Chapter 5
Setup and Configuration
RUGGEDCOM RSG2488
User Guide
122
RSTP States and Roles
The learning state is entered when the port is preparing to play an active part in the network. The port learns
addresses in this state but does not participate in frame transfer. In a network of RSTP bridges, the time spent in
this state is usually quite short. RSTP bridges operating in STP compatibility mode will spend six to 40 seconds in
this state.
After
learning
, the bridge will place the port in the forwarding state. The port both learns addresses and
participates in frame transfer while in this state.
IMPORTANT!
ROS introduces two more states - Disabled and Link Down. Introduced purely for purposes of
management, these states may be considered subclasses of the RSTP Discarding state. The Disabled
state refers to links for which RSTP has been disabled. The Link Down state refers to links for which
RSTP is enabled but are currently down.
Role
There are four RSTP port roles: Root, Designated, Alternate and Backup. If the bridge is not the root bridge, it
must have a single Root Port. The Root Port is the “best” (i.e. quickest) way to send traffic to the root bridge.
A port is marked as Designated if it is the best port to serve the LAN segment it is connected to. All bridges on
the same LAN segment listen to each others’ messages and agree on which bridge is the Designated Bridge.
The ports of other bridges on the segment must become either Root, Alternate or Backup ports.
3
2
1
3
C
1
2
1
2
3
D
4
2
2
1
3
4
5
6
3
Figure 84: Bridge and Port Roles
1.
Root Bridge
2.
Designated Bridge
3.
Designated Port
4.
Root Port
5.
Alternate Port
6.
Backup Port
A port is alternate when it receives a better message from another bridge on the LAN segment it is connected to.
The message that an Alternate Port receives is better than the port itself would generate, but not good enough to
convince it to become the Root Port. The port becomes the alternate to the current Root Port and will become the
new Root Port should the current Root Port fail. The Alternate Port does not participate in the network.