Multiple Stpds On A Port; Vlan Spanning Multiple Stpds - Extreme Networks ExtremeWare XOS Guide Manual

Multiple STPDs on a Port

Traditional 802.1d STP has some inherent limitations when addressing networks that have multiple
VLANs and multiple STPDs. For example, consider the sample depicted in Figure 11.
Figure 11: Limitations of traditional STPD
S1
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 carry 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 Figure 11, 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. Once 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. It 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.

VLAN Spanning Multiple STPDs

Traditionally, the mapping from VLANs to STP instances have been one-to-one, or many-to-one. In both
cases, a VLAN is wholly contained in a single instance. In practical deployment there are cases in which
a one-to-many mapping is desirable. In a typical large enterprise network, for example, VLANs span
multiple sites and/or buildings. Each site represents a redundant looped area. However, between any
two sites the topology is usually very simple.
ExtremeWare XOS 10.1 Concepts Guide
A B
S2
A B
A B
S1
A B
STP Configurations
S2
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