User guide

Per-VLAN Spanning Tree

ExtremeWare 7.2e Installation and User Guide 197

Alternatively, the same VLAN may span multiple large geographical areas (because they belong to the

same enterprise) and may traverse a great many nodes. In this case, it is desirable to have multiple STP

domains operating in a single VLAN, one for each looped area. The justifications include the following:

• The complexity of the STP algorithm increases, and performance drops, with the size and complexity

of the network. The 802.1d standard specifies a maximum network diameter of 7 hops. By

segregating a big VLAN into multiple STPDs, you reduce complexity and enhance performance.

• Local to each site, there may be other smaller VLANs that share the same redundant looped area

with the large VLAN. Some STPDs must be created to protect those VLAN. The ability to partition

VLANs allows the large VLAN to be “piggybacked” in those STPDs in a site-specific fashion.

Figure 31 has five domains. VLANs green, blue, brown, and yellow are local to each domain. VLAN red

spans all of the four domains. Using a VLAN that spans multiple STPDS, you do not have to create a

separate domain for VLAN red. Instead, VLAN red is “piggybacked” onto those domains local to other

VLANs.

Figure 31: VLAN Spanning Multiple STPDs

In addition, the configuration in Figure 31 has these features:

• Each site can be administered by a different organization or department within the enterprise.

Having a site-specific STP implementation makes the administration more flexible and convenient.

• Between the sites the connection usually traverse distribution switches in ways that are known

beforehand to be “safe” with STP. In other words, the looped areas are already well-defined.

Per-VLAN Spanning Tree

Switching products that implement Per-VLAN Spanning Tree (PVST) have been in existence for many

years and are widely deployed. To support STP configurations that use PVST, ExtremeWare has an

operational mode called PVST+.

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VLAN yellow

VLAN red

VLAN brown

VLAN red

VLAN green

VLAN red

VLAN blue