Specifications

210 ExtremeWare XOS 11.0 Concepts Guide
Spanning Tree Protocol
RSTP Terms
Table 37 describes the terms associated with RSTP.
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 38.
Table 37: RSTP terms
Term Description
Root port Provides the shortest path to the root bridge. All bridges except the root bridge,
contain one root port. For more information about the root port, see “Port Roles” on
page 210.
Designated port Provides the shortest path connection to the root bridge for the attached LAN
segment. Each LAN segment has only one designated port. For more information
about the designated port, see “Port Roles” on page 210.
Alternate port Supplies an alternate path to the root bridge and the root port. For more information
about the alternate port, see “Port Roles” on page 210.
Backup port Supports the designated port on the same attached LAN segment. Backup ports only
exist when the bridge is connected as a self-loop or to a shared-media segment. For
more information about the backup port, see “Port Roles” on page 210.
Edge ports Ports that connect to non-STP devices such as routers, endstations, and other hosts.
Root bridge The bridge with the best bridge identifier selected to be the root bridge. The network
has only one root bridge. The root bridge is the only bridge in the network that does
not have a root port.
Table 38: 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.