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Per-VLAN mode is the Default Spanning Tree setting on all multiservice switches. In this mode, the switch runs a separate spanning process for each VLAN. Each logical Spanning Tree has its own BPDUs which are tagged with the appropriate VLAN Identifier. In this configuration, the switch can participate in as many Spanning Tree domains as there are VLANs defined on the switch. This conforms to a "virtual" bridging model where the switch runs as if each VLAN is a separate logical bridge (separate Address Forwarding Tables, separate spanning trees, etc.). If spanning tree is not required on all VLANs, you can disable it for individual VLANs on a VLAN by VLAN basis. Also, different root bridges can be configured with different bridge priorities based upon VLAN. This will allow load sharing to occur based upon VLAN. Similarly link costs and priorities can be adjusted on a per-VLAN basis allowing further load sharing per VLAN.
Per VLAN Spanning Tree is the recommended method whenever many VLANs are passed over interswitch links, but not every VLAN resides on every interswitch link. If IEEE 802.1D Spanning Tree mode is used without care under these constraints, VLANs can be split due to blocking on inappropriate links. The only time that Per-VLAN Spanning Tree may become undesirable is when you have more VLANs to manage than the number of Spanning Tree Domains. The only other constraint is to limit the number of outgoing BPDUs.
It is required that the number of outgoing BPDUs be less than 500/second on all multiservice switches. This means that if you have 50 VLANs you cannot have more than 20 possible paths back to the root bridge (50 VLANS X 20 LINKS X 0.5 BPDU/sec. = 500), or if you have 100 VLANs you cannot have more that 10 possible paths back to the root bridge. These constraints are not generally exceeded in real networks. If these limits are exceeded, you must use 802.1D Spanning Tree mode.
This method can easily interoperate with legacy IEEE802.1D devices. The legacy devices become part of the VLAN associated with the VLAN port binding with which they are attached. In other words, if three P580 switches are in a network running VLANA and VLANB, and three legacy devices connect up through a port configured in VLAN A, the legacy devices will become part of the VLAN A Spanning Tree. The P580 switches will participate in two Spanning Tree domains, one for VLAN A containing three bridges and one for VLAN B containing 6 bridges. The legacy devices need not be VLAN aware.
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