About RPVST+

Comparing spanning tree options

Without spanning tree, having more than one active path between a pair of nodes causes loops in the network, which can result in duplication of messages, leading to a “broadcast storm” that can bring down the network.

The 802.1D spanning tree protocol operates without regard to a network's VLAN configuration, and maintains one common spanning tree throughout a bridged network. This protocol maps one loop-free, logical topology on a given physical topology. This results in the least optimal link utilization and longest convergence times.

The 802.1s multiple spanning tree protocol (MSTP) uses multiple spanning tree instances with separate forwarding topologies. Each instance is composed of one or more VLANs, which significantly improves network link utilization and the speed of reconvergence after a failure in the network’s physical topology. However, MSTP requires more configuration overhead and is more susceptible to dropped traffic due to misconfiguration.

Rapid spanning tree protocol (RSTP) requires less configuration overhead, provides faster convergence on point-to-point links, and speedier failure recovery with predetermined, alternate paths. The switches covered by this guide, use the IEEE Rapid Per-VLAN spanning tree Protocol (RPVST) standard. RPVST was introduced as an enhancement to Rapid spanning tree Protocol (RSTP) to improve the link utilization issue and require less configuration overhead. Basically, RPVST+ is RSTP operating per-VLAN in a single layer 2 domain. VLAN tagging is applied to the ports in a multi-VLAN network to enable blocking of redundant links in one VLAN while allowing forwarding over the same links for non-redundant use by another VLAN. Each RPVST+ tree can have a different root switch and therefore can span through different links. Since different VLAN traffic can take different active paths from multiple possible topologies, overall network utilization increases.

Another major advantage of RPVST+ is that it localizes topology change propagation to individual VLANs. Since there is a separate spanning tree for each VLAN, topology changes affecting a particular VLAN are propagated only inside that VLAN. The switch flushes the MAC addresses learned only on the affected VLAN and other VLAN traffic is not disturbed. This minimizes the network flooding caused by the spanning tree topology changes. This is a significant improvement in the case of a large, flat, layer 2 network. Note that in a network having a large number of per-VLAN spanning tree instances, RPVST+ can result in an increased load on the switch's CPU.

Understanding how RPVST+ operates

RPVST+ applies one RSTP tree per-VLAN. Each of these RSTP trees can have a different root switch and span the network through shared or different links. As shown in RSTP forming a single spanning tree across all VLANs since the active paths for traffic on different VLANs can use the same for different links, multiple topologies are possible, and overall network utilization increases.

RSTP forming a single spanning tree across all VLANs

RSTP forming a single spanning tree across all VLANs

The topology has four switches running RSTP. Switch “A” is the root switch. In order to prevent a loop, RSTP blocks the link between switch “B” and switch “D”. There are two VLANs in this network (VLAN 10 and VLAN 20). Since RSTP does not have VLAN intelligence, it forces all VLANs in a layer 2 domain to follow the same spanning tree. There will not be any traffic through the link between switch “B” and switch “D” and hence the link bandwidth gets wasted. On the other hand, RPVST+ runs different spanning trees for different VLANs. Consider the following diagrams.

RPVST+ creating a spanning tree for VLAN 10

RPVST+ creating a spanning tree for VLAN 10

RPVST+ creating a spanning tree for VLAN 20

RPVST+ creating a spanning tree for VLAN 20

The two topologies above are the same as the first topology, but now the switches run RPVST+ and can span different trees for different VLANs. Switch “A” is the root switch for the VLAN 10 spanning tree and switch “D” is the root switch for the VLAN 20 spanning tree. The link between switch “B” and switch “D” is only blocked for VLAN 10 traffic but VLAN 20 traffic goes through that link. Similarly the link between switch “A” and switch “C” is blocked only for VLAN 20 traffic but VLAN 10 traffic goes through that link. Here, traffic passes through all the available links, and network availability and bandwidth utilization increase.

Another major advantage of RPVST+ is that it localizes topology change propagation. Since there is a separate spanning tree for each VLAN, topology changes affecting a particular VLAN are propagated only inside that VLAN. The switch flushes the MAC addresses learned only on the affected VLAN, the traffic on other VLANs is not disturbed. This minimizes the network flooding due to spanning tree topology changes. This is a significant improvement in the case of a large, flat, layer 2 network.

The following figure shows a further example of shared links and redundant path-blocking in a network running RPVST+.

Sample RPVST+ network

Sample RPVST+ networkspanning treeRPVST+sample network

Working with the default RPVST+ configuration

In the factory default configuration, spanning tree operation is disabled. Configuring the spanning tree mode as RPVST+ on a switch and then enabling spanning tree automatically creates a spanning tree instance for each VLAN on the switch. Configuration with default settings is automatic, and in many cases does not require any adjustments. This includes operation with spanning tree regions in your network running STP, MSTP, or RSTP. Also, the switch retains its currently configured spanning tree parameter settings when spanning tree is disabled. Thus, if you disable, then later re-enable spanning tree, the parameter settings will be the same as before spanning tree was disabled.


[CAUTION: ]

CAUTION: The switch automatically senses port identity and type, and automatically defines spanning tree parameters for each type, and parameters that apply across the switch. Although these parameters can be adjusted, HP strongly recommends leaving these settings in their default configurations unless the proposed changes have been supplied by an experienced network administrator who has a strong understanding of RPVST+ operation.


Operating notes

Recommended application

RPVST+ is ideal in networks having less than 100 VLANs. In networks having 100 or more VLANs, MSTP is the recommended spanning tree choice due to the increased load on the switch CPU.

VLAN membership

A port will be part of a given VLAN spanning tree only if the port is a member of that VLAN.

RPVST+ interoperates with RSTP and MSTP on VLAN 1

Because a switch running RPVST+ transmits IEEE spanning tree BPDUs, it can interoperate with IEEE RSTP and MSTP spanning tree regions, and opens or blocks links from these regions as needed to maintain a loop-free topology with one physical path between regions.


[NOTE: ]

NOTE: RPVST+ interoperates with RSTP and MSTP only on VLAN 1.


Single spanning tree applications

One spanning tree variant can be run on the switch at any given time. On a switch running RPVST+, MSTP cannot be enabled. However, any MSTP-specific configuration settings in the startup configuration file will be maintained.

Exclusions

The following features cannot run concurrently with RPVST+:

  • Features that dynamically assign ports to VLANs:

    • GVRP

    • RADIUS-based VLAN assignments (802.1X, WebAuth, MKAC auth

    • Auth-VID/UnAuth-VID configuration on interfaces

    • MAC-Based VLANs

    • LLDP Radio Port VLAN

  • Switch Meshing

  • QinQ

  • Protocol VLANs

  • Distributed Trunking

  • Filter Multicast in rapid-PVST mode (The multicast MAC address value cannot be set to the PVST MAC address 01:00:0c:cc:cc:cd.)

GVRP

Spanning tree mode cannot be set to RPVST+ when GVRP is enabled, and GVRP cannot be enabled when RPVST+ is enabled.

RPVST+ operating limits

Virtual ports (vPorts) on a switch are determined by the number of physical ports on the switch, plus other factors. Exceeding the recommended number of vPorts can result in dropped BPDUs.

Allowing traffic on per-VLAN ID (PVID) mismatched links

The switch generates an Event Log message for a VID mismatch on an active RPVST+ VLAN only if ignore-pvid-inconsistency is disabled (the default).

If ignore-pvid-inconsistency is enabled on multiple switches connected by hubs, there could be more than two VLANs involved in PVID mismatches that will be ignored by RPVST+.

If there is an actual misconfiguration of port VLAN memberships in a network, then enabling ignore-pvid-inconsistency prevents RPVST+ from detecting the problem. This could result in packet duplication in the network because RPVST+ would not converge correctly.