Serve DHCP to a local and a remote subnet from one IOS router

Tasks

1. Address the interfaces and give R1 a route back to LAN B and R2 a default route to R1.
2. Exclude .1 to .10 in both LANs from DHCP.
3. Create one pool per LAN on R1 with gateway, DNS server, domain name and a 2-day lease.
4. Make R2 relay DHCP from LAN B to R1.
5. Prove that a client on each LAN gets the right address and options, and that LAN B's requests arrive through the relay.

Solution

1. 1Interfaces and routing on R1R1

   interface GigabitEthernet0/0/0
    ip address 10.0.12.1 255.255.255.252
    no shutdown
   interface GigabitEthernet0/0/1
    ip address 10.10.0.1 255.255.255.0
    no shutdown
   ip route 10.20.0.0 255.255.255.0 10.0.12.2

   Without the route to 10.20.0.0/24, R1 builds the OFFER for LAN B and cannot deliver it to the relay address.

2. 2Exclusions and pools on R1R1

   ip dhcp excluded-address 10.10.0.1 10.10.0.10
   ip dhcp excluded-address 10.20.0.1 10.20.0.10
   !
   ip dhcp pool LAN-A
    network 10.10.0.0 255.255.255.0
    default-router 10.10.0.1
    dns-server 10.0.0.53
    domain-name corp.example.com
    lease 2
   !
   ip dhcp pool LAN-B
    network 10.20.0.0 255.255.255.0
    default-router 10.20.0.1
    dns-server 10.0.0.53
    domain-name corp.example.com
    lease 2

   R1 picks the pool by matching the subnet of the receiving interface for local clients, and the giaddr written by the relay for remote clients. lease 2 is 2 days; the default is 1 day.

3. 3Relay on R2R2

   interface GigabitEthernet0/0/0
    ip address 10.0.12.2 255.255.255.252
    no shutdown
   interface GigabitEthernet0/0/1
    ip address 10.20.0.1 255.255.255.0
    ip helper-address 10.0.12.1
    no shutdown
   ip route 0.0.0.0 0.0.0.0 10.0.12.1

   The helper goes on the interface facing the clients. R2 writes 10.20.0.1 into giaddr, which is how R1 knows to use pool LAN-B.

Verify

R2# show ip interface GigabitEthernet0/0/1 | include Helper
Helper address is 10.0.12.1

R1# debug ip dhcp server packet
DHCPD: DHCPDISCOVER received from client 0100.5079.6668.14 through relay 10.20.0.1.
DHCPD: Sending DHCPOFFER to client 0100.5079.6668.14 (10.20.0.11).
DHCPD: unicasting BOOTREPLY for client 0050.7966.6814 to relay 10.20.0.1.
DHCPD: DHCPREQUEST received from client 0100.5079.6668.14.
DHCPD: Sending DHCPACK to client 0100.5079.6668.14 (10.20.0.11).
DHCPD: unicasting BOOTREPLY for client 0050.7966.6814 to relay 10.20.0.1.

R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address      Client-ID/              Lease expiration        Type       State      Interface
                Hardware address/
                User name
10.10.0.11      0100.5079.6668.0a       Nov 12 2026 09:31 AM    Automatic  Active     GigabitEthernet0/0/1
10.20.0.11      0100.5079.6668.14       Nov 12 2026 09:33 AM    Automatic  Active     GigabitEthernet0/0/0

PC20# ipconfig /all
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IPv4 Address. . . . . . . . . . . : 10.20.0.11(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : Tuesday, November 10, 2026 9:33:12 AM
Lease Expires . . . . . . . . . . : Thursday, November 12, 2026 9:33:12 AM
Default Gateway . . . . . . . . . : 10.20.0.1
DHCP Server . . . . . . . . . . . : 10.0.12.1
DNS Servers . . . . . . . . . . . : 10.0.0.53

Mistakes people make

• Putting ip helper-address on R2's link toward R1 instead of on the LAN B interface.
• Forgetting R1's route back to 10.20.0.0/24. DISCOVERs arrive, OFFERs go nowhere.
• Excluding addresses after clients already got them. Exclusions only stop future leases; clear the bindings or wait.
• Expecting ip dhcp excluded-address inside the pool. It is a global command.
• Leaving debug ip dhcp server packet running on a busy server.

On other platforms

feature dhcp
feature interface-vlan
ip dhcp relay
interface Vlan20
  no shutdown
  ip address 10.20.0.1/24
  ip dhcp relay address 10.0.12.1

{
  "Dhcp4": {
    "interfaces-config": { "interfaces": [ "eth0" ] },
    "valid-lifetime": 172800,
    "subnet4": [
      {
        "id": 20,
        "subnet": "10.20.0.0/24",
        "pools": [ { "pool": "10.20.0.11 - 10.20.0.254" } ],
        "option-data": [
          { "name": "routers", "data": "10.20.0.1" },
          { "name": "domain-name-servers", "data": "10.0.0.53" },
          { "name": "domain-name", "data": "corp.example.com" }
        ]
      }
    ]
  }
}