
To troubleshoot [QinQ tunnel](/vlans-8021q-tunneling-qinq) issues, verify the tunnel state using the following commands:

- `SW1#show dot1q-tunnel`
- `SW1#show interfaces trunk`

These commands help check the configuration and status of the [trunks](/switchport-trunk) and the QinQ tunnels. Additionally, perform a [Wireshark](/wireshark) capture to analyze packet size and [VLAN](/vlan) IDs in the [Ethernet](/ethernet) [frames](/ethernet-frame-types) to ensure they match expected results.

## Links

https://networklessons.com/switching/802-1q-tunneling-q-q-configuration-example/

VLAN - QinQ troubleshooting

Syslog - terminal monitor

Syslog relay or proxy

Syslog severity levels

Syslog

TCP - Congestion control

TCP - Determining the MSS

TCP - Header

TCP - SEQ and ACK values

TCP - Slow start

TCP - Three-way handshake

TCP - Urgent pointer field

TCP - Window Size Scaling

TCP - factors affecting segment size

TCP - role of checksum field

TCP IP Model

TCP MSS and window size

TCP RST flag

TCP SYN timeout

TCP header options

TCP phantom byte

TFTP

Tcl Shell

Telnet

Time to live

Time-based Access List (ACL) on Cisco IOS XR

Traceroute - Interpreting output

Traceroute asterisk and !H responses

Traditional Layer 2 Issues in VXLAN Networks

Transparent Cisco IOS Firewall use cases

Transport Layer port number

Troubleshooting - using Telnet to test transport layer ports

Troubleshooting Route Flapping on Cisco IOS

Troubleshooting a slow network

Troubleshooting high CPU and memory usage on a switch

UDP - Header

UDP - Maximum Datagram Size

Unicast Reverse Path Forwarding (uRPF)

Unicast traffic

Unknown unicast traffic

Upgrade your R&S skills to modern networking

VACL - Changes to ACL are active immediately

VACL - use cases

VACL vs ACL

VLAN - Extended Range

VLAN - Native VLAN best practices

VLAN - Native VLAN on a router on a stick

VLAN - Private VLAN

VLAN - VLAN IDs 0 and 4095

VLAN Access Lists

VLAN Mapping

VLAN SVI status

VLAN Tag

VLAN Trunk interfaces missing in show vlan output

VLAN control frames

VLAN

VLANs - 802.1Q tunneling (QinQ)

VLANs - Dynamic VLAN membership (VMPS)

VLANs - QinQ and CDP

VLANs - Vlans allowed and active in management domain

VLANs - when a tagged frame arrives on an access port

VLANs SVI

VMware Configuration Maximums

VPN - IKEv2 peer address of 0.0.0.0 0.0.0.0

VPN - Interesting Traffic

VPN - NAT Exemption

VPN - crypto keepalive

VPN - default gateway for site to site VPN

VPN - default gateway of VPN client

VPN - split tunneling

VPN DVTI tunnel source

VRF - Definition mode

VRF - VPNv4 address

VRF Communication and Route Target Configuration

VRF and L2TPv3 Configuration and Interaction

VRF capability vrf-lite command

VRF lite route leaking

VRF sharing across multiple routers

VRF use cases

VRRP - Virtual IP doesn't respond to pings

VRRP - load balancing

VRRP - support for authentication

VRRP - using the same ID on different interfaces

VRRP - using the same ID on subinterfaces

VRRP and sub-optimal routing

VRRP on sub-interfaces

VTP - Client can't modify VLANs

VTP - Code Field

VTP - Multiple Servers with VTP version 2

VTP - Per interface configuration

VTP - Pruning the Extended VLAN ID Range

VTP - Revision Numbers on transparent switch

VTP - Versions

VTP - domain name automatically changes

VTP - protecting switches from lower revision numbers

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