Extreme Networks ExtremeWare XOS 11.3, Manual

Page: 437 / 698

Rapid Spanning Tree Protocol
ExtremeWare XOS 11.3 Concepts Guide
437
RSTP Concepts
This section describes important RSTP concepts.
Port Roles
RSTP uses information from BPDUs to assign port roles for each LAN segment. Port roles are not user-
configurable. Port role assignments are determined based on the following criteria:
●
A unique bridge identifier (MAC address) associated with each bridge
●
The path cost associated with each bridge port
●
A port identifier associated with each bridge port
RSTP assigns one of four port roles to bridge ports in the network, as described in Table 59 .
When RSTP stabilizes, all:
●
Root ports and designated ports are in the forwarding state.
●
Alternate ports and backup ports are in the blocking state.
RSTP makes the distinction between the alternate and backup port roles to describe the rapid transition
of the alternate port to the forwarding state if the root port fails.
Ports that connect to non-STP devices are edge ports. Edge ports do not participate in RSTP, and their
role is not confirmed. Edge ports immediately enter the forwarding state.
Link Types
With RSTP, you can configure the link type of a port in an STPD. RSTP tries to rapidly move designated
point-to-point links into the forwarding state when a network topology change or failure occurs. For
rapid convergence to occur, the port must be configured as a point-to-point link.
Table 59: RSTP port roles
Port Role Description
Root Provides the shortest path to the root bridge. Each bridge has only one root port; the root bridge
does not have a root port. If a bridge has two or more ports with the same path cost, the port
with the best port identifier becomes the root port.
Designated Provides the shortest path connection to the root bridge for the attached LAN segment. To
prevent loops in the network, there is only one designated port on each LAN segment. To select
the designated port, all bridges that are connected to a particular segment listen to each other’s
BPDUs and agree on the bridge sending the best BPDU. The corresponding port on that bridge
becomes the designated port. If there are two or more ports connected to the LAN, the port with
the best port identifier (lowest MAC address) becomes the designated port.
Alternate Provides an alternate path to the root bridge and the root port.
Backup Supports the designated port on the same attached LAN segment. Backup ports exist only when
the bridge is connected as a self-loop or to a shared-media segment.