Evolution of STP
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Different Versions of STP
This topic details the many different versions of STP and other options for preventing loops in your network.
Up to now, we have used the term Spanning Tree Protocol and the acronym STP, which can be misleading. Many professionals generically use these to refer to the various implementations of spanning tree, such as Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP). In order to communicate spanning tree concepts correctly, it is important to refer to the implementation or standard of spanning tree in context.
The latest standard for spanning tree is contained in IEEE-802-1D-2004, the IEEE standard for Local and metropolitan area networks:Media Access Control (MAC) Bridges. This version of the standard states that switches and bridges that comply with the standard will use Rapid Spanning Tree Protocol (RSTP) instead of the older STP protocol specified in the original 802.1d standard. In this curriculum, when the original Spanning Tree Protocol is the context of a discussion, the phrase “original 802.1D spanning tree” is used to avoid confusion. Because the two protocols share much of the same terminology and methods for the loop-free path, the primary focus will be on the current standard and the Cisco proprietary implementations of STP and RSTP.
Several varieties of spanning tree protocols have emerged since the original IEEE 802.1D specification, as shown in the table.
|STP||This is the original IEEE 802.1D version (802.1D-1998 and earlier) that provides a loop-free topology in a network with redundant links. Also called Common Spanning Tree (CST), it assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.|
|PVST+||Per-VLAN Spanning Tree (PVST+) is a Cisco enhancement of STP that provides a separate 802.1D spanning tree instance for each VLAN configured in the network. PVST+ supports PortFast, UplinkFast, BackboneFast, BPDU guard, BPDU filter, root guard, and loop guard.|
|802.1D-2004||This is an updated version of the STP standard, incorporating IEEE 802.1w.|
|RSTP||Rapid Spanning Tree Protocol (RSTP) or IEEE 802.1w is an evolution of STP that provides faster convergence than STP.|
|Rapid PVST+||This is a Cisco enhancement of RSTP that uses PVST+ and provides a separate instance of 802.1w per VLAN. Each separate instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.|
|MSTP||Multiple Spanning Tree Protocol (MSTP) is an IEEE standard inspired by the earlier Cisco proprietary Multiple Instance STP (MISTP) implementation. MSTP maps multiple VLANs into the same spanning tree instance.|
|MST||Multiple Spanning Tree (MST) is the Cisco implementation of MSTP, which provides up to 16 instances of RSTP and combines many VLANs with the same physical and logical topology into a common RSTP instance. Each instance supports PortFast, BPDU guard, BPDU filter, root guard, and loop guard.|
A network professional, whose duties include switch administration, may be required to decide which type of spanning tree protocol to implement.
Cisco switches running IOS 15.0 or later, run PVST+ by default. This version incorporates many of the specifications of IEEE 802.1D-2004, such as alternate ports in place of the former non-designated ports. Switches must be explicitly configured for rapid spanning tree mode in order to run the rapid spanning tree protocol.
RSTP (IEEE 802.1w) supersedes the original 802.1D while retaining backward compatibility. The 802.1w STP terminology remains primarily the same as the original IEEE 802.1D STP terminology. Most parameters have been left unchanged. Users that are familiar with the original STP standard can easily configure RSTP. The same spanning tree algorithm is used for both STP and RSTP to determine port roles and topology.
RSTP increases the speed of the recalculation of the spanning tree when the Layer 2 network topology changes. RSTP can achieve much faster convergence in a properly configured network, sometimes in as little as a few hundred milliseconds. If a port is configured to be an alternate port it can immediately change to a forwarding state without waiting for the network to converge.
RSTP Port States and Port Roles
The port states and port roles between STP and RSTP are similar.
Click each button for a comparison between STP and RSTP port states and port roles.
As shown in the figure, there are only three port states in RSTP that correspond to the three possible operational states in STP. The 802.1D disabled, blocking, and listening states are merged into a unique 802.1w discarding state.
As shown in the figure, root ports and designated ports are the same for both STP and RSTP. However, there are two RSTP port roles that correspond to the blocking state of STP. In STP, a blocked port is defined as not being the designated or root port. RSTP has two port roles for this purpose.
As shown in the figure, the alternate port has an alternate path to the root bridge. The backup port is a backup to a shared medium, such as a hub. A backup port is less common because hubs are now considered legacy devices.
PortFast and BPDU Guard
When a device is connected to a switch port or when a switch powers up, the switch port goes through both the listening and learning states, each time waiting for the Forward Delay timer to expire. This delay is 15 seconds for each state, listening and learning, for a total of 30 seconds. This delay can present a problem for DHCP clients trying to discover a DHCP server. DHCP messages from the connected host will not be forwarded for the 30 seconds of Forward Delay timers and the DHCP process may timeout. The result is that an IPv4 client will not receive a valid IPv4 address.
When a switch port is configured with PortFast, that port transitions from blocking to forwarding state immediately, bypassing the usual 802.1D STP transition states (the listening and learning states) and avoiding a 30 second delay. You can use PortFast on access ports to allow devices connected to these ports, such as DHCP clients, to access the network immediately, rather than waiting for IEEE 802.1D STP to converge on each VLAN. Because the purpose of PortFast is to minimize the time that access ports must wait for spanning tree to converge, it should only be used on access ports. If you enable PortFast on a port connecting to another switch, you risk creating a spanning tree loop. PortFast is only for use on switch ports that connect to end devices.
In a valid PortFast configuration, BPDUs should never be received on PortFast-enabled switch ports because that would indicate that another bridge or switch is connected to the port. This potentially causes a spanning tree loop. To prevent this type of scenario from occurring, Cisco switches support a feature called BPDU guard. When enabled, BPDU guard immediately puts the switch port in an errdisabled (error-disabled) state on receipt of any BPDU. This protects against potential loops by effectively shutting down the port. The BPDU guard feature provides a secure response to invalid configurations because an administrator must manually put the interface back into service.
Alternatives to STP
STP was and still is an Ethernet loop-prevention protocol. Over the years, organizations required greater resiliency and availability in the LAN. Ethernet LANs went from a few interconnected switches connected to a single router, to a sophisticated hierarchical network design including access, distribution and core layer switches, as shown in the figure.
Depending on the implementation, Layer 2 may include not only the access layer, but also the distribution or even the core layers. These designs may include hundreds of switches, with hundreds or even thousands of VLANs. STP has adapted to the added redundancy and complexity with enhancements, as part of RSTP and MSTP.
An important aspect to network design is fast and predictable convergence when there is a failure or change in the topology. Spanning tree does not offer the same efficiencies and predictabilities provided by routing protocols at Layer 3. The figure shows a traditional hierarchical network design with the distribution and core multilayer switches performing routing.
Layer 3 routing allows for redundant paths and loops in the topology, without blocking ports. For this reason, some environments are transitioning to Layer 3 everywhere except where devices connect to the access layer switch. In other words, the connections between access layer switches and distribution switches would be Layer 3 instead of Layer 2, as shown in the next figure.
Although STP will most likely continue to be used as a loop prevention mechanism in the enterprise, on access layer switches, other technologies are also being used, including the following:
- Multi System Link Aggregation (MLAG)
- Shortest Path Bridging (SPB)
- Transparent Interconnect of Lots of Links (TRILL)
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