Wired Switching Fundamentals
Domain overview
Wired Switching Fundamentals tests the Layer 2 and Layer 3 mechanics that Mist Wired Assurance later observes and configures: Ethernet framing, VLANs and 802.1Q tagging, spanning tree, link aggregation, LLDP, PoE, 802.1X, and basic IP services. You need to know what a switch does to a frame before you can diagnose why Mist reports a port as unhealthy.
Core rules
- An access port carries one untagged VLAN; a trunk port carries multiple tagged VLANs plus one untagged native VLAN. A native-VLAN mismatch is a common silent failure.
- RSTP/MSTP block redundant paths; LACP bundles links and requires consistent speed, duplex, VLAN, and LAG mode on both ends.
- LLDP advertises neighbor identity and capabilities; LLDP-MED extends this for voice endpoints and PoE negotiation.
- PoE has classes and budgets; a port can be administratively up while PoE is denied because the power budget is exhausted.
- 802.1X is port-based authentication: the supplicant authenticates to the authenticator, the switch, via RADIUS before the port forwards normal traffic.
- DHCP relay forwards broadcasts to a server; ARP and MAC learning still matter for L2 forwarding.
Worked example
A trunk port allows VLANs 10 and 20, native VLAN 1, and also carries voice VLAN 30 tagged. An untagged frame from a PC arrives: the switch assigns VLAN 1 because that is the native VLAN. A tagged frame with VLAN 10 passes because 10 is in the allowed list. A tagged frame with VLAN 40 is dropped. If the voice phone tags VLAN 30 but 30 is not in the allowed list, voice fails even though data works. Check the allowed VLAN list, native VLAN, and PoE budget before blaming the phone.
Traps
Do not assume a link is healthy because it is up; duplex mismatch, CRC errors, and STP blocking can still break traffic. Do not confuse access mode with a trunk's native VLAN. Do not forget that 802.1X and PoE are separate from VLAN membership.