Netgear GS728TP User Manual download pdf (Page 522)

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Configuration Examples

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ProSafe M5300 Switch

An MST Region comprises of one or more MSTP Bridges with the same MST Configuration

Identifier, using the same MSTIs, and which have no Bridges attached that cannot receive

and transmit MSTP BPDUs. The MST Configuration Identifier has the following components:

1. Configuration Identifier Format Selector

2. Configuration Name

3. Configuration Revision Level

4. Configuration Digest: 16-byte signature of type HMAC-MD5 created from the MST

Configuration Table (a VLAN ID to MSTID mapping)

As there are Multiple Instances of Spanning Tree, there is a MSTP state maintained on a

per-port, per-instance basis (or on a per port per VLAN basis: as any VLAN can be in one and

only one MSTI or CIST). For example, port A can be forwarding for instance 1 while

discarding for instance 2. The port states have changed since IEEE 802.1D specification.

To support multiple spanning trees, a MSTP bridge has to be configured with an

unambiguous assignment of VLAN IDs (VIDs) to spanning trees. This is achieved by:

1. Ensuring that the allocation of VIDs to FIDs is unambiguous.

2. Ensuring that each FID supported by the Bridge is allocated to exactly one Spanning Tree

Instance.

The combination of VID to FID and then FID to MSTI allocation defines a mapping of VIDs to

spanning tree instances, represented by the MST Configuration Table.

With this allocation we ensure that every VLAN is assigned to one and only one MSTI. The

CIST is also an instance of spanning tree with a MSTID of 0.

An instance may occur that has no VIDs allocated to it, but every VLAN must be allocated to

one of the other instances of spanning tree.

The portion of the active topology of the network that connects any two bridges in the same

MST Region traverses only MST bridges and LANs in that region, and never Bridges of any

kind outside the Region, in other words connectivity within the region is independent of

external connectivity.

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