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Dual-Layer spanning tree mode is a variation of per-VLAN spanning tree mode with many of the same features. However, instead of using normal BPDUs, which are clear (free of VLAN tags) on clear links and tagged on tagged links, as is the case with Per-VLAN Spanning Tree mode, dual-layer uses a proprietary BPDU. This proprietary BPDU is sent to a special multicast address and contains information about which VLAN the BPDU is associated with. This has an advantage over per-VLAN spanning trees in that this method can support multiple VLANs over a non-tagging link, or when connecting to a bridge/router.
In this mode, legacy bridges remain in separate Spanning Tree domains, yet loops between the layer 3 and legacy domains cannot form. For example; if there are three P580 switches in a network running VLAN A and VLAN B, and three legacy devices connect up through a port configured in VLAN A, the legacy devices remain in their own legacy Spanning Tree and do NOT join the Multilayer Spanning Tree.
In the previous example, all three multiservice switches participate in two Spanning Tree domains:
The legacy devices need not be aware of the other spanning tree domains. If a loop forms between the Multilayer domains and the legacy domain, the proprietary BPDU is seen returning to the Multilayer domain through the legacy domain and the loop is blocked. The Dual-Layer Spanning Tree method is preferred when inter-operating with large legacy bridge networks. Convergence time is reduced, and management is simplified, by allowing the legacy bridges to remain in their own spanning tree domain
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