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Difficulty: HardDHCP Server, Client, and Relay Agent Operation and Configuration

A Cisco IOS router is configured as an authoritative DHCP server with IP address conflict detection enabled (using the default ping settings). Place the operational steps in the correct chronological order to show how the router handles an incoming broadcast DHCP DISCOVER packet from a new client.

  1. 1The DHCP server selects an unassigned IP address from the matching IP pool based on the client subnet.
  2. 2The DHCP server transmits ICMP Echo Request packets to the candidate IP address to verify it is inactive.
  3. 3Upon receiving no ICMP response within the timeout, the server unicasts or broadcasts a DHCP OFFER to the client.
  4. 4The DHCP server receives a DHCP REQUEST from the client identifying the selected offer and server ID.
  5. 5The DHCP server commits the IP binding to its database and transmits a DHCP ACK packet to the client.

Answer

The correct operational sequence is: 1) Select candidate IP address from pool based on client subnet -> 2) Send ICMP Echo Requests to verify address availability -> 3) Send DHCP OFFER to client after ICMP timeout -> 4) Receive client DHCP REQUEST accepting the offer -> 5) Create IP binding entry and send DHCP ACK.
When Cisco IOS conflict detection is active, the router evaluates an available IP address from the pool upon receiving a DHCP DISCOVER message. It probes the address using ICMP Echo Requests before transmitting a DHCP OFFER. If no reply is received, it sends the DHCP OFFER. The client responds with a DHCP REQUEST, and the server finishes the handshake by writing the binding and returning a DHCP ACK.

Step-by-Step Solution

1
Address Selection
The router identifies an available IP address within the pool range matching the incoming interface subnet.
The server must choose a candidate address prior to testing its availability.
2
Conflict Detection (Ping Check)
The router emits ICMP Echo Request pings to the candidate IP address (2 pings by default in Cisco IOS).
Cisco IOS conflict detection prevents assigning duplicate IP addresses already active on the local segment.
3
DHCP OFFER Transmission
Having received no ICMP Echo Reply, the router builds and sends a DHCP OFFER containing the candidate IP address, subnet mask, default router, and DNS settings.
Absence of ping reply confirms the IP is free to offer.
4
DHCP REQUEST Receipt
The client processes the offer and broadcasts a DHCP REQUEST packet to notify all local DHCP servers of its choice.
The client must explicitly request the offered parameters.
5
Binding Creation and DHCP ACK
The server records the MAC-to-IP binding in `show ip dhcp binding` and sends a DHCP ACK back to the client.
The lease is finalized only when the server acknowledges the client request and stores the active binding.

Key Concept

Cisco IOS DHCP Server Address Conflict Detection Sequence during DORA Process
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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