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Spanning-Tree Protocols Overview

The Spanning Tree Protocol (STP) is used to create a loop-free topology in Layer 2 networks.

STP is a Layer 2 protocol that calculates the best path through a switched network that contains redundant paths. STP uses bridge protocol data unit (BPDU) packets to exchange information with other switches. STP uses the information provided by the BPDUs to elect a root bridge, identify root ports for each switch, identify designated ports for each physical LAN segment, and prune specific redundant links to create a loop-free tree topology. The resulting tree topology provides a single active Layer 2 data path between any two end stations. In discussions of STP, the terms bridge and switch are used interchangeably.

The original Spanning Tree Protocol is defined in the IEEE 802.1D 1998 specification. A newer version called Rapid Spanning Tree Protocol (RSTP) was originally defined in the IEEE 802.1w draft specification and later incorporated into the IEEE 802.1D-2004 specification. A recent version called Multiple Spanning Tree Protocol (MSTP) was originally defined in the IEEE 802.1s draft specification and later incorporated into the IEEE 802.1Q-2003 specification.

RSTP provides faster reconvergence time than the original STP by identifying certain links as point to point and by using protocol handshake messages rather than fixed timeouts. When a point-to-point link fails, the alternate link can transition to the forwarding state without waiting for any protocol timers to expire.

MSTP provides the capability to logically divide a Layer 2 network into regions. Every region has a unique identifier and can contain multiple instances of spanning trees. All regions are bound together using a Common Instance Spanning Tree (CIST), which is responsible for creating a loop-free topology across regions, whereas the Multiple Spanning Tree Instance (MSTI) controls topology inside regions. MSTP uses RSTP as a converging algorithm and is fully interoperable with earlier versions of STP.

The VLAN Spanning Tree Protocol (VSTP) is compatible with the Per-VLAN Spanning Tree Plus (PVST+) and Rapid-PVST+ protocols supported on Cisco Systems routers and switches. VSTP maintains a separate spanning-tree instance for each VLAN. Different VLANs can use different spanning-tree paths and VSTP can support up to 4094 different spanning-tree topologies. When different VLANs can use different spanning-tree paths, the CPU processing resources being consumed increase as more VLANs are configured. VSTP BPDU packets are tagged with the corresponding VLAN identifier and are transmitted to the multicast destination media access control (MAC) address 01-00-0c-cc-cc-cd with a protocol type of 0x010b. VSTP BPDUs are tunneled by pure IEEE 802.1q bridges.

The MX-series routers support STP, RSTP, MSTP, and VSTP.

Note: All virtual switch routing instances configured on an MX-series router are supported using only one spanning-tree process. The Layer 2 control protocol process is named l2cpd.

For more information about the various versions of STP, see the appropriate IEEE specification.

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