Allied-telesis AT-S62 Manual de usuario descargar pdf (Pagina 492)

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Chapter 24: Multiple Spanning Tree Protocol

Section IV: Spanning Tree Protocols 492

When port 3 on Switch B receives a BPDU, the switch notes the port

sending the packet belongs only to CIST. Consequently, Switch B uses

CIST in determining whether a loop exists. The result would be that the

switch would determine that a loop exists because the other port is also

receiving BPDU packets from CIST 0. Switch B would block a port to

cancel the loop.

To avoid this issue, always assign all VLANs on a switch, including the

Default_VLAN, to an MSTI. This guarantees that all ports on the switch

have an MSTI ID and that helps to ensure that loop detection is based on

MSTI, not CIST.

Connecting VLANs Across Different Regions

Special consideration needs to be taken into account when connecting

different MSTP regions or an MSTP region and a single-instance STP or

RSTP region. Unless planned properly, VLAN fragmentation can occur

between the VLANS of your network.

As mentioned previously, only the CIST can span regions. A MSTI cannot.

Consequently, you may run into a problem if you use more than one

physical data link to connect together various parts of VLANs that reside

in bridges in different regions. The result can be a physical loop, which

spanning tree disables by blocking ports.

This is illustrated in Figure 171. The example show two switches, each

residing in a different region. Port 5 in Switch A is a boundary port. It is

an untagged member of the Accounting VLAN, which has been

associated with MSTI 4. Port 15 is a tagged and untagged member of

three different VLANs, all associated to MSTI 12.

If both switches were a part of the same region, there would be no

problem since the ports reside in different spanning tree instances.

However, the switches are part of different regions and MSTIs do not

cross regions. Consequently, the result would be that spanning tree

would determine that a loop exists between the regions, and Switch B

would block a port.

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