(node-debug)=
# Debugging Network Devices

It's usually trivial to enable debugging on devices running in a virtual lab:

1. Log in to the affected devices
2. Enable debugging
3. Monitor debugging outputs in log files or an SSH session

However, you might want to enable specific debugging printouts every time you start a lab topology. That's pretty easy to do on most devices that allow you to execute regular commands (for example, the **debug** command) within the configuration mode:

1. Create a [custom configuration template](custom-config) that enables debugging, for example:

   ```
   do debug ipv6 routing
   do debug bgp ipv6 unicast import events
   do debug bgp ipv6 unicast import updates
   do debug bgp ipv6 unicast updates out
   ```

2. Add the name of the custom configuration template to a node **config** list.

```{tip}
This approach allows you to automate debugging in a [multi-vendor environment](custom-config-multivendor)
```

Unfortunately (for debugging purposes), _netlab_ executes the custom configuration templates after configuring the network devices, which means you might lose the interesting part of the debugging information: what happens when the control-plane protocols start.

You can enable debugging *before* configuring network devices with this simple trick:

1. Execute **netlab up --no-config**
2. If needed, execute **netlab initial --ready** to ensure the network devices are fully operational
3. Log into the network devices and enable debugging, or use a custom configuration template and execute it with **netlab config _debugging_template_** (this approach yet again gives you multi-vendor capabilities).
4. Start device configuration with **netlab initial**

```{warning}
Some devices (for example, Arista EOS) accept debugging commands only after the corresponding control-plane protocol has been configured, making the above approach a non-starter.
```

(node-debug-attribute)=
## Device-Specific Debugging Attributes

Finally, you might be worried that the symptoms you're experiencing depend on the time after the device boots, so you want to enable debugging as soon as possible. In that case, you can use the device-specific **debug** attribute (on [devices for which we implemented it](platform-initial-extra)), which is a list of debugging parameters that will be executed at the very beginning of the initial device configuration.

**Caveats:**

* The contents of the **_device_.debug** attribute are device-specific. *netlab* currently does not have multi-vendor **debug** capabilites.
* The values of the **_device_.debug** attribute are not checked. They must be relevant to the underlying network device, or you'll get configuration errors during the initial configuration
* The initial device configuration template supplies the mandatory prefix (for example, **do debug**). You only have to list the debugging conditions, for example:

```
nodes:
  dut:
    module: [ bgp, ospf, routing ]
    bgp.as: 65000
    device: iol
    ios.debug:
    - ip routing
    - bgp ipv4 unicast updates
    frr.debug:
    - bgp updates
```