HP ProCurve 9304M Installation And Configuration Manual page 230

Installation and Basic Configuration Guide
Assignment of Port Roles
At system start-up, all 802.1W-enabled bridge ports assume a Designated role. Once start-up is complete,
802.1W algorithm calculates the superiority or inferiority of the RST BPDU that is received and transmitted on a
port.
On a root bridge, each port is assigned a Designated port role, except for ports on the same bridge that are
physically connected together. In these type of ports, the port that receives the superior RST BPDU becomes the
Backup port , while the other port becomes the Designated port .
On non-root bridges, ports are assigned as follows:
• The port that receives the RST BPDU with the lowest path cost from the root bridge becomes the Root port .
• If two ports on the same bridge are physically connected, the port that receives the superior RST BPDU
becomes the Backup port , while the other port becomes the Designated port .
• If a non-root bridge already has a Root port, then the port that receives an RST BPDU that is superior to those
it can transmit becomes the Alternate port .
• If the RST BPDU that a port receives is inferior to the RST BPDUs it transmits, then the port becomes a
Designated port .
• If the port is down or if 802.1W is disabled on the port, that port is given the role of Disabled port . Disabled
ports have no role in the topology. However, if 802.1W is enabled on a port with a link down and the link of
that port comes up, then that port assumes one of the following port roles: Root, Designated, Alternate, or
Backup.
The following example (Figure 8.1) explains role assignments in a simple RSTP topology.
NO TE: All examples in this document assume that all ports in the illustrated topologies are point-to-point links
and are homogeneous (they have the same path cost value) unless otherwise specified.
The topology in Figure 8.1 contains four bridges. Routing Switch 1 is the root bridge since it has the lowest bridge
priority. Routing Switch 2 through Routing Switch 4 are non-root bridges.
F i g u r e 8 . 1 S i m p l e 80 2. 1W T o p o l o g y
Routing Switch 1
Bridge priority = 100
Routing Switch 3
Bridge priority = 300
Ports on Routing Switch 1
All ports on Routing Switch 1, the root bridge, are assigned Designated port roles.
8 - 24
Port2
Port3
Port2
Port3
Port4
Port7 Port8
Bridge priority = 200
Port2
Port4
Port3
Port3
Bridge priority = 400
Port4
Routing Switch 2
Routing Switch 4