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DHCP Server Down (Existing Devices Work, New Ones Fail)

R
Written by Rohit Yadav

DHCP Server Down – Existing Devices Work but New Devices Fail

Environment: Restaurant Network

1. Purpose

Structured troubleshooting and resolution procedure for incidents where the DHCP (Dynamic Host Configuration Protocol) service is unavailable or malfunctioning, causing new devices to fail to obtain IP addresses while existing devices continue operating normally (Due to an existing lease allocation).

This SOP applies to restaurant environments using:

ISP modem/router providing DHCP
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2. Scope

This procedure applies to:

  • POS terminals

  • Back-office PCs

  • Guest Wi-Fi devices

  • Handheld ordering tablets

  • New network devices being added
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3. Background – How DHCP Works (DORA Process)

DHCP automatically assigns IP addresses and network configuration settings to devices when they connect to the network.

When a new device connects:

  1. The device broadcasts a DHCP Discover message.

  2. The DHCP server responds with an Offer.

  3. The device sends a Request.

  4. The DHCP server sends an Acknowledgment (ACK).

If the DHCP server does not respond, the device cannot obtain a valid IP address.

In most restaurant environments, DHCP is provided by: The ISP/PC61 modem/router
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4. Problem Description

This issue occurs when:

  • Existing POS terminals continue functioning.

  • Existing POS loses connectivity post reboot.

  • New POS terminals cannot connect.

  • Guest Wi-Fi devices show “Connected – No Internet.”

  • Devices receive IP address starting with 169.254.x.x (APIPA).

5. Business Impact

If DHCP remains down:

  • New POS installations fail.

  • Replacement devices cannot be configured.

  • Guest Wi-Fi becomes unusable.

  • Network expansion is blocked.

  • Operational delays occur.

Severity Level: Medium (High if replacing failed POS)
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6. Common Root Causes

  1. DHCP service stopped on PC61 or ISP modem router.

  2. DHCP scope exhausted (no IP addresses available in the pool).

  3. VLAN misconfiguration (if VLANs are used).

  4. Router DHCP service broken dur to firmware bug

  5. Network loop or broadcast storm interfering with DHCP

  6. Incorrect switch port configuration.

  7. A Rouge DHCP is causing incorrect IP and gateway allocation
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8. Detailed Troubleshooting Procedure

Step 1 – Confirm the Issue

On the affected device (Client), open Command Prompt and run:

  • ipconfig /all or Ifconfig (Linux)

Check for below details:

  • IP Address

  • DHCP Enabled

  • Default Gateway

  • DNS Servers

If IP shows 169.254.x.x, DHCP failure is confirmed.

Step 2 – Verify Existing Devices

Check at least one working POS or PC.

Run below command

  • ipconfig /all

If it shows a valid IP (e.g., 192.168.x.x), the network routing is functional.

This confirms DHCP service may be partially down or scope exhausted.

Step 3 – Test DHCP Renewal

On affected device, run:

  • ipconfig /release

  • ipconfig /renew

If renewal fails with: Unknown Network or you see APIPA range is allocated

Step 4 – Determine Where DHCP Is Hosted

Identify whether DHCP is provided by:

A. ISP Router
or
B. PC61 Router

Check router LAN Configurations (Vlan allocation and port type is it is access port or trunk)

Step 5 – Check for Switch Port or Cable issues

Ensure to check the below information:

  • Device connected to correct network port (connection between Router and Switch).

  • Port is not in wrong VLAN or is there is switch proper vlan tagging is configured.

  • Cable is functional.

Test with another working port or cable if needed.

Step 6 – Check for Broadcast Issues (Advanced)

If DHCP intermittently works:

  • Check for loops.

  • Verify no unmanaged switch added.

  • Check for any rogue dhcp device connected

Excess broadcast traffic can interfere with DHCP replies.

Step 7 – Check the DHCP pool status on Device page (current allocation)

We need to ensure that PC61 DHCP pool is not exhausted and there are available IP addresses

  • Go to Router page and look for DHCP column

  • Validate that DHCP IP pool is having available IP address

  • If pool is exhausted, ask admin to increase the IP dhcp pool range

Step 8 – Run the pcap on PC61 to capture Discover request

This step would help us to validate, if client DHCP discovery request is reaching to PC61 or not.

  • Validate if endpoint client (POS, Printer etc) sending discovery request

  • Run pcap and connect disconnect Wired cable or wireless clients to be able to capture the dhcp packets

  • Analyze the pcap using Wireshark
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Step 9 - Reboot/Reset Router

  • If everything so far looks good, try to reboot or rest the Pronto Router

9. Resolution Scenarios

Scenario A – IP Scope Exhausted (In case Pool is Exhausted)

Expand DHCP range.
Example:

  • Old range:
    192.168.1.100 – 192.168.1.150

  • New range:
    192.168.1.100 – 192.168.1.200

Scenario C – VLAN Misconfiguration

  • Ensure Router port is in correct VLAN.

  • Ensure VLAN allowed on uplink ports between Router and Switch along with Native Vlan

  • Ensure Switch port is allocated in correct vlan

Scenario D – Router Firmware Glitch (In case pcap shows discovery packets are coming from endpoint)

  • Reboot/reset the (PC61) router.

  • Escalate issue to Pronto Support
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10. Validation After Fix (End users check)

Confirm:

  • Affected device receives valid IP.

  • Default gateway assigned.

  • DNS servers assigned.

  • Internet access works.

  • POS processes test transaction.

Run:

  • ipconfig /all

  • Confirm no 169.254 address remains.

11. Preventive Measures

  • Monitor DHCP scope usage monthly.

  • Keep at least 20% free IP space.

  • Avoid connecting unmanaged switches without approval.

  • Maintain network diagram.
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12. Quick Diagnostic Summary

If new devices fail but old devices work:

  1. Check IP address (169.254?).

  2. Run ipconfig /renew.

  3. Verify DHCP service status.

  4. Check scope exhaustion.

  5. Validate new device connectivity.


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