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RSTP (Rapid Spanning Tree Protocol)

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RSTP Rapid Spanning Tree Protocol

In 2001, the IEEE introduced Rapid Spanning Tree Protocol (RSTP) as 802.1w. RSTP provides significantly faster spanning tree convergence after a topology change, introducing new convergence behaviour and bridge port roles to do this. RSTP was designed to be backwards-compatible with standard STP.

While STP can take 30 to 50 seconds to respond to a topology change, RSTP is typically able to respond to changes within 3 × Hello times (default: 3 times 2 seconds) or within a few milliseconds of a physical link failure. The so-called Hello time is an important and configurable time interval that is used by RSTP for several purposes; its default value is 2 seconds.

Standard IEEE 802.1D-2004 incorporates RSTP and obsoletes the original STP standard.

RSTP (Rapid Spanning Tree Protocol) Operation

RSTP adds new bridge port roles in order to speed convergence following a link failure. The number of states a port can be in has been reduced to three instead of STP’s original five.

RSTP Bridge Port Roles

  • Root Port: The port that receives the best BPDU on a bridge is the root port. This is the port that is the closest to the root bridge in terms of path cost. The STA elects a single root bridge in the whole bridged network (per-VLAN). The root bridge sends BPDUs that are more useful than the ones any other bridge sends. The root bridge is the only bridge in the network that does not have a root port. All other bridges receive BPDUs on at least one port.
Root Port
Root Port
  • Designated Port A port is designated if it can send the best BPDU on the segment to which it is connected. 802.1D bridges link together different segments, such as Ethernet segments, to create a bridged domain. On a given segment, there can only be one path toward the root bridge. If there are two, there is a bridging loop in the network. All bridges connected to a given segment listen to the BPDUs of each and agree on the bridge that sends the best BPDU as the designated bridge for the segment. The port on that bridge that corresponds is the designated port for that segment.
Designated Port
Designated Port
  • Alternate and Backup Port Roles: These two port roles correspond to the blocking state of 802.1D. A blocked port is defined as not being the designated or root port. A blocked port receives a more useful BPDU than the one it sends out on its segment. Remember that a port absolutely needs to receive BPDUs in order to stay blocked. RSTP introduces these two roles for this purpose.
    An alternate port receives more useful BPDUs from another bridge and is a port blocked. This is shown in this diagram:
    Alternate Port
    Alternate Port

    A backup port receives more useful BPDUs from the same bridge it is on and is a port blocked. This is shown in this diagram:
    Backup Port
    Backup Port

    This distinction is already made internally within 802.1D. This is essentially how Cisco UplinkFast functions. The rationale is that an alternate port provides an alternate path to the root bridge and therefore can replace the root port if it fails. Of course, a backup port provides redundant connectivity to the same segment and cannot guarantee an alternate connectivity to the root bridge. Therefore, it is excluded from the uplink group.

New Port States

The 802.1D is defined in these five different port states:

  1. Disabled
  2. Listening
  3. Learning
  4. Blocking
  5. Forwarding
STP (802.1D) Port State RSTP (802.1w) Port State Is Port Included in Active Topology? Is Port Learning MAC Addresses?
Disabled Discarding No No
Blocking Discarding No No
Listening Discarding Yes No
Learning Learning Yes Yes
Forwarding Forwarding Yes Yes

RSTP Switch Port States

  • Discarding: No user data is sent over the port.
  • Learning: The port is not forwarding frames yet, but is populating its MAC-address-table.
  • Forwarding: The port is fully operational.
  • Detection of root switch failure is done in 3 hello times, which is 6 seconds if the default hello times have not been changed.
  • Ports may be configured as edge ports if they are attached to a LAN that has no other bridges attached. These edge ports transition directly to the forwarding state. RSTP still continues to monitor the port for BPDUs in case a bridge is connected. RSTP can also be configured to automatically detect edge ports. As soon as the bridge detects a BPDU coming to an edge port, the port becomes a non-edge port.
  • RSTP calls the connection between two or more switches as a “link-type” connection. A port that operates in full-duplex mode is assumed to be point-to-point link, whereas a half-duplex port (through a hub) is considered a shared port by default. This automatic link type setting can be overridden by explicit configuration. RSTP improves convergence on point-to-point links by reducing the Max-Age time to 3 times Hello interval, removing the STP listening state, and exchanging a handshake between two switches to quickly transition the port to forwarding state. RSTP does not do anything differently from STP on shared links.
  • Unlike in STP, RSTP will respond to BPDUs sent from the direction of the root bridge. An RSTP bridge will “propose” its spanning tree information to its designated ports. If another RSTP bridge receives this information and determines this is the superior root information, it sets all its other ports to discarding. The bridge may send an “agreement” to the first bridge confirming its superior spanning tree information. The first bridge, upon receiving this agreement, knows it can rapidly transition that port to the forwarding state bypassing the traditional listening/learning state transition. This essentially creates a cascading effect away from the root bridge where each designated bridge proposes to its neighbors to determine if it can make a rapid transition. This is one of the major elements that allows RSTP to achieve faster convergence times than STP.
  • As discussed in the port role details above, RSTP maintains backup details regarding the discarding status of ports. This avoids timeouts if the current forwarding ports were to fail or BPDUs were not received on the root port in a certain interval.
  • RSTP will revert to legacy STP on an interface if a legacy version of an STP BPDU is detected on that port.

New BPDU Format

Few changes have been introduced by RSTP to the BPDU format. Only two flags, Topology Change (TC) and TC Acknowledgment (TCA), are defined in 802.1D. However, RSTP now uses all six bits of the flag byte that remain in order to perform:

  • Encode the role and state of the port that originates the BPDU.
  • Handle the proposal/agreement mechanism.

New BPDU Format
New BPDU Format

(Note: Bit 0 (Topology Change) is the least significant bit.)

Another important change is that the RSTP BPDU is now of type 2, version 2. The implication is that legacy bridges must drop this new BPDU. This property makes it easy for a 802.1w bridge to detect legacy bridges connected to it.

New BPDU Handling

  1. BPDU are Sent Every Hello-Time.
  2. Faster Aging of Information.
  3. Accepts Inferior BPDUs.

RSTP Convergence Process

Its convergence Process is divided into 3 steps:

  • Proposal: In this step they will exchange their BPDU. Type 2, Version 2.
  • Synchronization: In this step a switch will put its all trunk ports in discarding state. To avoid possible loops.
  • Agreement: A switch will elect for RP and block port. After election, switch will send a request to neighbor switch that you should be DP.

In this a switch will decide its own root port and will send a message to its neighbor switch that you should be a designated port.

RSTP Port Types

  • Edge Port → Switch to PC/Router/Server
  • Point-Point → Switch to Switch
  • Shared port → Switch to Hub (Ethernet Device)

(Note: RSTP (1:1) Single instance for single Vlan.)

Mixed STP and RSTP Networks

RSTP IEEE 802.1w is fully compliant with STP IEEE 802.1d. Your network can consist of bridges running both protocols. STP and RSTP in the same network can operate together to create a single spanning tree domain.

The switch monitors the traffic on each port for BPDU packets. When you set the switch to RSTP mode, all the ports operate in that mode and reject STP BPDU packets. When you set the switch to operate in STP-compatible mode, the ports can receive either RSTP or STP BPDU packets.

Rapid Spanning Tree Configuration Menu

  • Root Port: The active port on the switch that is communicating with the root bridge. If the switch is the root bridge for the LAN, then there is no root port and the root port parameter will be 0.
  • Root Path Cost: The sum of all the root port costs of all the bridges between the switch’s root port and the root bridge including the switch’s root port cost.
  • Time Since Topology Change: The time in seconds since the last topology change took place. When RSTP detects a change to the LAN’s topology or when the switch is rebooted, this parameter is reset to 0 seconds and begins incrementing until the next topology change is detected.
  • Topology Change Count: An integer that reflects the number of times RSTP has detected a topology change on the LAN since the switch was initially powered on or rebooted.

The following parameters refer to the designated root bridge:

  • Designated Root: This parameter includes two fields: the root bridge priority and the MAC address of the root bridge. For example, 1000 00C08F1211BB shows the root bridge priority as 1000, and 00C08F1211BB as the MAC address.
  • Hello Time: The hello time. See Hello Time and Bridge Protocol Data Units (BPDUs). This parameter affects only the root bridge.
  • Maximum Age: The maximum amount of time that BPDUs are stored before being deleted on the root bridge.
  • Forward Delay: The time interval between generating and sending configuration messages by the root bridge.

The following parameters refer to the switch:

  • Bridge ID: The MAC address of the bridge. The bridge identifier is use as a tie breaker in the selection of the root bridge when two or more bridges have the same bridge priority. You cannot change this setting.
  • Bridge Hello Time: This is the time interval between generating and sending configuration messages by the bridge. This parameter is active only when the switch is the root bridge.
  • Bridge Maximum Age: The length of time after which stored bridge protocol data units (BPDUs) are deleted by the bridge.
  • Bridge Forward Delay: This is the time interval between generating and sending configuration messages by the bridge.

Topology Information Menu

  • Trunk: The trunk of which the port is a member.
  • Link: Whether the link on the port is up or down.
  • Desig. Root: The designated root bridge to which the switch’s root port is actively connected.
  • Desig. Cost: The sum of all the root port costs on all bridges, including the switch, between the switch and the root bridge.
  • Desig. Bridge: An adjacent bridge to which the root port of the switch is actively connected.
  • Desig. Port: The root bridge to which the root port of the switch is actively connected.

Configuration Instruction

Topology
Topology
  • Step1: Configure SW1, SW2 and SW3 to run Rapid Per-VLAN Spanning Tree Protocol.

SW1(config)#spanning-tree mode rapid-pvst
SW1#show spanning-tree bridge


SW2(config)#spanning-tree mode rapid-pvst
SW2#show spanning-tree bridge


SW3(config)#spanning-tree mode rapid-pvst
SW3#show spanning-tree bridge

  • Step 2: Configure SW1 as the ROOT bridge for VLAN 1 and 10. Verify your configuration on SW2.

SW1(config)#spanning-tree vlan 1 root primary
SW1(config)#spanning-tree vlan 10 root primary


SW2#show spanning-tree vlan 1
SW2#show spanning-tree vlan 10

  • Step 3: Configure SW2 as the ROOT bridge for VLAN 20. Verify your configuration on SW1.

SW2(config)#spanning-tree vlan 20 root primary

SW1#show spanning-tree vlan 20

  • Step 4: Configure SW3 as the ROOT bridge for VLAN 30. Verify your configuration on SW1.

SW3(config)#spanning-tree vlan 30 root primary

SW1#show spanning-tree vlan 30

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