RUGGEDCOM RSG2488
User Guide
Chapter 5
Setup and Configuration
Edge Ports
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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.
Section 5.2.1.2
Edge Ports
A port may be designated as an Edge Port if it is directly connected to an end station. As such, it cannot create
bridging loops in the network and can thus directly transition to forwarding, skipping the listening and learning
stages.
Edge ports that receive configuration messages immediately lose their Edge Port status and become normal
spanning tree ports. A loop created on an improperly connected edge port is thus quickly repaired.
Because an Edge Port services only end stations, topology change messages are not generated when its link
toggles.
Section 5.2.1.3
Point-to-Point and Multipoint Links
RSTP uses a peer-peer protocol called Proposing-Agreeing to ensure transitioning in the event of a link failure.
This protocol is point-to-point and breaks down in multipoint situations, i.e. when more than two bridges operate
on a shared media link.
If RSTP detects this circumstance (based upon the port’s half duplex state after link up) it will switch off
Proposing-Agreeing. The port must transition through the learning and forwarding states, spending one forward
delay in each state.
There are circumstances in which RSTP will make an incorrect decision about the point-to-point state of the link
simply by examining the half-duplex status, namely:
• The port attaches only to a single partner, but through a half-duplex link.
• The port attaches to a shared media hub through a full-duplex link. The shared media link attaches to more
than one RSTP enabled bridge.
In such cases, the user may configure the bridge to override the half-duplex determination mechanism and force
the link to be treated in the proper fashion.
Section 5.2.1.4
Path and Port Costs
The STP path cost is the main metric by which root and designated ports are chosen. The path cost for a
designated bridge is the sum of the individual port costs of the links between the root bridge and that designated
bridge. The port with the lowest path cost is the best route to the root bridge and is chosen as the root port.
NOTE
In actuality the primary determinant for root port selection is the root bridge ID. Bridge ID is important
mainly at network startup when the bridge with the lowest ID is elected as the root bridge. After startup
(when all bridges agree on the root bridge’s ID) the path cost is used to select root ports. If the path
costs of candidates for the root port are the same, the ID of the peer bridge is used to select the port.
Finally, if candidate root ports have the same path cost and peer bridge ID, the port ID of the peer
