Multiple STPDs on a Port

Traditional 802.1D STP has some inherent limitations when addressing networks that have multiple VLANs and multiple STPD (Spanning Tree Domain)s.

For example, consider the sample depicted in the following figure.

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Limitations of Traditional STPD
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The two switches are connected by a pair of parallel links. Both switches run two VLANs, A and B. To achieve load-balancing between the two links using the traditional approach, you would have to associate A and B with two different STPDs, called S1 and S2, respectively, and make the left link carry VLAN A traffic while the right link carries VLAN B traffic (or vice versa). If the right link fails, S2 is broken and VLAN B traffic is disrupted.

To optimize the solution, you can use the Extreme Multiple Instance Spanning (EMISTP) mode, which allows a port to belong to multiple STPDs. EMISTP adds significant flexibility to STP network design. Referring to the figure above, using EMISTP, you can configure all four ports to belong to both VLANs.

Assuming that S1 and S2 still correspond to VLANs A and B respectively, you can fine-tune STP parameters to make the left link active in S1 and blocking in S2, while the right link is active in S2 and blocking in S1. Again, if the right link fails, the left link is elected active by the STP algorithm for S2, without affecting normal switching of data traffic.

Using EMISTP, an STPD becomes more of an abstract concept. The STPD does not necessarily correspond to a physical domain; it is better regarded as a vehicle to carry VLANs that have STP instances. Because VLANs can overlap, so do STPDs. However, even if the different STPDs share the entire topology or part of the redundant topology, the STPDs react to topology change events in an independent fashion.