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Layer 2 Control Protocol on ACX Series Routers


On ACX Series routers in a Layer 2 environment, you can configure various spanning-tree protocol versions to create a loop-free topology in Layer 2 networks.

A spanning-tree protocol is a Layer 2 control protocol (L2CP) that calculates the best path through a switched network containing redundant paths. A spanning-tree protocol uses bridge protocol data unit (BPDU) data frames to exchange information with other switches. A spanning-tree protocol 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 spanning-tree protocols, the terms bridge and switch are often used interchangeably.

The ACX Series Universal Access Routers support Spanning-Tree Protocol (STP), Rapid Spanning-Tree Protocol (RSTP), Multiple Spanning-Tree Protocol (MSTP), and VLAN Spanning-Tree Protocol (VSTP).

  • The original STP is defined in the IEEE 802.1D 1998 specification. A newer version called 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 MSTP was originally defined in the IEEE 802.1s draft specification and later incorporated into the IEEE 802.1Q-2003 specification. The VSTP is compatible with the Per-VLAN Spanning Tree Plus (PVST+) and Rapid-PVST+ protocols supported on Cisco Systems routers and switches.

  • 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 within regions. MSTP uses RSTP as a converging algorithm and is fully interoperable with earlier versions of STP.

  • VSTP maintains a separate spanning-tree instance for each VLAN. Different VLANs can use different spanning-tree paths. When different VLANs 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.

ACX Series routers can support up to 128 STP instances, which includes all instances of VSTP, MSTP, RSTP and STP.

  • Maximum number of VSTP instances supported is 128.

  • Maximum number of MSTP instances supported is 64.

  • Maximum number of RSTP instances supported is 1.

  • Maximum number of STP instances supported is 1.

The ACX Series routers supports enabling Spanning-Tree Protocol (STP), Rapid Spanning-Tree Protocol (RSTP), Multiple Spanning-Tree Protocol (MSTP), and VLAN Spanning-Tree Protocol (VSTP) on aggregated Ethernet interface .

For more information about the various versions of spanning-tree protocols, see the appropriate IEEE specification.