How to troubleshoot DHCP address leasing issues

[jdryan]

In an network, issues that are seen generally  
>  End devices don’t or are unable to get an IP.
>  End devices get an IP but are not able to reach out to internet.

Here, will be looking into the first one : End devices don’t or are unable to get an IP.

When this is seen, primarily check if the client/end device in picture is the only one not getting the IP or if there are other clients.
If there is only one client not getting an IP, then check on the client end to understand if there is a pre-requisite for the client to connect to the network like:
• Client needs to go through authentication in order to connect [ Color-less/dynamic network port set up ]
• Client needs to have the specific port settings for itself on the switch port for connection [ fully managed network ]
• Client is missing the necessary setting on itself for the connection.

If all the above are in place and the IP is still not being received and there are also lot of clients failing, then :
• Check if there are connected clients :
○ And if client(s) connecting from same or specific location are not getting an IP
○ On the server, if the pool is running dry [rare but can happen]

If the clients not getting IP, are from the same location or a specific location in the network :
• Check if the client is connecting to the right network. On the port check if the vlan assignment is correct and is associated correctly.
   ○ Like for PCs : its usually untagged or access vlan set up.
   ○ Switches and APs can have multiple vlans passing through.
   ○ Server usually would have one vlan associated to them, and the same would most of the time be untagged, this is in case of a server that’s not running any VMs
   ○ If its running VMs, then based on the Networks associated on VDS/VSS [ in case of ESXi ] confirm the association is right.

• DHCP, when the process starts, is an L2 protocol, so the request for IP is broadcasted. Once the right vlan association is confirmed on the end user port, would need to check if the vlan is allowed till core of the network correctly for the request to propagate and reach the Server.
   ○ This can also be tested by assigning an IP statically to the PC, as we know which network it’s a part of, and check for connectivity to the network via pings.
   ○ If the above step works, and post that IP is still not leased : then we would need to check on the server
   § To know if there is a bad reservation
   § If the scope is out of addresses
   § If the network has a scope mapped to it . etc.

End devices get an IP but are not able to reach out to internet and anywhere on the network:
Refer here to check network connection issues