Alcatel-Lucent 9500 MXC, User Manual, Page: 460 / 916

Chapter 7. Node and Terminal Plug-ins
Vol. IV-7-70 Alcatel-Lucent
• Port priority is contained within a Port ID, which comprises a port
priority setting, and the port number.
• Where RWPR costs to root are such that they cannot assist RSTP to set
a priority, the port with the lowest port ID is used to decide port states,
such as forwarding or blocking. Where ports are set for the same port
priority, spanning tree selects the port with the lowest port number as
the forwarding port.
• If path costs are not assigned, RSTP uses the port number within the
port ID in its selection of port cost/status.
10.6
Bridge ID
6
and Bridge Priority
are used to establish the root DAC GE
(root switch) in an RSTP network.
• The root switch is the logical center of the network. For a typical single
gateway ring (all traffic on the ring is sourced and sunk at one ‘gateway’
site) the gateway DAC GE must be elected as the root switch.
• The DAC GE with the lowest bridge priority number is elected as the
root switch. In the event this DAC GE fails or is withdrawn, the DAC
GE with the next lowest-value bridge priority is elected as the root
switch. The setable range is 0 to 65,535 - set each DAC GE in the
network such that the root DAC GE has the lowest number, with the
next highest number assigned to the DAC GE best positioned to act as
the root switch in the event of a failure of the gateway Node/DAC GE.
RWPR Cost (port/channel cost) and RWPR Priority (port/channel priority)
settings are used by spanning-tree to elect the network topology beneath the root
switch. The spanning-tree algorithm uses data from both to determine an
optimum network tree (optimum paths to/from root switch), with contesting ports
set for forwarding or blocking.
On a simple ring network where all paths are configured for the same Ethernet
capacity and Ethernet traffic loading on the ring is balanced, Cost and Priority
settings should be set such that traffic flows both ways around the ring from the
root DAC GE (root switch) to a DAC GE that is equi-distant from the root, at which
point RSTP will set one of the DAC GE channels (or ports) to forwarding and the
other to blocking. RWPR Cost can be set the same (e.g. 500) for all DAC GEs.
Similarly RWPR Priority can be set the same (e.g. 50). Doing so will ensure costs
are set equally (they total equally) in both directions around the network from the
root DAC GE.
The value ranges available for port cost and port priority allow selection of a wide
range of values. The actual value selected is not important on its own - what is
important are the values selected in conjunction with other DAC GEs (other
paths) on the network.
See Figure 7-25 for an example network.
6
Bridge ID is a unique identifier for the DAC GE switch function. The most significant bits are occupied
by the bridge priority setting, the remainder by the switch MAC address.