DHCP is a network service that automatically assigns IP addresses and other IP parameters to clients using a central server, with a relay agent across subnets if needed.
Facts
| Topic | Fact |
|---|---|
| Transport | UDP |
| Server port | UDP 67 |
| Client port | UDP 68 |
| IPv4 process | DORA (Discover, Offer, Request, Acknowledge) |
| Broadcast | The client always starts with broadcast |
| Lease | Time-limited IP assignment |
| Relay | Used when the server is in another subnet |
| Cisco relay command | ip helper-address |
| CCNP focus | Troubleshooting, relay, options, binding table |
DHCP roles
- Client: a device missing IP config, knows no server, starts with broadcast, stores the IP as a lease (PCs, IP phones, printers, WAN interfaces)
- Server: holds IP pools (scopes); hands out IP, mask, gateway, DNS, lease time; tracks MAC↔IP bindings (IOS router, Windows, ISC DHCP, firewalls)
- Relay agent: a router/L3 switch that receives the client broadcast, converts broadcast → unicast, forwards to the server and inserts GIADDR (gateway IP) – critical in multi-VLAN enterprise networks
DORA (IPv4)
- Discover: client → broadcast – “is there a DHCP server?”
- Offer: server → client – “here is an IP, want it?”
- Request: client → broadcast – “I want THIS IP”
- Acknowledge: server → client – “the IP is yours, lease starts now”
Lease renewal (CCNP)
| Time | Action |
|---|---|
| 50 % | Unicast Request to the server |
| 87.5 % | Broadcast Request |
| Expired | New DORA process |
Problem & solution
- Without DHCP: manual configuration, subnet/gateway errors, IP conflicts, slow administration
- With DHCP: central control, automatic assignment, no conflicts, fast onboarding
Special considerations
- DHCP uses broadcast → stopped at routers, needs relay on L3 devices
- Depends on VLAN design and IP addressing plan
- Security features: DHCP snooping, IP source guard, DAI
DHCP relay in depth (CCNP)
Client sends Discover (broadcast) → router receives → router unicasts to the server → server replies to the relay → relay forwards to the client. ip helper-address also forwards: TFTP (69), DNS (53), TACACS (49), NetBIOS and more.
Configuration
! DHCP server (router) – full pool
ip dhcp excluded-address 192.168.10.1 192.168.10.20
ip dhcp pool VLAN10
network 192.168.10.0 255.255.255.0
default-router 192.168.10.1
dns-server 8.8.8.8 1.1.1.1
domain-name firma.local
lease 7
! network = subnet matched against GIADDR/interface
! default-router = option 3, dns-server = option 6, lease in days
! Multiple pools – router picks by subnet / GIADDR
ip dhcp excluded-address 192.168.20.1 192.168.20.10
ip dhcp pool VLAN20
network 192.168.20.0 255.255.255.0
default-router 192.168.20.1
dns-server 8.8.4.4
! Relay (required with multiple VLANs) – inserts GIADDR so the server picks the right pool
interface vlan 10
ip address 192.168.10.1 255.255.255.0
ip helper-address 192.168.100.10
! Router as client (WAN/ISP)
interface g0/0
ip address dhcp
no shutdown
! Option 150 for IP phones (CCNP)
ip dhcp pool VOICE
network 192.168.30.0 255.255.255.0
default-router 192.168.30.1
option 150 ip 192.168.100.5DHCP options (CCNP)
| Option | Name | Function | Typical use |
|---|---|---|---|
| 43 | Vendor-specific information | Vendor data | AP / WLC discovery |
| 60 | Vendor class identifier | Identifies client type | IP phones / APs |
| 66 | TFTP server name | TFTP server (hostname/IP) | PXE / IP phones |
| 67 | Bootfile name | Boot file | PXE boot |
| 82 | Relay agent information | Inserted by the relay | Security / tracking |
| 150 | TFTP server address | Cisco IP phone TFTP | Voice VLAN (preferred over 66) |
Verification & troubleshooting
show ip dhcp binding
show ip dhcp pool
show ip dhcp server statistics
show running-config | section dhcp
debug ip dhcp server events
debug ip dhcp relayTypical faults: missing ip helper-address, wrong VLAN/interface, pool not matching the subnet, DHCP snooping blocking offers. In ACLs permit DHCP with:
permit udp any any eq bootpc
permit udp any any eq bootps