CISCO and STP Variants


Like many networking standards, the evolution of STP has been driven by the need to create industry-wide specifications when proprietary protocols become de facto standards. When a proprietary protocol becomes so prevalent that all competitors in the market need to support it, agencies like the IEEE step in and create a public specification. The evolution of STP has followed this same path, as seen in the table. When you read about STP on the Cisco.com site, you notice that there are many types or variants of STP. Some of these variants are Cisco proprietary and others are IEEE standards. You will learn more details on some of these STP variants, but to get started you need to have a general knowledge of what the key STP variants are. The table summarizes the following descriptions of the key Cisco and IEEE STP variants.
Cisco Proprietary
Per-VLAN spanning tree protocol (PVST) - Maintains a spanning-tree instance for each VLAN configured in the network. It uses the Cisco proprietary ISL trunking protocol that allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs. Because PVST treats each VLAN as a separate network, it can load balance traffic at Layer 2 by forwarding some VLANs on one trunk and other VLANs on another trunk without causing a loop. For PVST, Cisco developed a number of proprietary extensions to the original IEEE 802.1D STP, such as BackboneFast, UplinkFast, and PortFast. These Cisco STP extensions are not covered in this course. To learn more about these extensions, visit: http://www.cisco.com/en/US/docs/switches/lan/catalyst4000/7.4/configuration/guide/stp_enha.html.
Per-VLAN spanning tree protocol plus (PVST+) - Cisco developed PVST+ to provide support for IEEE 802.1Q trunking. PVST+ provides the same functionality as PVST, including the Cisco proprietary STP extensions. PVST+ is not supported on non-Cisco devices. PVST+ includes the PortFast enhancement called BPDU guard, and root guard. To learn more about BPDU guard, visit: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a008009482f.shtml.
To learn more about root guard, visit: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96b.shtml.
Rapid per-VLAN spanning tree protocol (rapid PVST+) - Based on the IEEE 802.1w standard and has a faster convergence than STP (standard 802.1D). Rapid PVST+ includes Cisco-proprietary extensions such as BackboneFast, UplinkFast, and PortFast.
IEEE Standards
Rapid spanning tree protocol (RSTP) - First introduced in 1982 as an evolution of STP (802.1D standard). It provides faster spanning-tree convergence after a topology change. RSTP implements the Cisco-proprietary STP extensions, BackboneFast, UplinkFast, and PortFast, into the public standard. As of 2004, the IEEE has incorporated RSTP into 802.1D, identifying the specification as IEEE 802.1D-2004. So when you hear STP, think RSTP. You will learn more about RSTP later in this section.
Multiple STP (MSTP) - Enables multiple VLANs to be mapped to the same spanning-tree instance, reducing the number of instances needed to support a large number of VLANs. MSTP was inspired by the Cisco-proprietary Multiple Instances STP (MISTP) and is an evolution of STP and RSTP. It was introduced in IEEE 802.1s as amendment to 802.1Q, 1998 edition. Standard IEEE 802.1Q-2003 now includes MSTP. MSTP provides for multiple forwarding paths for data traffic and enables load balancing. A discussion of MSTP is beyond the scope of this course. To learn more about MSTP, visit: http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_19_ea1/configuration/guide/swmstp.html.

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