Interswitch connectivity allows multiple switches to communicate and share VLAN information, ensuring seamless data transmission across different parts of a LAN. In this article, we’ll explore how to configure and verify Interswitch connectivity, step by step.

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What is Interswitch Connectivity?

Before discussing Configuring and Verifying Interswitch Connectivity, let us understand what Interswitch connectivity is, trunk port, 802.1q VLAN protocol and native VLAN.

Interswitch connectivity refers to the link established between two or more network switches. This connection, known as a trunk link, carries traffic for multiple VLANs, allowing hosts in the same VLAN but on different switches to communicate efficiently.

What is a Trunk Port?

A trunk port is a switch port configured to transmit and receive frames from multiple VLANs using VLAN tagging (IEEE 802.1Q). Each frame passing through the trunk is tagged with a VLAN ID, which tells the receiving switch which VLAN the frame belongs to.

Purpose of a Trunk Port

• To extend VLANs across multiple switches.
• To enable inter-switch connectivity between VLANs.
• To conserve physical ports by using one link for many VLANs.

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Steps to Configure Interswitch Connectivity

Let’s go through the configuration process using Cisco IOS commands.

Step 1: Create VLANs on Each Switch

Before connecting switches, ensure that the same VLANs exist on both devices.

Switch(config)# vlan 10
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit

Switch(config)# vlan 20
Switch(config-vlan)# name HR
Switch(config-vlan)# exit

Step 2: Assign Ports to VLANs

Connect end devices to specific VLANs by configuring access ports.

Switch(config)# interface fastEthernet 0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10
Switch(config-if)# exit

Switch(config)# interface fastEthernet 0/2
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 20
Switch(config-if)# exit

Step 3: Configure the Trunk Link

Next, configure the link between the switches as a trunk so it can carry multiple VLANs.

Switch(config)# interface gigabitEthernet 0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# switchport trunk native vlan 1
Switch(config-if)# no shutdown

Verification Commands

After configuration, verify that the interswitch link is functioning correctly.

Command	Description
show vlan brief	Displays all VLANs and their assigned interfaces.
show interfaces trunk	Shows trunk ports and VLANs allowed on them.
show interfaces switchport	Displays port mode (access or trunk).
show cdp neighbors	Verifies physical connection between switches.

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802.1Q Tagging

IEEE 802.1Q is a VLAN tagging protocol that inserts a small tag inside an Ethernet frame.
This tag contains the VLAN ID (VID) — a number that tells the receiving switch which VLAN the frame belongs to. It allows multiple VLANs to use the same physical link while keeping their data isolated.

Without 802.1Q tagging, switches would have no way to distinguish which VLAN a frame came from when multiple VLANs share a single trunk.

How 802.1Q Tagging Works

When a frame leaves a switch via a trunk port:

1. The switch adds a VLAN tag to the Ethernet frame header.
2. The tag contains VLAN information (typically VLAN ID 1–4094).
3. The frame is then sent across the trunk link.
4. The receiving switch reads the tag, identifies the VLAN, and forwards the frame only within that VLAN.

When the frame reaches an access port, the tag is removed before it is delivered to the end device — ensuring devices remain unaware of VLANs.

Structure of an 802.1Q Tag

The 802.1Q tag is 4 bytes long and is inserted between the Source MAC address and the Ether Type fields in an Ethernet frame.

802.1q Tag consists of multiple fields that make up VLAN tag.

• TPID (Tag Protocol Identifier): It is 16 bits or 2 bytes field which is always set to hexadecimal value of 0x8100. This value helps to identify the frame as VLAN Tag.
• TCI (Tag Control Information): It consists of three major fields:
  • PCP (Priority Code Point) is a 3-bit field that defines the Class of Service (CoS) or frame priority (values 0–7) used for traffic prioritization. It helps to handle the layer 2 critical information like voice traffic.
  • DEI (Drop Eligible Indicator) was formerly called CFI (Canonical Format Indicator). it is a 1-bit field that decides whether the frame can be dropped when there is congestion (1 = eligible to drop).
  • VID (VLAN ID) is a12-bit field that supports 4096 VLANs. It provides a unique identification of the VLAN that ranges from 1 to 4094. VLAN 0 and 4095 are reserved.

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Native VLAN

Native VLAN refers to the default VLAN that carries untagged traffic on a trunk port. It does not add an 802.1Q tag to Ethernet frames as they pass over a trunk link. It is used for untagged traffic between switches or other 802.1Q-compliant devices.

Native VLAN is defined in 802.1q as the backward compatibility for the old network device that does not support VLAN tagging.

Configuration of Native VLAN

Switch1 Configuration:

Switch1(config)# vlan 10
Switch1(config-vlan)# name SALES
Switch1(config)# interface Gig0/1
Switch1(config-if)# switchport mode trunk
Switch1(config-if)# switchport trunk native vlan 999
Switch1(config-if)# switchport trunk allowed vlan 10,20

Switch2 Configuration:

Switch2(config)# vlan 10
Switch2(config-vlan)# name SALES
Switch2(config)# interface Gig0/1
Switch2(config-if)# switchport mode trunk
Switch2(config-if)# switchport trunk native vlan 999
Switch2(config-if)# switchport trunk allowed vlan 10,20

Verifying Interswitch Connectivity

The folowing IOS commands are used to verify Interswitch connectvity

Command	Purpose
show interfaces trunk	View trunk ports and native VLANs
show interfaces switchport	Detailed VLAN info for a specific port
show vlan brief	Verify VLAN existence
CDP messages	Detect mismatched native VLANs

The output of show interfaces trunk command

The output of show interfaces switchport command:

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