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

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.

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

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.

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.

On this page

The DORA exchange (UDP 67/68)
DHCP roles on the wire
Broadcasts don't cross routers — DORA di…
DHCP relay (helper address)
IOS DHCP server

Handbook sections

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.