BigIron RX Series Configuration Guide 37553-1001810-01Configuring RSTP parameters 13For example, in Figure 62, Switch 10 and Switch 30 receive legacy BPDUs from Switch 20. Ports onSwitch 10 and Switch 30 begin sending BPDUs in STP format to allow them to operatetransparently with Switch 20.FIGURE 62 RSTP bridges with an 802.1D bridgeOnce Switch 20 is removed from the LAN, Switch 10 and Switch 30 receive and transmit BPDUs inthe STP format to and from each other. This state will continue until the administrator enables theforce-migration-check command to force the bridge to send RSTP BPDU during a migrate timeperiod. If ports on the bridges continue to hear only STP BPDUs after this migrate time period,those ports will return to sending STP BPDUs. However, when the ports receive RST BPDUs duringthe migrate time period, the ports begin sending RST BPDUs. The migrate time period isnon-configurable. It has a value of three seconds.NOTEThe IEEE standards state that RSTP bridges need to interoperate with 802.1D bridges. IEEEstandards set the path cost of RSTP bridges to be between 1 and 200,000,000; whereas path costof 802.1D bridges are set between 1 and 65,535. In order for the two bridge types to be able tointeroperate in the same topology, the administrator needs to configure the bridge path costappropriately. Path costs for either RSTP bridges or 802.1D bridges need to be changed; in mostcases, path costs for RSTP bridges need to be changed.Configuring RSTP parametersThe remaining RSTP sections explain how to configure the RSTP protocol on a device.You can enable or disable RSTP at the following levels:• Port-based VLAN – Affects all ports within the specified port-based VLAN. When you enable ordisable RSTP within a port-based VLAN, the setting overrides the global setting. Thus, you canenable RSTP for the ports within a port-based VLAN even when RSTP is globally disabled, ordisable the ports within a port-based VLAN when RSTP is globally enabled.• Individual port – Affects only the individual port. However, if you change the RSTP state of theprimary port in a trunk group, the change affects all ports in the trunk group.Enabling or disabling RSTP in a port-based VLANUse the following procedure to disable or enable RSTP on a device on which you have configured aport-based VLAN. Changing the RSTP state in a VLAN affects only that VLAN.Switch 10 Switch 20 Switch 30802.1W 802.1D 802.1W
