VLAN Spanning Tree Protocol

VLAN Spanning Tree Protocol (VSTP) is a proprietary protocol developed by Juniper Networks to allow each VLAN to have a completely independent STP configuration, providing STP control on a per-VLAN basis. For example, each VLAN can have its root bridge located in a different place. Cost values and priority values can be tuned on a per-VLAN basis. Per-VLAN control allows the network designer total flexibility when it comes to optimizing data flows within each VLAN. It also makes spanning-tree load-balancing possible. However, VSTP also creates a lot of chaotic traffic in the network as the BPDUs are exchanged for every VLAN.

Interoperability with Other Solutions

VSTP on Juniper Networks routers has some interoperability with other proprietary per-VLAN vendor solutions; specifically, VSTP can interoperate with per-VLAN spanning tree (PVST) and Rapid-PVST on IEEE 802.1Q non-proprietary trunks.

In PVST, the IEEE 802.1Q BPDUs are sent untagged on the common spanning-tree VLAN 1 for interoperability with other vendors. The CST BPDUs are sent to the IEEE standard bridge group: MAC Address 01-80-c2-00-00-00, DSAP 42, SSAP 42.

PVST BPDUs are tagged and sent to MAC address 01-00-0c-cc-cc-cd (SNAP HDLC protocol type 0x010b) for each VLAN on a trunk. PVST per-VLAN BPDUs are tunneled by pure IEEE 802.1Q bridges.

VLAN Spanning Tree Protocol Operation

In a Layer 2 bridging network with a single spanning-tree instance (SSTP), a single instance of STP or RSTP runs on all VLANs in a single bridged LAN environment.

There are certain network deployment scenarios that require different VLAN groups to be administered for trunk load-sharing or other purposes. MSTP can be used to achieve trunk load-balancing.

In Junos OS revision 9.0 and later, a single spanning tree running on each configured VLAN is supported to ensure that each VLAN has a loop-free data path through the Layer 2 network. VSTP and Rapid VSTP (RVSTP) can interoperate in these scenarios with other proprietary solutions such as PVST and Rapid PVST on non-proprietary trunks.

Virtual bridges can be defined within a routing instance by configuring the routing instance type as virtual-switch. Each virtual switch can run one instance of standard STP or RSTP common spanning-tree (CST) and zero or more STP or RSTP instances for each enabled VLAN for VSTP. BPDUs of the CST are sent without any IEEE 802.1Q tags (untagged frame) on a port. BPDUs for the VSTP are sent tagged with corresponding VLAN IDs.

Each instance of VSTP or RVSTP on a VLAN elects a single root bridge. The root bridge distributes the STP information associated with that VLAN to all other bridges in the network. Because different root bridges can be provisioned for each VLAN in the network, this feature enhancement can be used by the network administrator for load balancing.

To enable VSTP or RVSTP for a specific VLAN ID, the user must define a bridge domain or VPLS routing instance with a VLAN ID, and all of the logical interfaces assigned to each of them should have the same VLAN ID.

For each VSTP or RVSTP, you can separately specify bridge-priority, max-age, hello-time, forward-delay, port-priority, port-cost, port-mode and port-edge. Each of these parameters are the same as in standard STP or RSTP.

For interfaces configured at the global level, VSTP is enabled for all configured VLANs. If an interface is configured at both the global and the VLAN level, the configuration at the VLAN level overrides the global configuration.

Because VSTP is enabled per VLAN, a physical interface can be part of more than one VSTP instance under different virtual bridges, if each of those VSTP instances have different VLANs.

The Juniper Networks default implementation of STP is RSTP and the default operation of VSTP is RVSTP.

VSTP Configuration Constraints on Logical Interfaces

VSTP has some restrictions. It cannot be configured on logical interfaces that have:

• VLAN translations
• The VLAN range statement included
• Dual VLAN tags

VSTP Interoperability with Other Spanning-Tree Configurations

Virtual switches that have VLAN spanning trees enabled can still run standard STP and RSTP on the ports and can interoperate with standard IEEE 802.1Q bridges by sending untagged BPDUs to the IEEE standard bridge group (MAC address 01-80-c2-00-00-00).

VSTP does not directly interoperate with STP, RSTP, and MSTP. Instead, VSTP BPDUs are transparently tunneled by standard STP, RSTP, and MSTP bridges. To interoperate with standard IEEE 802.1Q bridges, RSTP must also be enabled on any ports where VSTP is enabled.

When interoperating with other vendor VLAN STP solutions, VSTP BPDUs that are tagged with another vendor’s multicast MAC address (for example, 01-00-0c-cc-cc-cd) are tunneled across the IEEE 802.1Q CST. This allows VSTP spanning tree information to be maintained by Juniper Networks devices even if they are separated by a cloud of standard IEEE 802.1Q network devices from other vendors.

The following is a VSTP configuration example:

Note: To force VSTP to run in STP mode. include the force-version stp statement at the [edit protocols vstp] hierarchy level.

Related Topics

• Decision Sequence for a Loop-Free STP Topology
• Key Concepts in Spanning Tree Protocols
• Multiple Spanning Tree Protocol
• Overview of Spanning Tree Protocol on Juniper Networks MX Series 3D Universal Edge Routers
• Port Roles in STP
• Rapid Spanning Tree Protocol Port States and Port Roles
• Restrictions and Cautions for Implementing STP
• Spanning Tree Protocol Operation
• Spanning Tree Protocol States
• STP Scaling and Performance on Juniper Networks MX Series 3D Universal Edge Routers