RSTP Link Types

Spanning Tree Link Type Shared

Link type provides a categorization for each port participating in RSTP. The link type can predetermine the active role that the port plays as it stands by for immediate transition to a forwarding state, if certain parameters are met. These parameters are different for edge ports and nonedge ports. Nonedge ports are categorized into two link types. Link type is automatically determined but can be overwritten with an explicit port configuration.

© 2006 Cisco Systems, Inc. Implementing Spanning Tree 3-39

RSTP Link Types

The table defines link types.

Link Type

Description

Point-to-point

Port operating in full-duplex mode. It is assumed that the port is connected to a single switch device at the other end of the link.

Shared

Port operating in half-duplex mode. It is assumed that the port is connected to shared media where multiple switches might exist.

Edge ports, the equivalent of PortFast-enabled ports, and point-to-point links are candidates for rapid transition to a forwarding state. Before the link type parameter can be considered for the purpose of expedient port transition, RSTP 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."

Alternative and backup ports: Do not use the link type parameter in most cases.

Designated ports: Make the most use of the link type parameter. Rapid transition to the forwarding state for the designated port occurs only if the link type parameter indicates a point-to-point link.

3-40 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Examining the RSTP BPDU

This topic differentiates the 802.1w use of the BPDU from 802.1D.

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