I have inherited a data centre with very little documentation. The network is split up into multiple vlans (and subnets which don't tally with the vlans). I have a lot of physical devices, some which have labels, but the lables do not resolve to DNS names / addresses.
If I could get an IP address as a minimum that would be a start - but ideally I'd want to see what services are provided/consumed.
In principle I suppose it should be possible to configure a computer (linux?) with 2 interfaces as a bridge, quickly swap around the rj-45s and tap the traffic going across it, but I don't know how to resolve the the direction of the traffic from a packet trace. And that's assuming I can find a laptop with 2 ethernet ports.
I suspect that the switches may have bridging ports but they are buried under a pile of spaghetti.
Any suggestions?
Discovery at the link layer
If you have multiple VLANs then you must have a switch that has created these VLANs and mapped them to ports.
a) If you can manage the switches (and this is massively preferred) then you can work out which ports are in which VLAN and, very likely, use the features of the switch to tap into each VLAN to a mirror port. If you put a wireshark** capture device on this port then everything going on on the network will become apparent. Flow direction is not very important to cataloguing the network but the source and destination address is clearly apparent in wireshark.
If you can't tap into the network using the switch then at least you will know which ports are in which VLAN and you could put a capture device in to each VLAN in turn. If you capture ARP only then you will soon learn the MAC addresses and IPs of every device on the VLAN. This should be very helpful and will also point to routing devices (which must exist).
Remember that MAC addresses give a big clue to the vendor and type of device.
b) If the switches are really inaccessible to you then you will need to place your capture device into literally every port in turn and run the ARP capture for a few minutes. Because ARP will not route out of a VLAN you will be able to deduce which groups of ports are in the same VLAN and this should be helpful.
Inventory
Once you have an idea about the subnets in each VLAN and which ones can route to each other, then you can carry out an inventory audit. I have found that Spiceworks on Windows will do an excellent job of identifying and fingerprinting a wide range of devices without credentials. You will need to feed it a list of subnets that it can access, though, and if some are not routed together you may have to move the Spiceworks server around between VLANs.
** Wireshark capture device
You can build a simple Linux laptop with one interface and install wireshark. Learn how to create capture filters to select only the traffic you want. Only capture the first 70 bytes or so. There are numerous tutorials eg Lifewire.