Spanning Tree Theory 2

Spanning Tree Theory

When a loop is detected, STP promptly takes action by blocking a link to resolve the loop. CST (Common Spanning Tree) 802.1d, representing the classic or legacy STP, supports only a single instance for all VLANs, resulting in a single topology with a root switch governing all VLANs in the network.

CST does not facilitate VLAN-based load-balancing, and it only supports the Active-Standby topology. This means one switch acts as the root switch for all VLANs, and if the active switch fails, the standby switch takes over as the root switch for all VLANs.

PVSTP+ is Cisco's implementation of 802.1d but supports a one-to-one instance to VLAN mapping.

Enhancements to PVSTP provide significant optimization for CST. However, it's worth noting that even PVSTP experiences slower convergence compared to MST and RSTP.

RSTP (802.1w) is the IEEE standard for the spanning tree mechanism. Its primary advantage is achieving rapid convergence through the Proposal Agreement handshake mechanism. MST also incorporates the same mechanism and convergence characteristics as RSTP.

Spanning Tree Toolkit

Some of the spanning treefeatures are explained below.

• PortFast: It allows an access port to bypass the listening and learning phasesduring spanning tree operation. Additionally, it prevents the transmission ofTCN (Topology Change Notification) BPDUs when there is a change in the port'sstate.

• UplinkFast: Rapid convergence within three-to-five seconds is ensured following a linkfailure by employing this feature.

• Backbone Fast: For certain topologies, this feature reduces convergence time by theMaxAge duration when dealing with indirect failures.

• Loop Guard: By requiring the presence of Bridge Protocol Data Units (BPDUs), thisfeature prevents the election of the alternate or root port. It also detectsunidirectional link failures when BPDUs are not received from the link.

• Root Guard: Designed to safeguard against external switches assuming the role of theroot switch, this feature ensures optimal switching and forwarding within layer2 spanning tree topologies.

• BPDU Guard: Upon receiving a BPDU (Bridge Protocol Data Unit), this featureautomatically disables a PortFast-enabled port.

• BPDU Filter: While not blocking the ports, this feature ensures that Bridge ProtocolData Units (BPDUs) are neither sent nor received. It primarily functions as amonitoring feature.

As depicted in the diagramshown below, the commonly deployed features can be seen.

Spanning Tree Features Placement

MST (Multiple SpanningTree) 802.1s is the industry standard, and its convergence mechanism resemblesRSTP with a proposal and agreement mechanism. It allows groups of VLANs to bemapped to specific STP instances.

In contrast to RPVST+,where you would require 100 instances for 100 VLANs, MST reduces the burden onswitches' CPU and memory resources, enhancing scalability.

By introducing MST regionsupport, it becomes feasible to interconnect data centers, although the STPdomain remains limited to each local data center. This setup can be likened toOSPF multi-area behavior, where Spanning Tree BPDUs are confined to the MSTregion.

MST efficiently supports asignificant number of VLANs, making it well-suited for large data centers orservice provider access networks. In service provider access networks, MST iscommonly used alongside QinQ (802.1ah Provider Bridging) or Mac in Mac (802.1aqProvider Backbone Bridging).

Even in modern networks,MST remains widely adopted in a number of data centers due to its comprehensiveLayer 2 support and the flexibility of configuring distinct MST regions fordifferent data centers. This approach effectively restricts STP BPDUs toindividual data centers, promoting efficient network management and control.

Spanning Tree Best Practices

• For rapid convergence during direct and indirectfailures, use RSTP or RPVST+ instead.

• When scalability is a concern, choose MST and group VLANsinto MST instances for large-scale VLAN deployments with reduced CPU usage.

• Avoid using 802.1d and CST; if adherence to standardprotocols is necessary, go with RSTP or MST instead.

• Maximize uplink capacity utilization by implementing VLANload-balancing.

• Though VLAN load-balancing may present challenges, iteffectively utilizes all available uplinks.

• Spanning tree selectively blocks certain links within thetopology to prevent switching loops. If using all available links is essential,consider grouping them into a bundle.

• Utilize the Cisco preparatory protocols PAGP and LACP tocombine multiple physical links into a logical bundle.

• LACP can be employed between two switches or multipleswitches as part of Multi-Chassis link aggregation or multi-chassisEtherChannel.

• Ensure the System ID, generated by System Priority andMAC address, is the same on both switches when setting up Multi-chassisEtherChannel.

• For easier troubleshooting, designate one distributionswitch as the primary root switch for odd VLANs and another distribution switchfor even VLANs to enhance predictability.

• Always enable STP on access-facing ports to protect thenetwork from unintentional attacks.

• Implement port-security as an STP loop avoidancemechanism at the edge of Layer 2 campus Ethernet networks.

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