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Figure23-14Typical Network Design with VLANs Spanning Too Many Links
Trunk
Core B
Core A
Trunk
Trunk
Trunk
Trunk
Dist2
Dist3
Users in VLAN 2
Users in VLAN 3
This is a very common design. Distribution switches are dual-attached to two core switches. Users
connected on distribution switches are in only a subset of the VLANs available in the network (here,
users connected on Dist2 are all in VLAN 2; Dist3 connects only users in VLAN 3). By default, trunks
carry all the VLANs defined in the VTP domain. Now only is Dist2 receiving unnecessary broadcast and
multicast traffic for VLAN 3, but it is also blocking one of its ports for VLAN 3. The result is that there
are three redundant paths between Core A and Core B. This means more blocked ports and increased
chances for a loop.
Important note: Prune any VLAN not needed off your trunks.
VTP pruning can help doing this, but that kind of plug-and-play feature is not really needed in the core
of the network.
Let's take the same example as previously shown in Figure 23-14. This time, we just use an access
VLAN to connect the distribution switches to the core, as shown in Figure 23-15.
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