So I've been tasked with creating a topology map of the network, but on some devices I find that CDP is either not an option, or has been explicitly disabled. How can I find neighbors in these situations? I know I can use LLDP or SNMP, but I'm not sure HOW to use those commands to find neighbors.
LLDP works just like CDP. So instead of
show cdp neig you use
show lldp neig. But chances are good that LLDP is disabled as well.
Without CDP, LLDP or physical access (following the cables) you'll have rely on the device configuration. Temporarily enabling one or both protocols is out of the option?
sh mac-address-table and
show arp you are able to see the device manufacture connected to a port. This might help to map your network but will take a lot of time.
You can to all those things either via the command line or using SNMP (best for writing scripts to do the discovery for you).
If CDP is disabled then mostly likely LLDP might be disabled too.
BTW, both of them are used to discover the L2 network. And the only viable option is to discover the L2 network is via ARP. ARP tables can be polled via SNMP. (In case you want to discover/map the L3 network, then you can do that by reading routing tables of your routers, starting from the seed router.)
Are you using any monitoring tool? They have functions to discover & map networks.