STP

The Spanning Tree Protocol is used to avoid L2 loops withing the same Ethernet broadcast domain. STP is needed because frames do not have Time To Live (TTL): if a loop is formed, frames will continue to flow between switches, which multiply them over and over until a network collapse. Because a link between switches must be redundant, a mechanism is needed to temporary disable all paths but one: this is the purpose of STP.

There are many STP protocols:

  • STP: Spanning Tree Protocol (802.1d), defined for simple networks with only one VLAN (802.1q was not developed yet). STP is also known as MST or Mono Spanning Tree (rarely used).
  • PVST: Per VLAN Spanning Tree (Cisco), defined by Cisco to enhance 802.1d and support multiple VLANs on ISL trunks.
  • CST: Common Spanning Tree (802.1q), defined for networks with more than one VLAN (and with 802.1q trunks), a single (common) instance configure just one topology for all VLANs.
  • PVST+: Per VLAN Spanning Tree Plus (Cisco), enhance PVST to support 802.1q trunks. PVST is the default STP on Cisco Catalyst switches and it is compatible with CST.
  • RSTP: Rapid Spanning Tree (802.1W), enhance STP reducing convergence time. It is compatible with STP.
  • RPVST+: Rapid Per VLAN Spanning Tree Plus (Cisco), enhance PVST+ reducing convergence time. It is compatible with RSTP and it is a default on Cisco Nexus switches.
  • MST: Multiple Spanning Tree (Cisco), is a 802.1s pre-standard.
  • MIST: Multiple Instance Spanning Tree (802.1s), uses RSTP.

Cisco Catalyst switches allows only 3 out of 7 discussed protocols:

  • pvst: it configures PVST+ (with 802.1q support), compatible with PVST, STP e CST.
  • rapid-pvst: compatible with RSTP.
  • mst: standard and pre-standard.

References

Posted on 27 May 2013 by Andrea.
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