Troubleshoot IPv4 and IPv6 DHCP (DHCP client, IOS DHCP server, DHCP relay, DHCP options)

📘CCNP Enterprise – ENARSI (300-410)

DHCP (Dynamic Host Configuration Protocol) is critical in IT networks. It automatically assigns IP addresses and other network settings (like default gateway, DNS) to devices. For Cisco exams, you need to understand how DHCP works, how to configure it on IOS devices, and how to troubleshoot it.

This section covers:

DHCP Client
IOS DHCP Server
DHCP Relay
DHCP Options
Troubleshooting Steps & Commands

1. DHCP Client

A DHCP client is any device that requests an IP address from a DHCP server.

Key Points:

Devices like PCs, servers, or routers can act as DHCP clients.
They send a DHCPDISCOVER message to find a DHCP server.
Server replies with DHCPOFFER, and then the client sends DHCPREQUEST.
Server finalizes with DHCPACK (acknowledgment) with IP info.

IPv4 vs IPv6:

IPv4: Traditional DHCP (DHCPv4) works as explained above.
IPv6: DHCPv6 is slightly different:
- Clients can use stateless (only get DNS) or stateful (get IP and other info) DHCPv6.
- Messages: SOLICIT → ADVERTISE → REQUEST → REPLY.

Common Issues:

Client does not get an IP → check interface status and DHCP server connectivity.
Wrong subnet → client may be connected to a VLAN without DHCP server support.
Lease issues → previous IP leases can cause conflicts.

Useful Commands (on Cisco IOS):

# Check IP address on client interface
show ip interface brief        # IPv4
show ipv6 interface brief      # IPv6

# Force client to request new DHCP IP
ip address dhcp
ipv6 address dhcp

2. IOS DHCP Server

A Cisco router or switch can act as a DHCP server.

Key Points:

Assigns IP addresses to clients automatically.
Can provide additional information via DHCP options (default gateway, DNS, domain name, TFTP, etc.).
Can manage IPv4 and IPv6 leases.

IPv4 Server Configuration Example:

# Step 1: Create a DHCP pool
ip dhcp pool LAN_POOL
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 8.8.8.8 8.8.4.4
 lease 7                      # 7-day lease

IPv6 Server Configuration Example:

ipv6 dhcp pool LAN_V6
 address prefix 2001:db8:1::/64
 dns-server 2001:4860:4860::8888

Common Issues:

DHCP pool exhausted → no IP left to assign.
Wrong subnet → clients get IP from wrong network.
Conflicting IPs → static IPs overlap with DHCP range.
Incorrect default gateway → clients cannot reach outside network.

Troubleshooting Commands:

show ip dhcp binding           # See IPv4 addresses leased
show ip dhcp pool              # Check pool utilization
show ipv6 dhcp binding         # IPv6 addresses leased

3. DHCP Relay

DHCP relay is used when clients and DHCP server are on different networks/VLANs.

How it Works:

Client sends DHCPDISCOVER → broadcast (cannot cross routers by default).
Router configured as DHCP relay agent forwards this to DHCP server (unicast).
Server replies → router forwards response back to client.

Configuration Example (IPv4):

interface GigabitEthernet0/1
 ip address 192.168.10.1 255.255.255.0
 ip helper-address 192.168.20.100  # DHCP server IP

Configuration Example (IPv6):

interface GigabitEthernet0/1
 ipv6 dhcp relay destination 2001:db8:1::1

Common Issues:

No ip helper-address → client cannot reach server.
Wrong IP in helper-address → relayed messages fail.
Relay traffic blocked by ACL → DHCP messages dropped.

Troubleshooting Commands:

show ip interface              # Verify helper-address
debug ip dhcp server events    # Monitor DHCP server activity
debug ip dhcp relay            # Track relay messages

4. DHCP Options

DHCP options provide additional network info to clients.

Examples:

Option 3 → Default gateway
Option 6 → DNS server
Option 15 → Domain name
Option 66 → TFTP server
Option 150 → Cisco TFTP server

IPv6 Options:

DNS server
Domain search list
SIP server

Troubleshooting Options:

Missing options → clients get IP but cannot reach network services.
Wrong option values → misconfigured gateway, DNS, or TFTP.

5. Troubleshooting DHCP Step-by-Step

Step 1: Verify Connectivity

Ping server from relay/router.
Ensure interface is up.

Step 2: Verify Client Configuration

ipconfig /all on PC
show ip interface brief on router

Step 3: Verify Server Configuration

show ip dhcp pool
Check lease availability
Ensure correct network/subnet

Step 4: Verify Relay (if used)

show ip interface → verify helper-address
debug ip dhcp relay

Step 5: Check Logs

Enable debug on server for detailed info:

debug ip dhcp server events
debug ip dhcp server packet

Step 6: Solve Common Problems

Problem	Likely Cause	Fix
Client no IP	Server down, helper missing, VLAN mismatch	Check server & relay
Wrong IP	Pool misconfigured	Correct DHCP pool
No DNS / gateway	DHCP options missing	Configure proper options
IP conflicts	Static IP overlap or wrong lease	Adjust pool or static IPs

Exam Tips

Remember DHCP sequence (Discover → Offer → Request → ACK) for IPv4 and IPv6.
Know key commands (show ip dhcp binding, show ip dhcp pool, debug ip dhcp server).
Understand relay configuration and why it is needed.
Recognize common mistakes: pool exhaustion, wrong helper address, wrong options.
Be able to troubleshoot both IPv4 and IPv6 DHCP scenarios.