Emistp Deployment Constraints - Extreme Networks ExtremeWare XOS Guide Manual

Spanning Tree Protocol
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
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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.
Local to each site, there may be other smaller VLANs that share the same redundant looped area
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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 30 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 30: VLAN spanning multiple STPDs
VLAN red
S1
VLAN green
VLAN red
S3
VLAN blue
In addition, the configuration in
Each site can be administered by a different organization or department within the enterprise.
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Having a site-specific STP implementation makes the administration more flexible and convenient.
Between the sites the connections usually traverse distribution switches in ways that are known
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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:
Although a physical port can belong to multiple STPDs, any VLAN on that port can be in only one
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domain. Put another way, a VLAN cannot belong to two STPDs on the same physical port.
Although a VLAN can span multiple domains, any LAN segment in that VLAN must be in the same
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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 31.
ExtremeWare XOS 11.1 Concepts Guide
S2
VLAN yellow
VLAN red
Figure 30 has these features:
VLAN red
S4
VLAN brown
EX_051
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