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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.

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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?

Using 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).

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  • Thanks so much for this answer, I'll try doing some digging. Don't want to upset anyone by opening the network for discovery.
    – EnduranceMan
    Mar 15, 2016 at 14:10
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If you enable LLDP on the devices which support it, you basically use it the same way as CDP on Cisco devices. How it is enabled or used on other devices is dependent on the device make/model/software version.

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  • What about in cases where LLDP isn't available as well?
    – EnduranceMan
    Mar 14, 2016 at 19:54
  • Then you are down to tracing cables.
    – Ron Maupin ♦
    Mar 14, 2016 at 19:56
  • So I had heard SNMP could be used to find neighbors as well, is that incorrect?
    – EnduranceMan
    Mar 14, 2016 at 19:58
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    It's nominally correct. If I was doing this, I'd use SNMP to query mac tables for the switch so I can see the L2 information and query the L3 device to which the neighbor is connected to line up IP to MAC.
    – vigilem
    Mar 14, 2016 at 20:01
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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.

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