VLANs

Trunk Ports

Sections: 

Overview: 

  • A trunk port is a switchport that is configured to carry traffic for multiple VLANs simultaneously across one or more physical links 
  • Efficient use of resources is a benefit to trunk links, reducing the number of physical connections needed between devices
  • Trunk links are used to connect switches, routers, and other devices that need to communicate across different VLANs or subnets
  • In a trunk port, the traffic that travels across the link is tagged with a VLANID using the IEEE 802.1Q trunking protocol
  • 802.1Q is used to identify the VLANs on a trunk link by adding the VLAN tag to an Ethernet frame
    • Note: ISL or Inter-Switch Link is a legacy Cisco proprietary trunking protocol that has since been replaced with 802.1Q

Lab Topology

Scenario:

  • In this lab topology scenario I will configure trunk ports on the switch uplinks to allow all defined VLANs to be carried across the links
  • When all trunk links are configured, I will demonstrate a Wireshark capture to analyze a 802.1Q VLAN tag in an Ethernet frame

Trunk Link Configuration

HQ-Access-SW1 

HQ-Distro-SW1

HQ-Distro-SW2

HQ-Core-SW1

To summarize the scenario, trunk links have been configured on all switch uplinks to carry multiple VLANs on the physical links.

  • Interface Configuration Commands
    • To specify trunking protocol
      • switchport trunk encapsulation <protocol>
    • To assign interface port to a trunk port
      • switchport mode trunk
    • To verify configured trunk ports
      • show interfaces trunk

Packet Captures

802.1Q Wireshark Capture 

Scenario:

  • In this scenario we will examine several Ethernet frames with a 802.1Q tag by sending ICMP traffic towards the Gateway-R1 router from the PCs in VLANs 10, 20, 60, and 80

802.1Q VLAN 10 - Engineering

802.1Q VLAN 20 - Finance

802.1Q VLAN 60 - MGMT

802.1Q VLAN 80 - Servers

To summarize the scenario, there is a VLANID tag in a 802.1Q header to differentiate the VLAN traffic is sourced from.