Chapter 5
Setup and Configuration
RUGGEDCOM ROS
User Guide
152
Point-to-Point and Multipoint Links
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
bridge is used to select the root port. In all cases the lower ID, path cost or port ID is selected as the
best.
How Port Costs Are Generated
Port costs can be generated either as a result of link auto-negotiation or manual configuration. When the link
auto-negotiation method is used, the port cost is derived from the speed of the link. This method is useful when
a well-connected network has been established. It can be used when the designer is not too concerned with the
resultant topology as long as connectivity is assured.
Manual configuration is useful when the exact topology of the network must be predictable under all
circumstances. The path cost can be used to establish the topology of the network exactly as the designer intends.
STP vs. RSTP Costs
The IEEE 802.1D-1998 specification limits port costs to values of 1 to 65536. Designed at a time when 9600 bps
links were state of the art, this method breaks down in modern use, as the method cannot represent a link speed
higher than 10 gigabits per second.
