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Each network device network is named edge (device 1 and 2) or non-edge (device 3
and 4) due to its location on the network. Only the network devices that stopped
forwarding data are referred to as edge. The spanning tree is generated according
to the following rules:
1.1.1.1.
Select the root node (Root Device):
Each network device receives a single
identifier with a configurable priority number. The device with the smallest
id is set as rooter. If two devices get the same ID, the device with the smaller
MAC address will become the rooter.
1.1.1.2.
Select the optimal path to the root device
: Every link in the network is
provided a cost, depending on physical criteria, such as the data
transmission rate. In figure 3.1. we assume the following:
Costs = 1000/Data.
If there is more than one path of a network device to the root node, the connection is
switched to the higher cost. This connection can take over the necessary data transfer in
the case of a failure. If you follow these rules, for example, the connection marked red in
Figure 3.1. can be kept as a spare connection.
Due to the generated network topology (spanning tree) and the application of Rule 2, each
port of a network device is provided with a port status (Port Role):
–
Root
: Forwarder port that connects the network device to the root device (enabled).
–
Designated
: forwarder port that connects the root device with other network devices
(enabled).
–
Backup
: port forwards data to network devices via redundant connections (disabled).
–
Alternate
: port that is not part of the tree (disabled).
The building of such a spanning tree implies knowledge about the entire network. In practice, the
method is implemented by means of the RST protocol. For this, the user simply selects the root
device. The root device then communicates with other network devices on the Bridge Protocol
Data Units (BPDUs) to exchange information on the ID of the network device and the path costs to
the root device. Then the port of a network device may be located in the following states:
- Discarding
: The port discards all frames and does not learn MAC addresses.
- Learning:
The port discards all frames, and learns MAC Addresses of other network devices.
- Forwarding
: The port forwards all incoming frames on (operational state).
When configuring RSTP among others, the following parameters can be set, see also section 4.2.5:
- Hello Time:
It indicates the interval in which the root device regularly informs the other network
devices using a BPDU.
- Maximum age of RSTP information:
The root device transmits RSTP information with aging '0
'and within the ''Hello Time“-interval. If a BPDU is received by the root device, any
network
