Cisco's use of explicit tagging creates significant scalability benefits.
However, there is a hidden downside to the advantage of every switch not needing to know what VLANs other switches are using—flooded traffic must be sent to every switch in the Layer 2 network. In other words, by default, one copy of every broadcast, multicast, and unknown unicast frame is flooded across every trunk link in a Layer 2 domain.
Two approaches can be used to reduce the impact of this flooding. First, note that if you are using campus-wide VLANs, this flooding problem also becomes campuswide. Therefore, one of the simplest and most scalable ways to reduce this flooding is to partition the network with several Layer 3 barriers that utilize routing (Layer 3 switching) technology. This breaks the network into smaller Layer 2 pockets and constrains the flooding to each pocket.
Where Layer 3 switching cannot prevent unnecessary flooding (such as with campuswide VLANs or within each of the Layer 2 pockets created by Layer 3 switching), a second technique of VLAN pruning can be employed. By using the clear trunk command discussed in Chapter 8, "Trunking Technologies and Applications," unused VLANs can be manually pruned from a trunk. Therefore, when a given switch needs to flood a frame, it only sends it out access ports locally assigned to the source VLAN and trunk links that have not been pruned of this VLAN. For example, an MDF switch can be configured to flood frames only for VLANs 1 and 2 to a given IDF switch if the switch only participates in these two VLANs. To automate the process of pruning, VTP pruning can be used. For more information on VTP pruning, please refer to Chapter 12, "VLAN Trunking Protocol."
One of the most important uses of manual VLAN pruning involves the use of a Layer 2 campus core, the subject of the next section.
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