31.1.2 Operating specifications

To use the Ring Protocol and Spanning Tree Protocols together, a virtual line must be connected between any two devices on which both functionalities exist. This virtual circuit is called a virtual link. Virtual links are built between two devices on a ring network. Building a virtual link requires a virtual link ID for identifying the virtual link, and a virtual link VLAN for sending and receiving control frames between virtual links.

Nodes using the Ring Protocol and Spanning Tree Protocols together comprise a Spanning Tree topology with devices that have the same virtual link ID as that of the local device. Device groups with the same virtual link ID are called bases, and each base comprises an independent Spanning Tree topology.

The following figure gives an overview of virtual links.

Figure 31-3: Overview of virtual links

<Structure of this section>

(1) Virtual-link VLAN

A virtual link VLAN is used to send and receive control frames between virtual links. One of the VLANs managed as a VLAN for data transfer for the ring port is used as the virtual link VLAN. A virtual link VLAN can use the same VLAN ID on multiple bases.

(2) Handling Ring Protocol Control VLAN

Control VLANs for the Ring Protocol are not subject to Spanning Tree Protocols.

Therefore, a tree of corresponding VLANs is not built for PVST+. Also, the transfer status for Single Spanning Tree and Multiple Spanning Tree is not applied.

(3) Ring port status and configuration settings

The transfer status of the VLAN for data transfer for a ring port is determined by the Ring Protocol.

For example, when the Blocking status is determined by a Spanning Tree topology, if the Ring Protocol determines it to be Forwarding, the status of the port is Forwarding. Therefore, when a topology is built in which the ring port is Blocking for the Spanning Tree Protocol, a loop might occur. This means that for a Spanning Tree Protocol used with the Ring Protocol and for which the ring port is always Forwarding, the initial value of the bridge priority for the Switch is automatically raised so that the Switch becomes the root bridge or next item in priority. Any values set by configuration will be used for operation.

The following table describes the value set for bridge priority.

Table 31-1: Bridge priority setting values
Configuration items	Related configuration	Initial value
Bridge priority	spanning-tree single priority spanning-tree vlan priority spanning-tree mst root priority	0

Note that the port for a virtual link runs with a fixed value because values set by configuration are not applied.

The following table describes the values set for virtual link ports.

Table 31-2: Virtual link port settings
Configuration items	Related configuration	Initial value (fixed)
Link type	spanning-tree link-type	point-to-point
Port priority	spanning-tree port-priority spanning-tree single port-priority spanning-tree vlan port-priority spanning-tree mst port-priority	0
Path cost	spanning-tree cost spanning-tree single cost spanning-tree vlan cost spanning-tree mst cost	1

(4) About Spanning Tree functionality on ring ports

The following Spanning Tree functionality does not work for ring ports.

BPDU filter
BPDU guard
Loop guard functionality
Root guard functionality
PortFast functionality

(5) Clearing MAC address table when the spanning tree topology is changed

When the topology is changed for a Spanning Tree Protocol, a flush control frame is sent so that MAC address table entries are cleared for the entire single ring or multi-ring network. Each device receiving this in the ring network clears MAC address table entries for ring ports for which the Ring Protocol is running. Note that the base device for which the topology change occurs clears MAC address table entries through the Spanning Tree Protocol.

(6) About Temporary Blocking on Ports Other Than Ring Ports

When one of the following events occurs on a device using both the Ring Protocol and a Spanning Tree Protocol, ports for the Spanning Tree Protocol other than for the ring port are temporarily put in Blocking status.

Switch startup (including restarting of the switch)
Application of the configuration file to a running configuration
Executing the restart vlan command
Executing the restart spanning-tree command

When the topology within an access network is built before control frames can be sent and received by the Spanning Tree Protocol over a virtual link, no ports are changed to Blocking, because this alone will not cause a loop configuration. However, because a loop configuration will occur across the ring network and access network if this is left as is, this functionality is used to temporarily set the Blocking status to prevent loops. This functionality can also run on ports for which PortFast functionality is set, Blocking by this function is performed as follows, depending on the type and status of the Spanning Tree Protocol being used.

For PVST+/STP, if the Switch is the root bridge in Rapid PVST+/Rapid STP/MSTP

30 seconds after the event occurred
When the Switch is a designated bridge in Rapid PVST+/Rapid STP/MSTP

4 seconds after the event was received BPDU through the virtual link

To run this functionality effectively, configure the setting values within the ranges shown in the table below. If these values are not set within range, loops might occur temporarily.

Table 31-3: Setting values when a port other than a ring port is temporarily placed in blocking status
Configuration items	Related configuration	Value set
Reception hold time for Ring Protocol flush control frames	forwarding-shift-time	10 seconds or less (default value of 10 seconds)
Spanning Tree control frame sending interval	spanning-tree single hello-time spanning-tree vlan hello-time spanning-tree mst hello-time	2 seconds or less (default value of 2 seconds)