Netgear XCM8810 - 8800 SERIES 10-SLOT CHASSIS SWITCH ユーザーズマニュアル
Chapter 18. STP
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NETGEAR 8800 User Manual
Figure 37. Limitations of Traditional STPD
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 carries VLAN B traffic (or vice
versa). If the right link fails, S2 is broken and VLAN B traffic is disrupted.
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 carries 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 Multiple Instance Spanning (EMISTP) mode, which
allows a port to belong to multiple STPDs. EMISTP adds significant flexibility to STP network
design. Referring to
allows a port to belong to multiple STPDs. EMISTP adds significant flexibility to STP network
design. Referring to
, 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. 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.
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. 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. The STPD 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.
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.
VLANs 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.
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.
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
belong to the same enterprise) and may traverse a great many nodes. In this case, it is
EX_050
S1
S2
A
A
B
B
S1
S2
A
A
B
B