378 BigIron RX Series Configuration Guide53-1001810-01Configuring RSTP parameters13The priority parameter specifies the preference that RSTP gives to this port relative toother ports for forwarding traffic out of the topology. You can specify a value from 0 – 655352, inincrements of 4. If you enter a value that is not divisible by four the software rounds to the nearestvalue that is. The default is 128 (the terminal shows this too). A higher numerical value means alower priority; thus, the highest priority is 8.Set the admin-edge-port to enabled or disabled. If set to enabled, then the port becomes an edgeport in the domain.Set the admin-pt2pt-mac to enabled or disabled. If set to enabled, then a port is connected toanother port through a point-to-point link. The point-to-point link increases the speed ofconvergence. This parameter, however, does not auto-detect whether or not the link is a physicalpoint-to-point link.The force-migration-check parameter forces the specified port to sent one RST BPDU. If only STPBPDUs are received in response to the sent RST BPDU, then the port will go return to sending STPBPDUs.ExampleSuppose you want to enable RSTP on a system with no active port-based VLANs and change thehello-time from the default value of 2 to 8 seconds. Additionally, suppose you want to change thepath and priority costs for port 5 only. To do so, enter the following commands.BigIron RX(config)# spanning-tree 802-1w hello-time 8BigIron RX(config)# spanning-tree 802-1w ethernet 5 path-cost 15 priority 64Fast port spanWhen STP is running on a device, message forwarding is delayed during the spanning treerecalculation period following a topology change. The STP forward delay parameter specifies theperiod of time a bridge waits before forwarding data packets. The forward delay controls thelistening and learning periods of STP reconvergence. You can configure the forward delay to avalue from 4 – 30 seconds. The default is 15 seconds. Thus, using the standard forward delay,convergence requires 30 seconds (15 seconds for listening and an additional 15 seconds forlearning) when the default value is used.This slow convergence is undesirable and unnecessary in some circumstances. The Fast PortSpan feature allows certain ports to enter the forwarding state in four seconds. Specifically, FastPort Span allows faster convergence on ports that are attached to end stations and thus do notpresent the potential to cause Layer 2 forwarding loops. Because the end stations cannot cause1 Gigabit per second 20,000 2,000 – 200,000,00010 Gigabits per second 2,000 200 – 20,000100 Gigabits per second 200 20 – 2,0001 Terabits per second 20 2 – 20010 Terabits per second 2 1 – 20TABLE 76 Recommended path cost values of RSTP (Continued)Link speed Recommended (default) RSTP pathcost valuesRecommended RSTP path cost range
