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DT Etherlink IV
User Manual
detect paths availability in a ring and quickly select active path, discarding other paths. All RSTP-
enabled devices exchange information about topology change in so-called BPDU packets.
Root Bridge
One switch in a RSTP-enabled network must act as Root Bridge. Root Bridge selection will be done
automatically according to Bridge ID – a unique ID that each member of RSTP network has. Bridge
ID is a combination of Switch MAC address and Bridge Priority. Switch with smallest Bridge Priority
will act as Root Bridge. If two or more switches have same priority, Switch with less MAC address
will become Root Bridge.
Port Roles
After Root Bridge has been selected, other switches define their ports role. The port that has the
shortest path to Root Bridge will become Root Port. The opposite port on the other switch will
become Designated Port. The Root Bridge has Designated Ports only, while other switches have
one Root port and could have Designated Port connected to other switches
A Root and Designated ports are active, they learn and forward packets. Other ports have blocking
state. They could act as Alternate port or as Backup port. Please take a look to a picture below:
Figure 3.16 Port role definition in RSTP.
When a failure appears on a network, an alternative path will be selected and port roles will be
changed. Please take a look to a picture below:
Figure 3.17 Change of Port role upon network failure.
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