I85PREP
⌘ K
Ruankao Network Engineer (Intermediate) study materials
Ruankao Network Engineer (Intermediate) study resources

Ruankao Network Engineer (Intermediate): Data Link Layer and LAN Technologies

Ruankao Network Engineer (Intermediate) · Work through your exam, one topic at a time.

All guide sections

Data Link Layer and LAN Technologies

This domain tests Ethernet mechanics, switching behaviour, VLANs, spanning tree, wireless LANs and point-to-point link protocols. It is the most configuration-adjacent of the lower-layer domains.

Ethernet framing and MAC

  • Frame: 7-byte preamble, 1-byte SFD, 6-byte destination and source MAC, 2-byte Type/Length, 46–1500-byte payload, 4-byte FCS. Minimum frame 64 bytes, maximum 1518 bytes (excluding preamble), so the minimum payload is 46 bytes after padding.
  • MAC addresses are 48 bits; the first 24 bits form the OUI.
  • CSMA/CD applies to half-duplex shared media only; full-duplex switched links do not use it.

Switching and VLANs

  • A switch learns source MACs into its CAM table, floods unknown, broadcast and multicast frames, and filters by destination port.
  • Each switch port is its own collision domain; a VLAN is a broadcast domain.
  • 802.1Q inserts a 4-byte tag; the VID field is 12 bits, giving usable IDs 1–4094 (0 and 4095 reserved). Access ports carry one untagged VLAN; trunk ports carry many tagged VLANs plus one untagged native VLAN.
  • Inter-VLAN routing needs a Layer 3 device: router sub-interfaces (one per VLAN, 802.1Q encapsulation) or a Layer 3 switch with SVIs.

Spanning Tree

  • 802.1D elects one root bridge by lowest bridge ID (priority in 4096 steps, default 32768, then MAC). Each non-root bridge selects one root port by lowest path cost, then lowest sender bridge ID, then lowest port ID.
  • Port states: blocking, listening, learning, forwarding; convergence takes 30–50 s. RSTP (802.1w) reduces this to seconds with discarding/learning/forwarding states.

WLAN and link protocols

  • 802.11b/g use 2.4 GHz, 802.11a/ac use 5 GHz, 802.11n uses both with MIMO, 802.11ax is Wi-Fi 6. Access uses CSMA/CA with acknowledgement.
  • Security evolution: WEP (broken) → WPA/TKIP → WPA2/AES-CCMP → WPA3.
  • PPP carries LCP for link setup, NCP for network-layer parameters, and PAP or CHAP for authentication (CHAP uses a challenge and is the safer choice). HDLC is a simpler bit-oriented framing protocol.

Worked example

Three switches form a triangle. SW1 priority 4096, SW2 and SW3 default 32768 with MACs 00-00-0C-AA-AA-AA and 00-00-0C-BB-BB-BB. SW1 wins on priority and becomes root. On SW2, the path to SW1 is one hop (cost 19 for 100 Mbps), while the path via SW3 is two hops (38), so SW2's direct link to SW1 is its root port and its link to SW3 is blocked on the higher port ID side.

Traps that cost marks

  • Claiming a switch blocks broadcasts.
  • Quoting VLAN range as 1–4095 rather than 1–4094.
  • Forgetting that a native VLAN mismatch on a trunk causes a CDP/STP warning and possible traffic leakage.
  • Using CSMA/CD to explain full-duplex switched Ethernet.