Overview

Enable DHCP Relay Agent on Cisco routers (Cisco IOS). This allows a single DHCP server to support DHCP clients on multiple networks.

Network Diagram

Figure: DHCP Relay Agent Configuration Example [Cisco] Network Diagram
Figure: DHCP Relay Agent Configuration Example [Cisco] Network Diagram

Configuration Condition

  • Create the required DHCP pools on DHCP-Server. DNS Server and Domain Name are common. DNS Server “8.8.8.8” Domain Name “n-study.com”.
  • CL1/CL2/CL3 IP addresses are automatically configured with DHCP.
  • Allows R1 to forward DHCP messages from DHCP clients to DHCP-Server.

Initical Configuration

This is an excerpt of the initial configuration for R1/DHCP-Server; only the hostname is configured for CL1/CL2/CL3.

R1 Configuration Excerpt(Click)

hostname R1
!
interface Ethernet0/0
 ip address 192.168.1.1 255.255.255.0
 ip helper-address 192.168.4.100
!
interface Ethernet0/1
 ip address 192.168.2.1 255.255.255.0
!
interface Ethernet0/2
 ip address 192.168.3.1 255.255.255.0
!
interface Ethernet0/3
 ip address 192.168.4.1 255.255.255.0

DHCP-Server Configuration Excerpt(Click)

hostname DHCP-Server
!
interface Ethernet0/0
 ip address 192.168.4.100 255.255.255.0
!
ip route 0.0.0.0 0.0.0.0 192.168.4.1

Configuration and Verification

Step1:DHCP Server Configuration

Create the following DHCP pools so that DHCP-Server can distribute IP addresses to DHCP clients on 192.168.1.0/24, 192.168.2.0/24 and 192.168.3.0/24.

Pool nameNetwork addressDefault gateway
192.168.1.0192.168.1.0/24192.168.1.1
192.168.2.0192.168.2.0/24192.168.2.1
192.168.3.0192.168.3.0/24192.168.3.1

In addition, a DHCP pool is created to distribute DNS server IP address and domain name.

Pool nameNetwork addressDNS ServerDomain name
DNS_DOMAIN192.168.0.0/168.8.8.8n-study.com

DHCP-Server DHCP pool configuration

ip dhcp pool 192.168.1.0
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.1
!
ip dhcp pool 192.168.2.0
 network 192.168.2.0 255.255.255.0
 default-router 192.168.2.1
!
ip dhcp pool 192.168.3.0
 network 192.168.3.0 255.255.255.0
 default-router 192.168.3.1
!
ip dhcp pool DNS_DOMAIN
 network 192.168.0.0 255.255.0.0
 domain-name n-study.com
 dns-server 8.8.8.8

Related article

The details of the commands for configuring a Cisco router as a DHCP server are described in the following article.

N-Study
Cisco IOS DHCP Server Configuration and Verification Commands | N-Study
https://www.n-study.com/en/tcp-ip/cisco-ios-dhcp-server-configuration/
You can use a Cisco router as a DHCP server to distribute IP addresses to DHCP clients.This section describes the configuration and verification commands to make a Cisco router (Cisco IOS) act as a DHCP server.

Step2:DHCP Client configuration

Ensure that the IP address of Ethernet0/0 on CL1/CL2/CL3 is automatically configured with DHCP.

CL1/CL2/CL3 DHCP Client

interface Ethernet0/0
 ip address dhcp

CL1/CL2/CL3 and DHCP-Server are not on the same network. Therefore, DHCP-Server cannot distribute TCP/IP configuration information to CL1/CL2/CL3.

Related article

The following article describes the configuration commands for obtaining a Cisco router’s IP address with DHCP.

N-Study
Cisco DHCP Client Configuration : “ip address dhcp” | N-Study
https://www.n-study.com/en/ip-addressing/cisco-dhcp-client/
This section describes the commands for configuring a Cisco router as a DHCP client.

Step3:R1 DHCP Relay Agent configuration

Configure R1 as a DHCP Relay Agent so that it can forward broadcast DHCP messages from CL1/CL2/CL3 to the DHCP-Server. On the interface to which CL1/CL2/CL3 are connected, use the ip helper-address command to specify the IP address of the forwarding DHCP-Server.

R1 DHCP Relay Agent configuration

interface Ethernet0/0
 ip helper-address 192.168.4.100
!
interface Ethernet0/1
 ip helper-address 192.168.4.100
!
interface Ethernet0/2
 ip helper-address 192.168.4.100

Related article

The following article describes the configuration commands for DHCP Relay Agent.

N-Study
DHCP Relay Agent | N-Study
https://www.n-study.com/en/tcp-ip/dhcp-relay-agent/
In principle, the DHCP server and DHCP clients must be connected on the same network. The DHCP Relay Agent is the function to avoid this restriction.This section explains how the DHCP Relay Agent works and the configuration commands on a C

Step4:DHCP Verification

Verify the IP addresses of CL1/CL2/CL3.

DHCP Verification Client

CL1#show ip interface brief | include DHCP
Ethernet0/0                192.168.1.3     YES DHCP   up                    up

CL2#show ip interface brief | include DHCP
Ethernet0/0                192.168.2.2     YES DHCP   up                    up

CL3#show ip interface brief | include DHCP
Ethernet0/0                192.168.3.2     YES DHCP   up                    up

You can see that CL1/CL2/CL3 are obtaining their IP addresses with DHCP.

Also, verify the IP address assigned to the client on the DHCP-Server.

DHCP Verification Server 

DHCP-Server#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address          Client-ID/              Lease expiration        Type
                    Hardware address/
                    User name
192.168.1.2         0063.6973.636f.2d61.    Nov 05 2022 04:49 AM    Automatic
                    6162.622e.6363.3030.
                    2e30.3230.302d.4574.
                    302f.30
192.168.2.2         0063.6973.636f.2d61.    Nov 05 2022 04:19 AM    Automatic
                    6162.622e.6363.3030.
                    2e30.3330.302d.4574.
                    302f.30
192.168.3.2         0063.6973.636f.2d61.    Nov 05 2022 04:19 AM    Automatic
                    6162.622e.6363.3030.
                    2e30.3430.302d.4574.
                    302f.30

Packet Capture

DHCP DISCOVER(CL1→R1)

DHCP DISCOVER(CL1→R1) Packet Capture Text(Click)

Frame 15: 334 bytes on wire (2672 bits), 334 bytes captured (2672 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00), Dst: Broadcast (ff:ff:ff:ff:ff:ff)
Internet Protocol Version 4, Src: 0.0.0.0, Dst: 255.255.255.255
User Datagram Protocol, Src Port: 68, Dst Port: 67
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
    Option: (57) Maximum DHCP Message Size
    Option: (61) Client identifier
    Option: (12) Host Name
    Option: (55) Parameter Request List
    Option: (255) End

DHCP DISCOVER(R1→DHCP-Server)

DHCP DISCOVER(R1→DHCP-Server) Packet Capture Text(Click)

Frame 17: 334 bytes on wire (2672 bits), 334 bytes captured (2672 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:01:30 (aa:bb:cc:00:01:30), Dst: aa:bb:cc:00:05:00 (aa:bb:cc:00:05:00)
Internet Protocol Version 4, Src: 192.168.1.1, Dst: 192.168.4.100
User Datagram Protocol, Src Port: 67, Dst Port: 67
Dynamic Host Configuration Protocol (Discover)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Discover)
    Option: (57) Maximum DHCP Message Size
    Option: (61) Client identifier
    Option: (12) Host Name
    Option: (55) Parameter Request List
    Option: (255) End

DHCP OFFER(DHCP-Server→R1)

DHCP OFFER(DHCP-Server→R1) Packet Capture Text(Click)

Frame 20: 342 bytes on wire (2736 bits), 342 bytes captured (2736 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:05:00 (aa:bb:cc:00:05:00), Dst: aa:bb:cc:00:01:30 (aa:bb:cc:00:01:30)
Internet Protocol Version 4, Src: 192.168.4.100, Dst: 192.168.1.1
User Datagram Protocol, Src Port: 67, Dst Port: 67
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 192.168.1.3
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (51) IP Address Lease Time
    Option: (58) Renewal Time Value
    Option: (59) Rebinding Time Value
    Option: (1) Subnet Mask (255.255.255.0)
    Option: (3) Router
    Option: (15) Domain Name
    Option: (6) Domain Name Server
    Option: (255) End
    Padding: 00

DHCP OFFER(R1→CL1)

DHCP OFFER(R1→CL1) Packet Capture Text(Click)

Frame 17: 342 bytes on wire (2736 bits), 342 bytes captured (2736 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:01:00 (aa:bb:cc:00:01:00), Dst: Broadcast (ff:ff:ff:ff:ff:ff)
Internet Protocol Version 4, Src: 192.168.1.1, Dst: 255.255.255.255
User Datagram Protocol, Src Port: 67, Dst Port: 68
Dynamic Host Configuration Protocol (Offer)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 192.168.1.3
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Offer)
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (51) IP Address Lease Time
    Option: (58) Renewal Time Value
    Option: (59) Rebinding Time Value
    Option: (1) Subnet Mask (255.255.255.0)
    Option: (3) Router
    Option: (15) Domain Name
    Option: (6) Domain Name Server
    Option: (255) End
    Padding: 00

DHCP REQUEST(CL1→R1)

DHCP REQUEST(CL1→R1) Packet Capture Text(Click)

Frame 18: 346 bytes on wire (2768 bits), 346 bytes captured (2768 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00), Dst: Broadcast (ff:ff:ff:ff:ff:ff)
Internet Protocol Version 4, Src: 0.0.0.0, Dst: 255.255.255.255
User Datagram Protocol, Src Port: 68, Dst Port: 67
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 0.0.0.0
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
    Option: (57) Maximum DHCP Message Size
    Option: (61) Client identifier
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (50) Requested IP Address (192.168.1.3)
    Option: (12) Host Name
    Option: (55) Parameter Request List
    Option: (255) End

DHCP REQUEST(R1→DHCP-Server)

DHCP REQUEST(R1→DHCP-Server) Packet Capture Text(Click)

Frame 21: 346 bytes on wire (2768 bits), 346 bytes captured (2768 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:01:30 (aa:bb:cc:00:01:30), Dst: aa:bb:cc:00:05:00 (aa:bb:cc:00:05:00)
Internet Protocol Version 4, Src: 192.168.1.1, Dst: 192.168.4.100
User Datagram Protocol, Src Port: 67, Dst Port: 67
Dynamic Host Configuration Protocol (Request)
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 1
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 0.0.0.0
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (Request)
    Option: (57) Maximum DHCP Message Size
    Option: (61) Client identifier
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (50) Requested IP Address (192.168.1.3)
    Option: (12) Host Name
    Option: (55) Parameter Request List
    Option: (255) End

DHCP ACK(DHCP-Server→R1)

DHCP ACK(DHCP-Server→R1) Packet Capture Text(Click)

Frame 22: 342 bytes on wire (2736 bits), 342 bytes captured (2736 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:05:00 (aa:bb:cc:00:05:00), Dst: aa:bb:cc:00:01:30 (aa:bb:cc:00:01:30)
Internet Protocol Version 4, Src: 192.168.4.100, Dst: 192.168.1.1
User Datagram Protocol, Src Port: 67, Dst Port: 67
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 192.168.1.3
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (51) IP Address Lease Time
    Option: (58) Renewal Time Value
    Option: (59) Rebinding Time Value
    Option: (1) Subnet Mask (255.255.255.0)
    Option: (3) Router
    Option: (15) Domain Name
    Option: (6) Domain Name Server
    Option: (255) End
    Padding: 00

DHCP ACK(R1→CL1)

DHCP ACK(R1→CL1) Packet Capture Text(Click)

Frame 19: 342 bytes on wire (2736 bits), 342 bytes captured (2736 bits) on interface -, id 0
Ethernet II, Src: aa:bb:cc:00:01:00 (aa:bb:cc:00:01:00), Dst: Broadcast (ff:ff:ff:ff:ff:ff)
Internet Protocol Version 4, Src: 192.168.1.1, Dst: 255.255.255.255
User Datagram Protocol, Src Port: 67, Dst Port: 68
Dynamic Host Configuration Protocol (ACK)
    Message type: Boot Reply (2)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0x00000b32
    Seconds elapsed: 0
    Bootp flags: 0x8000, Broadcast flag (Broadcast)
    Client IP address: 0.0.0.0
    Your (client) IP address: 192.168.1.3
    Next server IP address: 0.0.0.0
    Relay agent IP address: 192.168.1.1
    Client MAC address: aa:bb:cc:00:02:00 (aa:bb:cc:00:02:00)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type (ACK)
    Option: (54) DHCP Server Identifier (192.168.4.100)
    Option: (51) IP Address Lease Time
    Option: (58) Renewal Time Value
    Option: (59) Rebinding Time Value
    Option: (1) Subnet Mask (255.255.255.0)
    Option: (3) Router
    Option: (15) Domain Name
    Option: (6) Domain Name Server
    Option: (255) End
    Padding: 00

TCP/IP