Siemens RUGGEDCOM ROX II, User Manual

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Chapter 14
Network Redundancy
RUGGEDCOM ROX II
CLI User Guide
630 RSTP States and Roles
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!
Purely for purposes of management, RUGGEDCOM ROX II introduces two more states: Disabled and
Link Down . 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 other's 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
C
1
2
1
2 3
3
D
4
2 2
1
3
4
5 6 3
Figure 36: 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.
A port is a Backup Port when it receives a better message from the LAN segment it is connected to, originating
from another port on the same bridge. The port is a backup for another port on the bridge and will become active
if that port fails. The Backup Port does not participate in the network.