Assignment Of Port Roles - Dell PowerConnect B-RX Configuration Manual

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Overview of Rapid Spanning Tree Protocol
RSTP algorithm uses this information to determine if the RST BPDU received by a port is superior to
the RST BPDU that the port transmits. The two values are compared in the order as given above,
starting with the Root bridge ID. The RST BPDU with a lower value is considered superior. The
superiority and inferiority of the RST BPDU is used to assign a role to a port.
If the value of the received RST BPDU is the same as that of the transmitted RST BPDU, then the
port ID in the RST BPDUs are compared. The RST BPDU with the lower port ID is superior. Port roles
are then calculated appropriately.
The port's role is included in the BPDU that it transmits. The BPDU transmitted by an RSTP port is
referred to as an RST BPDU, while it is operating in RSTP mode.
Ports can have one of the following roles:
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Assignment of port roles

At system start-up, all RSTP-enabled bridge ports assume a Designated role. Once start-up is
complete, RSTP 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:
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The following example
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.
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Root – Provides the lowest cost path to the root bridge from a specific bridge
Designated – Provides the lowest cost path to the root bridge from a LAN to which it is
connected
Alternate – Provides an alternate path to the root bridge when the root port goes down
Backup – Provides a backup to the LAN when the Designated port goes down
Disabled – Has no role in the topology
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 RSTP is disabled on the port, that port is given the role of Disabled port.
Disabled ports have no role in the topology. However, if RSTP 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.
(Figure 40) explains role assignments in a simple RSTP topology.
BigIron RX Series Configuration Guide
53-1001810-01