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CCNA Self-Study (ICND Exam): Extending Switched Networks with Virtual LANs

Chapter Description

Steve McQuerry discusses how VLANs operate to provide more effective networks by controlling broadcasts in your network.

Per-VLAN Spanning Tree

One of the things that must be considered with VLANs is the function of the Spanning Tree Protocol (STP). STP is designed to prevent loops in a switch/bridged topology to eliminate the endless propagation of broadcast around the loop. With VLANs, there are multiple broadcast domains to be considered. Because each broadcast domain is like a unique bridged internetwork, you must consider how STP will operate.

The 802.1Q standard defines one unique Spanning Tree instance to be used by all VLANs in the network. STP runs on the Native VLAN so that it can communicate with both 802.1Q and non-802.1Q compatible switches. This single instance of STP is often referred to as 802.1Q Mono Spanning Tree or Common Spanning Tree (CST). A single spanning tree lacks flexibility in how the links are used in the network topology. Cisco implements a protocol known as Per-VLAN Spanning Tree Plus (PVST+) that is compatible with 802.1Q CST but allows a separate spanning tree to be constructed for each VLAN. There is only one active path for each spanning tree; however, in a Cisco network, the active path can be different for each VLAN.


The term Mono Spanning Tree is typically not used anymore because the IEEE 802.1s standard has now defined a Multiple Spanning Tree (MST) protocol that uses the same acronym.

Because a trunk link carries traffic for more than one broadcast domain and switches are typically connected together via trunk links, it is possible to define multiple Spanning Tree topologies for a given network. With PVST+, a root bridge and STP topology can be defined for each VLAN. This is accomplished by exchanging BPDUs for each VLAN operating on the switches. By configuring a different root or port cost based on VLANs, switches could utilize all the links to pass traffic without creating a bridge loop. Using PVST+, administrators can use ISL or 802.1Q to maintain redundant links and load balance traffic between parallel links using the Spanning Tree Protocol. Figure 3-15 shows an example of load balancing using PVST+.

Figure 15Figure 3-15 PVST Load Balancing

Cisco developed PVST+ to allow running several STP instances, even over an 802.1Q network by using a tunneling mechanism. PVST+ utilizes Cisco devices to connect to a Mono Spanning Tree zone, typically another vendor's 802.1Q-based network, to a PVST+ zone, typically a Cisco ISL-based network. No specific configuration is needed to achieve this. PVST+ provides support for 802.1Q trunks and the mapping of multiple spanning trees to the single spanning tree of standard 802.1Q switches running Mono Spanning Tree.

The PVST+ architecture distinguishes three types of regions:

  • A PVST region (PVST switches using ISL only)

  • A PVST+ region (PVST+ using ISL and/or 802.1Q between Cisco switches)

  • A Mono Spanning Tree region (Common or Mono Spanning Tree using 802.1Q and exchanging BPDUs on the Native VLAN only between a Cisco and Non-Cisco switches using 802.1Q)

Each region consists of a homogenous type of switch. You can connect a PVST region to a PVST+ region using ISL ports. You can also connect a PVST+ region to a Mono Spanning Tree region using 802.1Q ports.

At the boundary between a PVST region and a PVST+ region, the mapping of Spanning Tree is one-to-one. At the boundary between a Mono Spanning Tree region and a PVST+ region, the Spanning Tree in the Mono Spanning Tree region maps to one PVST in the PVST+ region. The one it maps to is the CST. The CST is the PVST of the Native VLAN (VLAN 1 by default).

On a 802.1Q trunk, BPDUs can be sent or received only by the Native VLAN. Using PVST+, Cisco can send its PVST BPDUs as tagged frames using a Cisco multicast address as the destination. When a non-Cisco switch receives the multicast, it is flooded (but not interpreted as a BPDU, thus maintaining the integrity of CST). Because it is flooded, it will eventually reach Cisco switches on the other side of the CST domain. This allows the PVST fames to be tunneled through the MST region. Tunneling means that the BPDUs are flooded through the Mono Spanning Tree region along the single spanning tree present in the Mono Spanning Tree region.

PVST+ networks must be in a tree-like structure for proper STP operation.

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