Dell PowerConnect B-FCXs, Configuration Manual

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PowerConnect B-Series FCX Configuration Guide 229
53-1002266-01
Configuring STP related features
8
Assignment of port roles
At system start-up, all 802.1W-enabled bridge ports assume a Designated role. Once start-up is
complete, the 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 26 ) explains role assignments in a simple RSTP topology.
NOTE
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 26 contains four bridges. Switch 1 is the root bridge since it has the lowest
bridge priority. Switch 2 through Switch 4 are non-root bridges.