rstp convergence

RSTP- IEEE 802.1w is the advanced version of STP, providing faster convergence than CST but holding with the same single root bridge in the topology. Most parameters in RSTP are the same as STP, so users who understand 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 to better understand the Rapid PVST+.

The figure below shows a network running RSTP. The topology is the same as that used in the PVST+. Switch 1 is the root bridge with two designated ports. Port F0/0 on Switch2 and Port F0/0 are both root ports. Port F0/1 is an alternate port that discards traffic. The topology has no blocking port because RSTP has no blocking state. The port states in RSTP are discarding, learning, or forwarding.

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 or backup port, it can change to a forwarding state without delay.

Rapid Spanning-Tree Protocol is preferred for avoiding loops in a switched network environment. The Cisco-proprietary improvement includes many differences from the original 802.1D, such as improvements in BPDUs.

The improved BPDU only carries and sends information about port roles to neighboring switches and requires 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 incompatible with Rapid Spanning-Tree Protocol.

Rapid Spanning-Tree Protocol (802.1w) replaces the original 802.1D while accommodating compatibility with old protocols. Much of the original 802.1D terminologies are unchanged. 802.1w is also proficient in interoperating with legacy switches per port. Because the RSTP elects a root bridge like the original 802.1D, it can safely transit the port to the forwarding state without 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 indicates RSTP and the flags field uses all 8 bits. The original 802.1D Spanning-Tree Protocol uses type 0, version 0 BPDUs. However, RSTP can communicate directly with the original 802.1D STP. RSTP sends BPDUs and settles the flag byte differently. The flag byte information is as follows.

• Bits 0 and 7 are for topology changes in the topology and also for acknowledgment. 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 port’s role and state. Bits 4 and 5 also encode the port role using a 2-bit code.

BPDUs are the mechanism that keeps the connection between switches with a Root Bridge alive. Three successively missed BPDUs indicate lost connectivity between a Root Bridge and its neighboring root or designated bridge. So, fast aging 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. When enabled, it immediately transitions to the forwarding state. The edge port concept is related to the PVST+ PortFast feature. An edge port is directly connected to an end station, and no switching device is assumed to be connected.

The edge port escapes the listening and learning state, so port transit will be made to the forwarding state immediately. The edge port functions similarly to a port configured with the Cisco Port Fast command.

Non-edge ports

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

Link Types

The link type defines the category for each port joining RSTP. The 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. The immediate transition assumes 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 ports and point-to-point connections are the successors for a rapid transition to a forwarding state. However, RSTP must determine the port role before considering the link-type parameter.
• Root ports do not use the link-type parameter and can rapidly transition to the forwarding state when the port is in sync.
• Alternate and backup ports mainly do not use the link-type parameter.
• Designated ports mostly used the link-type parameter. Only when the link-type parameter is set to point-to-point does the designated port rapidly transition to the forwarding state.