Extreme Networks ExtremeWare 7.2e, Installation And User Manual

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STP Configurations
ExtremeWare 7.2e Installation and User Guide 201
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 33 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 33: VLAN Spanning Multiple STPDs
In addition, the configuration in Figure 33 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.
EMISTP and PVST+ Deployment Constraints
NOTE
While EMISTP and PVST+ greatly enhances STP capability, these features must deployed with care. As
stated before, a VLAN can span multiple STPDs. However, on the Summit 400, there is a hardware
limitation that restricts each physical port to a single STPD. If the Summit 400 port is already a member
of an STPD, then that port cannot be in another VLAN that is in a different STPD, or not in a STPD at
all. EMISTP and PVST+ are supported only in compatibility mode.
Newly created EMISTP VLANs are
not associated with STPD s0 by default.
EW_083
S1
S3
VLAN yellow
VLAN red
VLAN red
VLAN brown
S2
S4
VLAN red
VLAN green
VLAN red
VLAN blue