RSTP and Rapid PVST+

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RSTP- IEEE 802.1w is the advance version of STP providing faster convergence than CST but holds with the same a single root bridge in the topology. Most parameters in RSTP have the same to STP, so users that are understood STP can effortlessly configure the RSTP. Rapid PVST+ is the Cisco proprietary of RSTP on a per-VLAN basis. In Rapid PVST+ an independent instance of RSTP runs for each VLAN. So we will discuss the RSTP in detail for better understanding the Rapid PVST+.

The figure below shows a network running RSTP. The topology is the same we have used in the PVST+. Switch1 is the root bridge with two designated ports. Port F0/0 on Switch2 and Port F0/0 both are root ports. The port F0/1 is an alternate port which discards the traffic. There is no blocking port in the topology because RSTP does not have a blocking state. The port state in RSTP are discarding, learning, or forwarding.

RSTP and Rapid PVST+ 3

RSTP speeds up the convergence in the properly configured network. It also increases the recalculation of the spanning tree when the changes occur in the network topology. It also redefines the type of ports and their state. If a port is configured as an alternate port or a backup port, it can change to a forwarding state without any delay.

Rapid Spanning-Tree Protocol is the preferred protocol for avoiding loops in a switched network environment. The cisco-proprietary improvement includes many differences in the original 802.1D, such as improvement in BPDUs. The improved BPDU only carry and send information about port roles to neighbouring switches, and require no additional configuration. It performs better than the earlier Cisco-proprietary versions. The enhancements to the original 802.1D, such as Uplink Fast and Backbone Fast, are not compatible with Rapid Spanning-Tree Protocol.

Rapid Spanning-Tree Protocol (802.1w) replace the original 802.1D while accommodating compatibility with old protocols. Much of the original 802.1D terminologies unchanged.802.1w is also proficient to interoperate with legacy switches on a per-port basis. Because the RSTP is elect a root bridge in exactly the same way as the original 802.1D. RSTP is can transit the port safely to the forwarding state without any further delay to rely on a timer configuration.

RSTP BPDUs

RSTP uses the same BPDU format as the original IEEE 802.1D, except that the version field is set to 2, which indicate RSTP and the flags field uses all 8 bits. The original 802.1D Spanning-Tree Protocol uses type 0, version 0 BPDUs. But, RSTP can communicate directly with the original 802.1D STP. RSTP sends BPDUs and settles the flag byte in a different way. The flag byte information is as under.

  • Bits 0 and 7 are for topology changes in the topology and also for acknowledgement. Bit 0 and 7 also used in the original 802.1D.
  • Bits 1 and 6 are used for rapid convergence.
  • Bits 2 to 5 encode the role and state of the port. Bit 4 and 5 also used for encoding the port role using a 2-bit code.

BPDUs is the mechanism which keep-alive the connection between switches with Root Bridge. Three successively missed BPDUs point to lost connectivity between a Root Bridge and its neighbouring root or designated bridge. So, fast-ageing detects failures quickly. Protocol information immediately becomes aged on a port if Hello packets are not received for three consecutive Hello times or if the max-age timer expires.

RSTP Edge Ports

An RSTP edge port is never proposed to be connected to another switch. It immediately transitions to the forwarding state when enabled. The edge port concept is related to the PVST+ PortFast feature. An edge port is directly connected to an end station and assumes that there is no switching device is connected to it. So the edge port escapes the listening and learning state. Therefore port transit to the forwarding state without any delay. The edge port functions similarly to a port configured with the Cisco Port Fast command.

Non-edge ports

Ports that may be connected to other switch devices on the network, and should not be configured as edge ports.

Link Types

The link type defines the category for each port join RSTP. Link-type depends on what is attached to each port. The link type also determines whether the port can immediately transit to a forwarding state or not. The immediate transition is assuming certain conditions. The conditions are different for both edge and non-edge ports. non-edge ports are categorized into two types of links: point-to-point links and shared links. The link type is dynamically determined; however, we can override the link type with an explicit port configuration using the spanning-tree link-type parameter command. Characteristics of port roles regarding link type are:

  • Edge port connection, as well as point-to-point connection, is the successor for a rapid transition to a forwarding state. but, before considering the link-type parameter, RSTP must determine the port role.
  • Root ports not using the link-type parameter and able to make a rapid transition to the forwarding state as soon as the port is in sync.
  • Alternate and backup ports mainly not used the link-type parameter.
  • Designated ports mostly used the link-type parameter. A rapid transition to the forwarding state for the designated port only occurs when the link-type parameter is set to point-to-point.
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