Configuring Spanning Tree Protocol (STP) and IronSpan Features
December 2005
© Foundry Networks, Inc.
7 - 19
Bridges and Bridge Port Roles
A bridge in an 802.1W rapid spanning tree topology is assigned as the root bridge if it has the highest priority
(lowest bridge identifier) in the topology. Other bridges are referred to as non-root bridges.
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’s 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,
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 7.1) explains role assignments in a simple RSTP topology.
Содержание FastIron Edge Switch X424
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