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3.2. Interswitch Connectivity (Trunking)

đź’ˇ First Principle: A trunk link carries traffic for multiple VLANs over a single physical connection by tagging each frame with its VLAN ID. Without trunks, you'd need a separate physical link for every VLAN between switches. The exam tests trunk configuration, native VLANs, and troubleshooting mismatches.

Imagine you have 10 VLANs and need to connect two switches. Without trunking, you'd need 10 cables—one for each VLAN. That's wasteful and doesn't scale. Trunking solves this by multiplexing all VLAN traffic onto a single link, with tags that say "this frame belongs to VLAN 10."

What breaks without proper trunking? Inter-switch VLAN communication fails completely. A user in VLAN 10 on Switch A can't reach a server in VLAN 10 on Switch B if the trunk is misconfigured—traffic either gets dropped or ends up in the wrong VLAN. Native VLAN mismatches are even more insidious: frames silently land in unexpected VLANs, creating security holes and phantom connectivity issues.

Why trunk configuration matters:
  • Allowed VLANs: Only VLANs you explicitly permit will traverse the trunk (security)
  • Native VLAN: Untagged traffic goes here—mismatches cause serious problems
  • Encapsulation: 802.1Q is the standard; ISL is Cisco-proprietary and deprecated

The native VLAN trap: This is a frequent exam topic and real-world troubleshooting issue. If Switch A sends untagged traffic (native VLAN 1) and Switch B expects untagged traffic on VLAN 99, frames end up in the wrong VLAN. Devices can't communicate, and you'll spend hours chasing phantom connectivity issues.