Rstp Terms; Rstp Concepts - Extreme Networks ExtremeWare XOS Guide Manual

RSTP Terms

Table 28 describes the terms associated with RSTP.
Table 28: RSTP terms
Term
root port
designated port
alternate port
backup port
edge ports
root bridge

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 29.
Table 29: RSTP port roles
Port Role
Root
Designated
Alternate
ExtremeWare XOS 10.1 Concepts Guide
Description
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 149.
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 149.
Supplies an alternate path to the root bridge and the root port. For more information
about the alternate port, see "Port Roles" on page 149.
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 149.
Ports that connect to non-STP devices such as routers, endstations, and other hosts.
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.
Description
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.
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.
Provides an alternate path to the root bridge and the root port.
Rapid Spanning Tree Protocol
149