When you use allClosed VLAN mode on a Multilayer Switching Module in your system, you can enable the module to ignore the Spanning Tree Protocol (STP) mode on a per-VLAN basis; that is, ignore STP blocked ports. (When STP detects multiple paths to a destination, it blocks all but one of the paths.)

If you have configured router port IP interfaces on your Multilayer Switching Modules (which causes the module to generate router port VLANs owned by the router IP interfaces), ignore STP mode is automatically enabled and you cannot disable it.

Important Considerations

• Ignore STP mode is disabled by default for static VLANs.

• You can use this mode only when the Multilayer Switching Module is in allClosed mode.

• Ignore STP mode is useful when you have redundant router connections between modules that have STP enabled. In this situation, if you want to create multiple VLANs and use one VLAN for routing, you can configure your module to ignore the STP blocking mode for that VLAN. This setting avoids disruptions to routing connectivity based on the STP state.

• To disable STP blocking on a per-port basis with allOpen or allClosed VLANs, use the bridging option (bridge port stpState on the Administration Console). See the Chapter 9 for bridging information.

  Ignore STP mode affects bridging as well as routing. If you have STP enabled on the system and you have redundant bridged paths between systems with different VLANs, STP blocks one of the paths unless you enable Ignore STP mode.

Figure 22 shows two paths available through the default VLAN and IP VLAN2 if end station A wants to communicate with end station B. STP blocks the routed as well as bridged traffic for the one path through IP VLAN2 unless you enable Ignore STP Mode for IP VLAN2 (because the backplane ports have higher priority). With the blocking removed for IP routed traffic, the best path is used. (In this configuration, the switch fabric module resides in slot 7 but is logically represented above the other modules.)

IP VLAN2 has routing interfaces defined on both Multilayer Switching Modules (22.2.2.2 on the first Multilayer Switching Module and 22.2.2.3 on the second Multilayer Switching Module).

Figure 22 Ignore STP Mode