Jeremy’s IT Lab - CCNA Lecture Notes

Introduction

• Continuation of the topic of VLANs (Virtual Local Area Networks)
• Review from previous lesson (Day 16): Basics of VLANs and basic configurations
• Today’s focus: Advanced basics necessary for understanding and using VLANs

Key Topics Covered

Trunk Ports

• Definition: Unlike access ports which belong to a single VLAN, trunk ports carry traffic from multiple VLANs over a single interface.
• Purpose: Allow switches to forward traffic from multiple VLANs over a single physical interface to save resources.

802.1Q Encapsulation

• Function: Adds a tag to Ethernet frames to identify which VLAN they belong to when sent over trunk ports.
• Structure: Inserts a 4-byte tag between the source MAC address and type/length fields of the Ethernet frame.
  • Components:
    • TPID (16 bits, always 0x8100 to indicate a dot1q-tagged frame)
    • TCI, which includes:
      • PCP (3 bits, Priority Code Point for prioritizing traffic)
      • DEI (1 bit, Drop Eligible Indicator for network congestion)
      • VID (12 bits, VLAN ID, identifies the VLAN the frame belongs to)
• VLAN Range: 1-4094 (0 and 4095 are reserved). Extended range: 1006-4094 not supported by older devices.

Network Topology

• Two switches involved with split VLANs for departments across physical locations.
• Trunk ports replace separate connections for each VLAN between switches and routers.

Trunk Port Configuration

• Commands:
  • SWITCHPORT MODE TRUNK: Configures interface as a trunk.
  • SWITCHPORT TRUNK ENCAPSULATION dot1q: Defines encapsulation type (necessary on some switches).
  • SHOW INTERFACES TRUNK: Checks trunk configurations.
  • SWITCHPORT TRUNK ALLOWED VLAN: Limits VLANs on trunk for security and performance.
  • SWITCHPORT TRUNK NATIVE VLAN: Changes native VLAN (frames in native VLAN are untagged).

Native VLAN

• Function: Frames in the native VLAN are not tagged.
• Importance: Native VLAN configuration must match between switches to avoid issues.

Router on a Stick (ROAS)

• Concept: Used for inter-VLAN routing using a single physical interface on the router divided into subinterfaces.
• Configuration:
  • Subinterface named to match VLAN numbers (recommended).
  • ENCAPSULATION DOT1Q <VLAN>: Router treats frames tagged with specified VLAN as arriving on that subinterface.

Review and Quiz

• Trunk Ports: Carry traffic over multiple VLANs.
• 802.1Q Encapsulation: Identifies VLAN of frame.
• Router on a Stick: Efficient inter-VLAN routing using subinterfaces.

Practice Questions

1. Command to send untagged VLAN10 frames: switchport trunk native vlan 10.
2. Restore default VLAN allowance on trunk: switchport trunk allowed vlan all.
3. Fix command rejection when setting trunk: switchport trunk encapsulation dot1q.
4. Field identifying VLAN ID in 802.1Q tag: VID.
5. VLAN10 not appearing in allowed VLANs: Check if VLAN10 exists on the switch.

Supplementary Materials

• Flashcards for review.
• Packet tracer lab for practice.

Acknowledgements

• Thanks to JCNP-level channel members for support.
• Encouragement to subscribe, like, comment, and share to spread the free CCNA series.