Overview of Spanning Tree Protocol on Juniper Networks MX Series 3D Universal Edge Routers

The Spanning Tree Protocol (STP) is defined in the Institute of Electrical and Electronics Engineers (IEEE) Standard 802.1D. STP examines the network topology and calculates a spanning-tree structure that encompasses all bridges in a given Layer 2 network domain. It can disable redundant paths by pruning links that are not part of the tree, leaving a unique single path from each source to any other destination in the network.

STP consists of distance vector protocols that use distance or hop count as the primary metric for determining the best forwarding path. It uses maximum-age parameters to help avoid loops that count to infinity.

This document describes the major Spanning Tree Protocol versions supported on Juniper Networks MX Series 3D Universal Edge Routers using Junos OS Release 8.4 or later, including:

• Spanning Tree Protocol (STP), specified in IEEE 802.1D 1998
• Rapid Spanning Tree Protocol (RSTP), specified in IEEE 802.1w; standardized in IEEE 802.1D 2004
• Multiple Spanning Tree Protocol (MSTP), specified in IEEE 802.1s; standardized in IEEE 802.1Q 2003
• Virtual LAN Spanning Tree Protocol (VSTP), Juniper Networks solution for using STP with multiple VLANs

Both Juniper Networks EX Series Ethernet Switches and MX Series routers run Junos OS and support a similar set of Layer 2 features with some variations. The EX Series switches support three standard versions: STP, RSTP, and MSTP. The MX Series routers add support for VSTP, which is compatible with Per-VLAN Spanning Tree Plus (PVST+) and Rapid-PVST+ protocols supported on other vendors’ routers and switches.

Understanding the Requirement for Spanning Tree Protocols

MX Series routers are used for deploying Layer 2 bridged networks. STPs are essential for these types of deployments, where they are used for allowing Layer 2 bridged networks to include spare, redundant links that provide automatic backup paths if an active link fails. However, whenever there are redundant paths between a source and a destination, there is the potential for forwarding loops. Forwarding loops must be avoided because they result in broadcast storms in the network. STPs are required to block redundant forwarding paths to prevent forwarding loops.

This document helps you decide which STP version can and should be used on your MX Series routers to ensure a loop-free topology.

Related Topics

• Decision Sequence for a Loop-Free STP Topology
• Key Concepts in Spanning Tree Protocols
• Multiple Spanning Tree Protocol
• 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
• VLAN Spanning Tree Protocol