RSTP Port States

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RSTP provides rapid convergence following the failure or re-establishment of a switch, switch port, or link. An RSTP topology change will cause a transition in the appropriate switch ports to the forwarding state through explicit handshakes or a proposal and agreement process and synchronization.

With RSTP, the role of a port is separated from the state of a port. For example, a designated port may be in the discarding state, even though this condition would be temporary. However, in a stable, active topology, the port role still determines the underlying final port state. For example, a designated port will still need to end up in the forwarding state.

The RTSP port states correspond to the three basic operations of a switch port: discarding, learning, and forwarding.

The port states have these characteristics:

■ Discarding: This state is seen in both a stable active topology and during topology synchronization and changes. The discarding state prevents the forwarding of data frames from "breaking" the continuity of a Layer 2 loop.

■ Learning: This state is seen in both a stable active topology and during topology synchronization and changes. The learning state accepts data frames to populate the MAC table in an effort to limit flooding of unknown unicast frames.

■ Forwarding: This state is seen only in stable active topologies. The forwarding switch ports determine the topology. Following a topology change or during synchronization, the forwarding of data frames is only achieved by a proposal and agreement process.

In addition to these characteristics, the port states all accept and receive BPDU frames for processing.

Identifying RSTP Port Roles

This topic discusses the RSTP port role functions.

This topic discusses the RSTP port role functions.

State And Explain Functions Ports

The port role defines the ultimate objective state of a switch port and, therefore, the handling of data frames. Yet the port role and the port state are able to transition independently of each other. RSTP uses these definitions for port roles:

Root port: This is the switch port on every nonroot switch that is the closest to the root switch in terms of path cost. There can only be one root port on every switch. The root port assumes the forwarding state in a stable active topology.

Designated port: This is the switch port on every switch that connects the Ethernet segments to the root port toward the root switch. There can only be one designated port per segment. The designated port assumes the forwarding state in a stable active topology. All switches connected to a given segment listen to all BPDUs and determine the switch that will be the designated switch for a particular Ethernet segment.

Alternate port: This is a switch port that offers an alternate path toward the root switch. The alternate port assumes a discarding state in a stable active topology. An alternate port will be present on nondesignated switches and will make a transition to a designated port if the current designated path fails.

Backup port: This is an additional switch port on the designated switch with a redundant link to the segment for which the switch is designated. A backup port has a higher port ID than the designated port on the designated switch. The backup port assumes the discarding state in a stable active topology.

Disabled port: This is a port that has no role within the operation of spanning tree.

Establishing the additional port roles allows RSTP to define a standby switch port before a failure or topology change. The alternate port moves to the forwarding state if there is any failure in the path through the designated port and root port on a designated switch, including the failure of the designated switch itself.

Identifying RSTP Link Types

This topic matches the correct RSTP link type with the appropriate function.

Rapid Transition to Forwarding

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© 2004, Cisco Systems, Inc. All rights re

© 2004, Cisco Systems, Inc. All rights re

The IEEE 802.1w standard includes parameters that allow for the immediate transition to the forwarding state if certain conditions are met. These parameters fall into two separate and distinct groups, yet they are used to accomplish the same result, which is the transition to the forwarding state. The parameters in the one group determine if the switch port is an edge port. The parameters in the other group determine the type of link to which the switch port is connected. These parameters are configurable.

An edge port is defined as a port that is not connected to another bridge device. If the edge port parameter is "true," the port is immediately transitioned to the forwarding state. If an edge port receives a topology change notification BDPU, the parameter is immediately changed to "false," a TCN is sent, and the spanning tree algorithm runs.

The link type parameter is either point-to-point or shared. The value of the parameter can be configured or automatically determined. A port operating in full-duplex mode is point-to-point, while a port operating in half-duplex mode is considered to be on a shared medium by default. You can override the automatic link type setting with an explicit configuration.

Before the link type parameter is used, the spanning tree algorithm must determine the port role.

Root ports do not use the link type parameter. Root ports are able to make a rapid transition to the forwarding state as soon as the port is in "sync." The dependency for a root port to sync and directly move from the discarding state to the forwarding state is the explicit handshakes that occur between the root port and the upstream designated port.

In most cases, alternate and backup ports do not use the link type parameter RSTP keeps track of ports that provide alternative paths to the root bridge. If a root port fails, RSTP can quickly make an alternative port the new root port. This new root port is placed in the forwarding state without delay.

The link type parameter is most important for the designated port. Rapid transition to the forwarding state for the designated Port occurs only if the link type parameter indicates a point-to-point link.

Examining RSTP BPDU

This topic identifies the features of RSTP BPDUs.

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Port role Proposal

Topology Change ACK Agreement

F(i rrii ng

Learning

00 Unknown

01 Altemaig/Backup 10 Root j

.11 Designated

RSTP has these features:

■ RSTP uses all 6 remaining bits of the flag byte to do the following:

— Encode the role and state of the port originating the BPDU

— Handle the proposal and agreement mechanism

RSTP BPDUs are defined as type 2, version 2 so an 802.1w bridge can detect legacy bridges.

■ BPDUs are now sent every hello time.

■ With RSTP, a bridge now sends a BPDU with its current information every hello time period (2 seconds by default), even if it does not receive any BPDUs from the root bridge.

Protocol information can be immediately aged on a port if hellos are not received for three consecutive hello times or if the max-age timer expires. Because BPDUs are now used as a "keepalive" mechanism, three consecutively missed BPDUs indicate lost connectivity between a bridge and its neighboring root or designated bridge. This fast aging of the information allows quick failure detection.

Transitioning Rapidly to the Forwarding State

This topic identifies the features of transitioning rapidly to the forwarding state.

This topic identifies the features of transitioning rapidly to the forwarding state.

RSTP significantly speeds up the recalculation process after a topology change occurs in the network. RSTP works by designating an alternate port and a back up port. These ports are allowed to immediately enter the forwarding state rather than passively waiting for the network to converge.

BPDUs are transmitted out all nonedge switch ports. If hellos are not received for three consecutive hello time periods, RSTP protocol information will be immediately aged out and the switch port is out of sync with the rest of the switched domain. Because the BPDUs form neighbor relationships, the switch that is not receiving BPDUs for the three hello time periods can positively identify which connection is lost and that switch port is "marked" as requiring synchronization when the link comes back up. To avoid Layer 2 loops when the link is reinstated, the switch port will default to the designated port and discarding state without any current protocol configuration information.

Note A physical link loss is detected in less than three hello time periods.

The RSTP BPDU is now used to implement the process of an explicit handshake, which quickly propagates through a switched domain and ensures a stable loop-free topology during convergence.

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