Learn how DHCP clients discover servers, choose an address offer, request configuration, and renew a lease. You will also connect the protocol exchange to address pools and basic Cisco IOS XE allocation behavior.
What you will be able to do
- Describe the roles of DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, and DHCPACK in obtaining a network address.
- Explain how a client selects an offer and informs servers that were not selected.
- Distinguish DHCPREQUEST behavior during initial selection from behavior during lease renewal.
- Interpret the addressing details and broadcast choices used in common DHCP messages.
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1. What DHCP accomplishes
DHCP coordinates the assignment of a network address between a client and a server. Instead of choosing an address alone, the client participates in a short message exchange that identifies an offer and confirms the choice.
The exchange is useful because the server can select an address from its available configuration, while the client can compare the parameters offered by one or more servers. The result is a client-server decision rather than an unannounced address change.
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2. Starting with discovery
A client begins its search with DHCPDISCOVER. After receiving that message, a DHCP server chooses a network address for the requesting client and sends DHCPOFFER with configuration parameters.
Think of DHCPOFFER as a proposal, not the client’s final acceptance. The server has identified an address it can offer, but the client still needs to evaluate the available proposals.
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3. Choosing among offers
The client selects one server by comparing the configuration parameters in the DHCPOFFER messages it receives. Its decision is therefore based on the offered configuration, rather than simply on which response arrived first.
The client announces its decision with DHCPREQUEST. That message identifies the selected server, allowing the chosen server to continue while other servers learn that their proposals were declined.
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4. The initial selection message
During the SELECTING stage, DHCPREQUEST carries two important choices: the selected server identifier and the offered address placed in the requested IP address field.
The client IP address is zero in this form of DHCPREQUEST. This separates a request for a newly selected offer from a message sent by a client that already has an address.
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5. Reaching the server
A client normally broadcasts DHCPDISCOVER, DHCPREQUEST, and DHCPINFORM. Broadcasting lets the client communicate when it does not yet know which DHCP server is available.
There is one important contrast. If the client knows a server’s address, it may direct DHCPDISCOVER or DHCPREQUEST to that address instead of using the IP broadcast address.
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6. DHCPREQUEST has several jobs
DHCPREQUEST is not limited to accepting a new offer. A client can use it to accept an offer, check a previously allocated address, or ask for a lease extension.
The message’s meaning depends on the client’s stage and situation. During initial selection, it names the chosen offer; later, it can support address verification or renewal.
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7. Renewing an existing lease
When the client reaches T1, it enters RENEWING and sends DHCPREQUEST to its server using unicast. This is different from the initial selection exchange, where the client commonly broadcasts while searching for a server.
After DHCPACK arrives, the client determines the lease expiration by adding the lease duration to the transmission time of its DHCPREQUEST. The client can therefore calculate when its current allocation ends.
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8. What happens to unused offers
An offer does not remain reserved forever simply because a server sent it. If the server receives no DHCPREQUEST, it should make the offered address available again.
This behavior returns an unaccepted address to the available supply. It prevents a proposal that never became a client choice from remaining unavailable indefinitely.
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9. Address pools in Cisco IOS XE
Cisco IOS XE DHCP assigns client addresses from predefined address groups inside DHCP address pools. A pool therefore provides the organized source from which the server supplies addresses.
Before assigning a pool address, the Cisco DHCP server pings that address twice by default. This check is a platform behavior associated with allocation, while the discover-and-request exchange describes how the client and server agree on an offer.
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10. Recap before Practice and Prove
DHCP begins with a client discovery and a server offer. The server chooses a network address and includes configuration parameters in DHCPOFFER.
The client compares offers, selects one server, and uses DHCPREQUEST to identify that server and request the offered address. Other servers treat the request as notice that their offers were not chosen.
During SELECTING, DHCPREQUEST includes the selected server and requested address, while the client IP address is zero. A client normally broadcasts while searching unless it knows the server’s address.
At T1, the client enters RENEWING and unicasts DHCPREQUEST to extend its lease. After DHCPACK, it calculates expiration from the request transmission time and lease duration.
Cisco IOS XE draws addresses from predefined groups within DHCP pools and pings a pool address twice by default before assignment. In Practice and Prove, trace the message purpose, destination choice, and address state at each stage.