CFWorx

How DHCP works

· in Cisco U

DHCP hands out IPv4 configuration automatically: address, mask, gateway, DNS, and more. Manually addressing every host stops being feasible fast.

It’s a client-server model, and nearly anything on a TCP/IP network can be a client: PCs, IP phones, printers, even Blu-Ray players. The intro and DORA summary live in common ports.

Allocation types

The lease timer counts down from the ACK on the server. Clients renew before expiry; they don’t normally wait to lose the address.

DORA at packet level

The Offer fills yiaddr (your IP address) with the proposed address and chaddr (client hardware address) with the client’s MAC, so the client knows it’s the target.

The Request is still broadcast and still sourced from 0.0.0.0. With several offers on the table, the broadcast plus the Server Identifier option tells every server who won and who was declined.

After the ACK the client enters the bound state and the address is live.

DHCP relay (ip helper-address)

The problem: Discover is a broadcast, and routers kill broadcasts. A server on another subnet never hears the client.

The fix is a relay agent. On the router interface that receives the client broadcasts:

console
Router1(config)# interface GigabitEthernet0/1
Router1(config-if)# ip helper-address 10.0.0.1

The router converts the broadcast into a unicast to the server and forwards the reply back. The command goes on the client-facing interface, not the WAN side.

The relay sources the forwarded packet from its own client-facing interface address, which is how the server knows which subnet’s pool to draw from (the “magic” is the relay stamping the client’s subnet into the packet, the giaddr field). It also inserts option 82 (remote ID + circuit ID) toward the server and strips it from the reply.

helper-address actually forwards several UDP broadcast types, BOOTP/DHCP among them.

Router as DHCP server

Useful at branch offices: it keeps DHCP chatter off the WAN instead of shouting across low-bandwidth links to a central server. Works on IOS routers and Catalyst switches.

console
Router(config)# ip dhcp excluded-address 10.1.50.1 10.1.50.50
Router(config)# ip dhcp pool Customer
Router(dhcp-config)# network 10.1.50.0 /24
Router(dhcp-config)# default-router 10.1.50.1
Router(dhcp-config)# dns-server 10.1.50.1
Router(dhcp-config)# domain-name cisco.com
Router(dhcp-config)# lease 0 12

Pool names are case sensitive. network takes the subnet, not a host address, and the default gateway is usually an address on this router.

excluded-address (global config, note: not in the pool) carves out the range you’re keeping for statics, here .1 through .50.

The lease syntax is lease days [hours] [minutes], so 0 12 means 12 hours. Default is one day; infinite is allowed.

Verify with show ip dhcp pool for pool state and show ip dhcp binding for the IP-to-MAC lease table.

Router as DHCP client

Covered in common ports: ip address dhcp on the interface. One addition from this lesson: if the server hands over a default gateway option, the router injects a default route pointing at it. Verify with show ip interface brief, where the Method column shows DHCP.

Windows client-side commands

bash
ipconfig /all         # full config incl DHCP server and DNS, per adapter
ipconfig /renew       # re-run DHCP (optionally per adapter)
ipconfig /release     # send DHCPRELEASE, drop the address
ipconfig /displaydns  # show the host's DNS cache
ipconfig /flushdns    # clear the DNS cache (stale IP for a name)
ipconfig /?           # help

The typical dance after moving networks: /release, then /renew.

Security concerns

DHCP has zero authentication built in, so it’s a soft target.