Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 464)

434 Multi-Service IronWare Switching Configuration Guide
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Configuring RSTP parameters
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For example, in Figure 94 , Switch 10 and Switch 30 receive legacy BPDUs from Switch 20. Ports on
Switch 10 and Switch 30 begin sending BPDUs in STP format to allow them to operate
transparently with Switch 20.
FIGURE 94 RSTP bridges with an 802.1D bridge
Once Switch 20 is removed from the LAN, Switch 10 and Switch 30 receive and transmit BPDUs in
the STP format to and from each other. This state will continue until the administrator enables the
force-migration-check command to force the bridge to send RSTP BPDU during a migrate time
period. 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 during
the migrate time period, the ports begin sending RST BPDUs. The migrate time period is
non-configurable. It has a value of three seconds.
NOTE
The IEEE standards state that RSTP bridges need to interoperate with 802.1D bridges. IEEE
standards set the path cost of RSTP bridges to be between 1 and 200,000,000; whereas path cost
of 802.1D bridges are set between 1 and 65,535. In order for the two bridge types to be able to
interoperate in the same topology, the administrator needs to configure the bridge path cost
appropriately. Path costs for either RSTP bridges or 802.1D bridges need to be changed; in most
cases, path costs for RSTP bridges need to be changed.
Configuring RSTP parameters
The remaining RSTP sections explain how to configure the RSTP protocol on a Brocade 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 or
disable RSTP within a port-based VLAN, the setting overrides the global setting. Thus, you can
enable RSTP for the ports within a port-based VLAN even when RSTP is globally disabled, or
disable 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 the
primary port in a LAG group, the change affects all ports in the LAG group.
RSTP in a LAG
The RSTP standard indicates that by default the path cost is determined by link speed. For a port
having 1G the path cost is 20,000 and for 10G the path cost is 2,000. However, if a LAG is made
consisting of n 1G ports where n is less than 10, the path cost remains as 20,000. The standard
does not indicate pathcost explicitly for LAG interfaces or if the bandwidth is in intermediate value.