CCNA 200-301 v1.1 reference — IP Services

DHCP

The DORA exchange, lease lifecycle, IOS DHCP server pools, client addressing and the relay agent that forwards broadcasts across subnets.

Objective 4.3Objective 4.6v1.1 exam target

Quick Reference Handbook

DORA and the lease

The DORA exchange (UDP 67/68)

Discover — client broadcasts to find servers.
Offer — servers offer an address, mask, gateway, DNS, lease time.
Request — client asks for the offered address.
Acknowledge — server confirms; lease begins.

A lease is a loan: the client renews (unicast, at ~50% of the lease) or rebinds (broadcast, at ~87.5%) before expiry. 169.254.x.x on a client means DORA completed zero offers — DHCP is unreachable or exhausted.

DHCP roles on the wire

Role	Does	Port
Client	Broadcasts Discover, accepts a lease	UDP 68
Server	Offers and acknowledges leases	UDP 67
Relay agent	Forwards broadcasts across subnets as unicast, sets giaddr	Gateway interface

IOS DHCP server

Serve the LAN subnet from the router — Privileged EXEC → Global configuration

Cisco IOS / IOS XE

configure terminal
ip dhcp excluded-address 192.168.10.1 192.168.10.20
ip dhcp pool LAN
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 192.168.10.53
 lease 7
 end

Verify

show ip dhcp binding
show ip dhcp pool
show ip dhcp conflict

Bindings appear as clients lease addresses.
Conflicts list indicates the pool is handing out contested addresses.

Changes device configuration.

Client and relay

Broadcasts don't cross routers — DORA dies at the subnet boundary. The relay agent (ip helper-address) converts the broadcast into a unicast to the server, filling giaddr with the receiving interface's address so the server picks the right scope. Clients think they talked to a local server.

DHCP relay (helper address)

Forward DHCP broadcasts from the LAN to a central server — Privileged EXEC → Interface configuration

Cisco IOS / IOS XE

The DHCP server already hosts a scope for this subnet.

configure terminal
interface gigabitethernet0/2
 ip helper-address 192.168.50.10
 end

Verify

show ip interface gi0/2
show ip interface brief

Helper addresses listed on the interface.
The server sees a unicast REQUEST relayed with giaddr set — this is how it picks the right scope.

ip helper-address forwards several UDP services by default — the scope can be limited with ip forward-protocol.
The server needs a route back to the relay's interface address.

Changes device configuration.

DHCP client on a router interface

Get an address from DHCP on the WAN interface — Privileged EXEC → Interface configuration

Cisco IOS / IOS XE

configure terminal
interface gigabitethernet0/1
 ip address dhcp
 end

Verify

show ip interface brief
show dhcp lease

The interface shows 'DHCP' as its address method with a leased address.

Changes device configuration.

Where the helper lives

Topology: Clients VLAN 10 (192.168.10.0/24), DHCP server 192.168.50.10

Client broadcasts Discover on VLAN 10.
The L3 gateway interface (SVI/router port on that VLAN) is where the helper goes — that's the device hearing the broadcast.
Relayed unicast reaches the server; server answers via giaddr → offer returns through the same gateway.

Put the helper on the interface that receives the client's broadcast — the gateway, not the server side.

Verification

Common mistakes

Putting ip helper-address on the server's side — the helper belongs on the client's subnet interface.
Forgetting ip dhcp excluded-address — the router hands out its own gateway IP.
Expecting broadcasts to route — without the relay, remote servers never see the Discover.
Treating APIPA as a valid lease — it's the universal DHCP-failed marker.

Clients on one VLAN get no lease

Observation: VLAN 20 clients get APIPA; VLAN 10 works on the same server.

Hypotheses

No helper on VLAN 20's gateway interface
No scope for VLAN 20's subnet on the server
Server has no route back to VLAN 20's gateway

Check	Indicates
`show run interface` for VLAN 20	Helper present?
Server scopes	A missing scope explains zero offers even with relay working.
Server → giaddr route	Offer can't return if the server can't reach the relay's gateway IP.

Correction: Add the helper/scope/route as indicated.

VLAN 20 clients lease addresses with correct gateway and DNS.

On this page

DORA and the lease
IOS DHCP server
Client and relay
Verification

Related resources

Practise this topic Source topic 39: DHCP (Dynamic Host Configuration Protocol)Practise topic 39 deck

Sources

Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
RFC 2131 — Dynamic Host Configuration Protocol — reviewed 2026-11-02
Cisco — IP Addressing Services Command Reference — reviewed 2026-11-02

Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.