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Understanding VSTP for EX Series Switches

Juniper Networks EX Series Ethernet Switches provide Layer 2 loop prevention through Spanning Tree Protocol (STP), Rapid Spanning Tree Protocol (RSTP), Multiple Spanning Tree Protocol (MSTP), and VLAN Spanning Tree Protocol (VSTP). The default factory configuration for EX Series switches uses RSTP. If you use VLANs, however, we recommend that you enable MSTP unless your network requires the device compatibility provided by VSTP. Switches configured to run VSTP automatically assign each VLAN to one spanning-tree instance that runs RSTP. While this approach is useful to optimize network usage in small networks with a limited number of VLANs, a VSTP configuration in a network with several hundred VLANs can overload switch CPUs. MSTP ensures that your network is not slowed down by the increased network traffic caused by hundreds of VLANs, each with its own spanning-tree instance.

VSTP and RSTP are the only spanning-tree protocols that can be configured concurrently on a switch. The maximum number of VLANs that can be supported by VSTP depends upon whether you are using Junos OS for EX Series and QFX Series switches with support for the Enhanced Layer 2 Software (ELS) configuration style or Junos OS that does not support ELS. For ELS details, see Getting Started with Enhanced Layer 2 Software.

VSTP Guidelines and Configuration Constraints

When you configure VSTP, note the following guidelines and constraints:

  • The number of VLANs supported depends on the specific switch platform:
    • On EX Series (other than EX9200) and QFX switches running Junos OS that supports ELS—VSTP can support up to 510 VLANs.
    • On EX9200 switches—VSTP can support up to 4000 VLANs.
    • On EX Series switches running Junos OS that does not support ELS—VSTP can support up to 253 VLANs.
  • We recommend that you enable VSTP on all VLANs that can receive VSTP bridge protocol data units (BPDUs).
  • If a VLAN is configured for VSTP/MSTP, all the interfaces belonging to that VLAN must also be explicitly configured for VSTP/MSTP.
  • The vlan all option is supported on EX Series and QFX Series that run Junos OS without ELS.
  • When you configure VSTP with the set protocol vstp vlan all command, VLAN ID 1 is not set; it is excluded so that the configuration is compatible with Cisco PVST+. If you want VLAN ID 1 to be included in the VSTP configuration on your switch, you must set it separately with the set protocol vstp vlan 1 command.
  • You cannot configure VSTP on logical interfaces that have:
    • VLAN translations
    • VLAN range statement
    • Dual VLAN tags

Selecting a Spanning-Tree Protocol

The default factory configuration for EX Series switches is RSTP, a faster version of STP. To determine which spanning-tree protocol is best for your situation, see Table 1.

Table 1: Selecting a Spanning-Tree Protocol

ProtocolAdvantagesDisadvantages

RSTP

  • Rapid Spanning Tree Protocol is the default switch configuration and is recommended for most network configurations because it converges more quickly than STP after a failure.
  • Voice and video work better with RSTP than they do with STP.
  • RSTP is backward compatible with STP so switches do not all have to run RSTP.
  • RSTP supports more ports than MSTP or VSTP.
  • RSTP does not work with 802.1D 1998 bridges.
  • RSTP is not recommended for multiple VLAN networks because it is not VLAN-aware—as a result, all VLANs within a LAN share the same spanning tree. This limits the number of forwarding paths for data traffic.

STP

  • Spanning Tree Protocol works with 802.1D 1998 bridges.
  • RSTP is backward compatible with STP so switches do not all have to run STP.
  • STP is slower than RSTP.
  • STP is not recommended for multiple VLAN networks because it is not VLAN-aware—as a result, all VLANs within a LAN share the same spanning-tree. This limits the number of forwarding paths for data traffic.

MSTP

  • Multiple Spanning Tree Protocol works with most VLANs.
  • RSTP and STP are recognized as distinct Spanning Tree regions by MSTP.
  • Some protocols require compatibility that is not provided by MSTP. In this case, use VSTP.
  • MSTP supports a limited number of ports.
  • MSTP uses more CPU than RSTP and does not converge as fast as RSTP.

VSTP

  • VLAN Spanning Tree Protocol works with VLANs that require device compatibility.
  • VSTP and RSTP are the only spanning-tree protocols that can be configured concurrently on a switch.
  • With VSTP there can be only STP instance per VLAN, whereas MSTP lets you combine multiple VLANs in one instance.
  • VSTP supports a limited number of ports compared to RSTP.
  • VSTP uses more CPU than RSTP and does not converge as fast as RSTP.
 

Related Documentation

 

Modified: 2016-03-15

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