MST General Overview Configuring Spanning Tree Parameters
page 6-14 OmniSwitch AOS Release 7 Network Configuration Guide June 2013
Flat Mode MSTP
In the above flat mode MSTP example:
• Both switches are running in the flat mode and using MSTP.
• VLANs 100 and 150 are not associated with an MSTI. They are controlled by the default CIST
instance 0 that exists on every switch.
• VLANs 200 and 250 are associated with MSTI 2 so their traffic can traverse a path different from that
determined by the CIST.
• Ports are blocked the same way they were blocked in the flat mode STP/RSTP example; all port
connections are compared to each other across VLANs to determine which connection provides the
best path.
However, because VLANs 200 and 250 are associated to MSTI 2, it is possible to change the port path
cost for ports 2/12, 3/6, 4/8 and/or 5/2 so that they provide the best path for MSTI 2 VLANs, but do not
carry CIST VLAN traffic or cause CIST ports to transition to a blocking state.
Another alternative is to assign all VLANs to an MSTI, leaving no VLANs controlled by the CIST. As
a result, the CIST BPDU contains only MSTI information.
See “Sample MSTI Configuration” on page 6-49 for more information about how to direct VLAN traffic
over separate data paths using MSTP.
2/1
3/1
4/2
4/8
5/2
5/1
||
VLAN 100
VLAN 250
MSTI-2
VLAN 200
VLAN 150
CIST-0
VLAN 100
VLAN 250
MSTI-2
VLAN 200
VLAN 150
CIST-0
||
2/12
3/6
||
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