Spanning Tree Protocol
ExtremeWare XOS 11.3 Concepts Guide
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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:
â—Ź 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 seven hops. By
segregating a big VLAN into multiple STPDs, you reduce complexity and enhance performance.
â—Ź 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 35 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 35: VLAN spanning multiple STPDs
In addition, the configuration in Figure 35 has these features:
â—Ź 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.
â—Ź Between the sites the connections 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.
EMISTP Deployment Constraints
Although EMISTP greatly enhances STP capability, these features must deployed with care. This section
discusses configuration issues that, if not followed, could lead to an improper deployment of EMISTP.
This section also provides the following restrictive principles to abide by in network design:
â—Ź Although a physical port can belong to multiple STPDs, any VLAN on that port can be in only one
domain. Put another way, a VLAN cannot belong to two STPDs on the same physical port.
â—Ź Although a VLAN can span multiple domains, any LAN segment in that VLAN must be in the same
STPD. VLANs traverse STPDs only inside switches, not across links. On a single switch, however,
bridge ports for the same VLAN can be assigned to different STPDs. This scenario is illustrated in
Figure 36.
EX_051
S1
S3
VLAN yellow
VLAN red
VLAN red
VLAN brown
S2
S4
VLAN red
VLAN green
VLAN red
VLAN blue
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