Dot1Q obscuring underlying CiscoDiscovery/CiscoDiscoveryInfo

I hope this is the right place to pose my question; I searched for a forum for asking How Do I questions, but couldn't seem to find one.

I'm currently using `gopacket` to capture network discovery packets. I've been able to successfully collect `LLDP` information across our environments, but `CDP` has proven to be a bit trickier. In essence what I've tried is the following:

- verify packet existence using `tcpdump` (with filter `ether[20:2] == 0x2000`)
- make a packet source using `gopacket/pcap` and a BPF (identical to what is used in my `tcpdump`tests)
- capture a single packet
- parse the appropriate struct as:

```
cdpL := p.Layer(layers.LayerTypeCiscoDiscoveryInfo)
if cdpL == nil {
return r, errors.New("nil CDP Info layer")
}
cdi, ok := cdpL.(*layers.CiscoDiscoveryInfo)
if !ok {
return r, errors.New("cannot parse CDP Info layer")
}
if cdi == nil {
return r, errors.New("no packets found")
}
```

Using packet captures from `tcpdump` in both the working and failing environments I created some unit tests that use this `testdata` to verify behavior. After digging around a bit I noticed the main difference in the packets captured in these environments is that the broken one has a `Dot1Q` layer, as:

```
Ethernet > Dot1Q
```

vs the working environment which has:

```
Ethernet > LLC > SNAP > CiscoDiscovery > CiscoDiscoveryInfo
```

My question is this: upon visual inspection of the bytes in the top-most layer payload in each case does eventually have something that looks like a `CDP` packet in it. My confusion stems from a few things:

- What's the right approach to get past the `Dot1Q` layer? I've tried searching to see if I could pass in a sub-slice into some decode method, but I failed to find such a thing. Am I missing something obvious? I feel like if I do the parsing myself I've jettisoned 50% of the utility of this package, but that is the next thing I'm going to try.
- `tcpdump` doesn't seem to be confused by the `Dot1Q` layer; do I have something misconfigured?

Sadly I don't feel comfortable sharing any more of the code I've written, but am happy to answer questions to get to the bottom of this.

Thanks for any pointers here.