Cisco VLAN Troubleshooting Commands And VLAN Issues

VLAN Troubleshooting

In order to find and fix connectivity problems caused by Virtual Local Area Networks (VLANs), VLAN troubleshooting entails a methodical approach to examining the setup and condition of network devices. Given that VLAN issues can impede host connectivity and proper network operation, this ability is essential for network engineers. The physical layer is usually where the troubleshooting process begins, and it then moves on to the data link (Layer 2) and routed (Layer 3) layers.

Common VLAN Issues

There are several possible causes of VLAN connectivity issues. Typical problems include:

Physical Connectivity Problems

• Damaged or broken cables: Optical cables or wires may be pinched, filthy, or broken.
• Defective Network Interface Cards (NICs) or connectors.
• Electrical systems interfering.

These frequently appear as unidirectional links, in which packets only move in one way.

Unreliable communication may result from a host-to-switch connectivity issue. To make sure the physical layer (Layer 1) is operational, it is advised to inspect the LED lights on physical switches.

Also Read About What Is VLAN Trunking? How It Works, Advantages & Protocols

Duplex Mismatch

• An Ethernet link requires that both sides be set up for the same speed and duplex setting (auto, full, or half).
• A basic ping test is insufficient for diagnosing a duplex mismatch since it can function at low packet speeds but malfunction at larger ones.
• Runt frames on a full-duplex interface coupled to a half-duplex device or late collisions on a half-duplex interface interacting with a full-duplex interface are examples of symptoms.
• Sluggish traffic loops or collision domain connectivity might also be the culprit, which results in packet loss and sluggish data transfer rates.
• For the majority of devices, “auto” is the recommended setting. Setting the interface’s bandwidth appropriately and checking data routing are the answers.

Incorrect VLAN Configuration (Layer 2 Data Link Errors)

Incorrect VLAN ID assignment for a port: This may result in a port being in the incorrect subnet or VLAN, which would prevent communication.

Omitting the voice VLAN ID: Leaving out the voice VLAN ID on IP phone connection ports. Cisco phones frequently employ two VLANs on a single port (one for voice and one for PC traffic). If the voice VLAN is configured incorrectly or is absent, the phone may not function or register.

Missing VLANs on switches: A port may become dormant if a VLAN associated with it is removed.

Native VLAN Mismatch: When a switch’s interfaces have distinct native VLANs, untagged traffic may be diverted. Cisco Discovery Protocol errors occur when native VLAN mismatches are detected.

Switch access port issues: An access port may not be attached to a VLAN.

Trunking Issues

Permitted VLAN list mismatch: Both ends of switch-to-switch trunking links must allow the same VLANs. Isolated VLAN situations where some endpoints function but others do not can arise from a mismatch.

Trunks have a native VLAN mismatch: On both ends of the trunk, the native VLAN which manages frames without a VLAN ID should match and be constant throughout the network.

Encapsulation type mismatch: The encapsulation type (such as 802.1Q or ISL) must be the same on both sides when trunking between switches. Trunking between Cisco and non-Cisco switches requires the IEEE standard 802.1Q.

Dynamic Trunking Protocol (DTP) issues: DTP modes should be examined if a trunk isn’t developing since some combinations like two switches set up with dynamic auto won’t form a trunk.

Trunk misconfiguration: The trunk may not be in trunk mode or may not be carrying the necessary VLANs.

Forwarding Loops in a Switched Network

• Loops can happen even when Spanning Tree Protocol (STP) is used.
• By quickly forwarding Ethernet frames and using up interface bandwidth and switch CPUs, a loop causes network congestion and stops working.
• In order to isolate the loop, troubleshooting entails segmenting the network into smaller domains. To avoid loops, vendors provide technologies like BPDU Guard, Loop Guard, Root Guard, and Unidirectional Link Detection.

Unicast Flooding

• Rarely, a switch could “forget” where an endpoint is in a VLAN, flooding all VLAN ports with traffic intended for that MAC address.
• This may occur if the router IP address-to-MAC address cache of the VLAN is different from the switch’s MAC address-to-port cache timer.
• Slow end systems on the impacted VLAN and packet counters increasing at the same pace on every interface inside the VLAN are symptoms.
• Using vendor-specific features to restrict unknown unicast flooding or setting the MAC address-to-port timer marginally higher than the IP address-to-MAC timer are two possible solutions.

Layer 3 Routing Problems

External Layer 3 network access fails, although VLAN works.

Endpoint configuration errors: The improper subnet mask, IP address, or VLAN ID puts endpoints in the wrong subnet or VLAN.

Gateway configuration issues: The default gateway may lack a Switch Virtual Interface (SVI) or have the wrong router “up” or configured.

No inter-VLAN routing: Devices in separate VLANs cannot connect without a router or Layer 3 switch for inter-VLAN routing. Layer 3 gateways, Switch Virtual Interfaces, are needed for each VLAN to communicate.

Also Read About CISCO Switch Configuration: A Safe and Effective Setup Guide

List VLAN troubleshooting steps

• It is advised to use a divide-and-conquer strategy while troubleshooting networks. Important actions consist of:
• Examine the symptoms to ascertain whether the issue is with the application, data link, physical, or routed layer.

Verify VLAN Configuration on Switches

• Check VLAN Existence: Use show vlan or show vlan brief to verify the VLAN is generated and active on all switches.
• show interface [interface_id] switchport to verify the end device’s port’s VLAN assignment. Make sure it’s designated and accessible.

Inspect VLAN Trunking Configuration

• Show interfaces trunk to verify inter-switch linkages as trunks.
• Check Allowed VLANs: Make sure the trunk allows the problematic VLAN.
• Make that both endpoints’ native VLANs match.
• Check encapsulation consistency (e.g., 802.1Q).

Verify IP Addressing and Gateway

• Endpoint IP: Check the endpoint’s IP and subnet mask against its VLAN.
• Default Gateway: Check the device’s default gateway and Layer 3 interface (Router-on-a-stick or SVI) for its VLAN.

Examine Layer 3 Routing (Inter-VLAN Communication)

• SVIs: Set up a “Up/Up” Switch Virtual Interface (SVI) for every VLAN on Layer 3 switches. Confirm with display interfaces vlan (#).
• Routing Table: Show ip route to check all VLAN subnet routes.

Use Network Tools for Diagnosis

• Show mac address-table (Cisco) or show bridge mac-table (Juniper) to see which MAC addresses are on specific VLANs and interfaces.
• Display arp to check MAC address mappings.
• Ping: To check connection both inside and between VLANs, Ping IP addresses.
• To make sure switches are finding one another, use the Cisco Discovery Protocol (CDP)/Link Layer Discovery Protocol (LLDP) show cdp neighbours command.

Consider Other Factors

• Examine any ports that may be restricted by Spanning Tree Protocol (STP) that might be impeding the use of a link.
• Security Policies: Verify that no traffic between the VLANs is being blocked by access control lists (ACLs) or other security policies.
• Automation of Configuration Validation: This can identify possible issues without requiring modifications.

VLAN Troubleshooting Commands

To diagnose VLAN problems, a few show commands are necessary:

• show vlan or show vlan brief: shows every VLAN along with its corresponding interfaces and status (active, act/unsup).
• show interface [interface_id] switchport: Shows trunking encapsulation, negotiation status, administrative/operational mode, access VLAN, and native VLAN.
• show interfaces trunk: Summarises all trunk ports’ mode, encapsulation, status, native VLAN, and allowed/active VLANs. Comparing each switch’s allowed VLANs is easier.
• show interfaces vlan (#): Checks if a VLAN Switch Virtual Interface (SVI) exists, is up or down, has an IP address, and is a subnet.
• show running-config or show run interface: displays the current configuration, confirming port settings, IP phone voice VLAN setup, and other interface-specific parameters.
• show ip route: Verifies that all required VLAN subnets have routing entries, particularly for inter-VLAN connectivity.
• show mac address-table: Provides a list of MAC table entries along with the corresponding port and VLAN.
• show cdp neighbors: Used to verify switch discovery in trunking setups.

Network troubleshooting is a useful ability that is acquired with practice.

Also Read About Link Aggregation Control Protocol (LACP) & How LACP Works?