multiple Instance STP (MISTP), and with existing Cisco PVST+ and rapid per-VLAN spanning-tree plus (Rapid PVST+). A device stack appears as a single spanning-tree node to the rest of the network, and all stack members use the same device ID.

Multiple Spanning Tree Protocol Configuration Guidelines • When you enable MST by using the spanning-tree mode mst global configuration command, RSTP is automatically enabled. • For configuration guidelines about UplinkFast, BackboneFast, and cross-stack UplinkFast, see the relevant sections in the Related Topics section. Configuring Multiple Spanning-Tree Protocol Information About Multiple Spanning Tree Protocol Configuring Multiple Spanning-Tree Protocol 2

Стр. 2

Multiple Spanning-Tree Regions

• When the device is in MST mode, it uses the long path-cost calculation method (32 bits) to compute the path cost values. With the long path-cost calculation method, the following path cost values are supported: Path Cost Value Speed 2,000,000 10 Mb/s 200,000 100 Mb/s 20,000 1 Gb/s 2,000 10 Gb/s 200 100 Gb/s Root Switch Configuration The switch maintains a spanning-tree instance for the group of VLANs mapped to it. A device ID, consisting of the switch priority and the switch MAC address, is associated with each instance. For a group of VLANs, the switch with the lowest device ID becomes the root switch.

When you configure a switch as the root, you modify the switch priority from the default value (32768) to a significantly lower value so that the switch becomes the root switch for the specified spanning-tree instance. When you enter this command, the switch checks the switch priorities of the root switches. Because of the extended system ID support, the switch sets its own priority for the specified instance to 24576 if this value will cause this switches to become the root for the specified spanning-tree instance. If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own priority to 4096 less than the lowest switch priority.

(4096 is the value of the least-significant bit of a 4-bit switch priority value. For more information, see Bridge ID, Switch Priority, and Extended System ID . If your network consists of switches that support and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software. The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning-tree primary root.

Use the diameter keyword, which is available only for MST instance 0, to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time. Multiple Spanning-Tree Regions For switches to participate in multiple spanning-tree (MST) instances, you must consistently configure the switches with the same MST configuration information.

A collection of interconnected switches that have the same MST configuration comprises an MST region. The MST configuration controls to which MST region each device belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. You configure Configuring Multiple Spanning-Tree Protocol Root Switch Configuration Configuring Multiple Spanning-Tree Protocol 3

Стр. 3

Operations Within an Multiple Spanning Tree Region

the device for a region by specifying the MST region configuration on it. You can map VLANs to an MST instance, specify the region name, and set the revision number. For instructions and an example, select the "Specifying the MST Region Configuration and Enabling MSTP" link in Related Topics. A region can have one or multiple members with the same MST configuration. Each member must be capable of processing RSTP bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network, but each region can support up to 65 spanning-tree instances. Instances can be identified by any number in the range from 0 to 4094.

You can assign a VLAN to only one spanning-tree instance at a time. Internal Spanning Tree, Common and Internal Spanning Tree, and Common Spanning Tree Unlike PVST+ and Rapid PVST+ in which all the spanning-tree instances are independent, the MSTP establishes and maintains two types of spanning trees: • An internal spanning tree (IST), which is the spanning tree that runs in an MST region. Within each MST region, the MSTP maintains multiple spanning-tree instances. Instance 0 is a special instance for a region, known as the internal spanning tree (IST). All other MST instances are numbered from 1 to 4094. The IST is the only spanning-tree instance that sends and receives BPDUs.

All of the other spanning-tree instance information is contained in M-records, which are encapsulated within MSTP BPDUs. Because the MSTP BPDU carries information for all instances, the number of BPDUs that need to be processed to support multiple spanning-tree instances is significantly reduced. All MST instances within the same region share the same protocol timers, but each MST instance has its own topology parameters, such as root switch ID, root path cost, and so forth. By default, all VLANs are assigned to the IST. An MST instance is local to the region; for example, MST instance 1 in region A is independent of MST instance 1 in region B, even if regions A and B are interconnected.

• A common and internal spanning tree (CIST), which is a collection of the ISTs in each MST region, and the common spanning tree (CST) that interconnects the MST regions and single spanning trees. The spanning tree that is computed in a region appears as a subtree in the CST that encompasses the entire switched domain. The CIST is formed by the spanning-tree algorithm running among switches that support the IEEE 802.1w, IEEE 802.1s, and IEEE 802.1D standards. The CIST inside an MST region is the same as the CST outside a region. Operations Within an Multiple Spanning Tree Region The IST connects all the MSTP switches in a region.

When the IST converges, the root of the IST becomes the CIST regional root. It is the switch within the region with the lowest device ID and path cost to the CIST root. The CIST regional root is also the CIST root if there is only one region in the network. If the CIST root is outside the region, one of the MSTP switches at the boundary of the region is selected as the CIST regional root. When an MSTP switch initializes, it sends BPDUs claiming itself as the root of the CIST and the CIST regional root, with both of the path costs to the CIST root and to the CIST regional root set to zero. The switch also initializes all of its MST instances and claims to be the root for all of them.

If the switch receives superior MST root information (lower device ID, lower path cost, and so forth) than currently stored for the port, it relinquishes its claim as the CIST regional root. Configuring Multiple Spanning-Tree Protocol Internal Spanning Tree, Common and Internal Spanning Tree, and Common Spanning Tree Configuring Multiple Spanning-Tree Protocol 4

Стр. 4