Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide User Manual
Page 272
Unique roles are assigned to ports on the root and non-root bridges. Role assignments are based on
the following information contained in the Rapid Spanning Tree Bridge Packet Data Unit (RST BPDU):
• Root bridge ID
• Path cost value
• Transmitting bridge ID
• Designated port ID
The 802.1W 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 role is included in the BPDU that it transmits. The BPDU transmitted by an 802.1W port is
referred to as an RST BPDU, while it is operating in 802.1W mode.
Ports can have one of the following roles:
• 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
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 (
) 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.
Spanning Tree Protocol
272
FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide
53-1003086-04