Spanning Tree rapidly branches out

Spanning Tree Protocol is the tried and tested method for providing path redundancy while eliminating loops in bridged networks.

Spanning Tree Protocol, supported on most bridges and switches, is the tried and tested method for providing path redundancy while eliminating loops in bridged networks. Loops create never-ending data paths, resulting in excessive system overhead.

An essential element of enterprise and carrier infrastructure, Spanning Tree Protocol is defined in the IEEE 802.1D standard. The 802.1w amendment to that standard, Rapid Reconfiguration of Spanning Tree, improves upon STP by providing rapid reconfiguration capability via Rapid Spanning Tree Protocol (RSTP).

Both standards support, preserve and maintain the quality of bridged LAN or media access control (MAC) service. However, networks that support RSTP offer greater resiliency and availability.

As more LAN-based voice and multimedia applications are being deployed, the underlying reliability and performance of the LAN infrastructure become critical. Availability of each network component is essential. Not only should redundancy be built into the WAN links, ISPs and routed network, but there also should be redundant bridged paths. Bridges/switches must be able to detect failures and reconfigure instantaneously. RSTP was designed with this in mind.

Convergence time is key

The key difference between STP and RSTP is the time it takes to converge. Once a link is lost or the topology has changed, STP requires 30 to 60 seconds to detect the changes and reconfigure, which affects network performance. When properly implemented, RSTP reduces the time it takes to reconfigure and restore service on link failures and restorations to subsecond levels, while retaining compatibility with equipment based on STP.

Rapid Spanning Tree is a distributed algorithm that selects a single bridge/switch to act as the spanning tree’s root. The algorithm assigns port roles to individual ports on each bridge/switch. Port roles determine whether the port is to be part of the active topology connecting the bridge or switch to the root bridge (a root port), or connecting a LAN through the bridge/switch to the root bridge (a designated port).

Regardless of their roles, ports can serve as alternate or back-up ports that provide connectivity in the case of failure – when bridges, switches, bridge ports or entire LANs fail or are removed. Bridge Protocol Data Units define the role of a port, and they contain information about the switch and ports.

State machines associated with port roles maintain and change the port states that control the processing and forwarding of frames by a MAC relay entity. Port states can be in a discarding, learning or forwarding status. The 802.1D states of disabled, blocking, and listening have been merged into a unique 802.1w discarding state. States are assigned to support and maintain the quality of the MAC service.

Properly configured port state in a bridge or switch reduces the probability of data loops and the duplication and misordering of frames.

RSTP speeds recovery

RSTP ensures rapid recovery of connectivity following the failure of a bridge/switch, bridge port or LAN. A new root port can transition rapidly to the forwarding port state. Explicit acknowledgements between bridges and switches in the LAN allow designated ports to transition rapidly to the forwarding port state.

Bridge ports can be configured so that they can transition directly to the forwarding port state on re-initialization of the bridge/switch. This is useful when a specific bridge port is connected to a LAN segment that is at the edge of the LAN, such as when no other bridges or switches are reachable by that LAN segment.

Ideally, RSTP should be the default spanning tree implementation in a network. Because of the compatibility between STP and RSTP, transition from STP to RSTP is seamless. However, RSTP employs rapid state transitions that tend to increase the rates of frame duplication and misordering. Therefore, for RSTP to work properly with STP, you should use the RSTP Force Protocol Version parameter to disable rapid transitions.

Hayes is director of technology partners and security product management for Alcatel, and also is a Certified Information Systems Security Professional. He can be reached at jeff.hayes@alcatel.com.