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

A network router receives a broadcast DHCP Discover packet from a client host on interface GigabitEthernet0/1. The interface is configured with the command `ip helper-address 192.168.10.5` to forward requests to a central DHCP server located in a different subnet. Which TWO actions does the router perform when processing and relaying this broadcast message? (Select TWO.)

  1. It converts the client incoming broadcast packet into a unicast packet directed to 192.168.10.5.Answer
  2. It populates the gateway IP address (giaddr) field in the DHCP packet header with the IP address of GigabitEthernet0/1.Answer
  3. C
    It requires the helper command to be configured on the outbound interface connected directly to the DHCP server.
  4. D
    It floods the original layer 2 broadcast frame out all router interfaces to locate the DHCP server.

Answer

The router converts the client's incoming broadcast packet into a unicast packet directed to the configured IP helper address (192.168.10.5), and it populates the gateway IP address (giaddr) field with the IP address of its client-facing interface (GigabitEthernet0/1).
When a router receives a client DHCP broadcast on an interface configured with `ip helper-address`, it acts as a DHCP Relay Agent. It converts the broadcast into a unicast IP packet destined for the helper address and populates the `giaddr` (gateway IP address) field in the DHCP message header with the IP address of the receiving interface. This informs the DHCP server which IP subnet pool to draw from.

Step-by-Step Solution

1
Examine the client DHCP broadcast reception
The client sends a broadcast DHCP Discover message which arrives on interface GigabitEthernet0/1.
DHCP clients initially do not possess an IP address and must communicate using local broadcast.
2
Process the ip helper-address configuration
The router intercepts the UDP broadcast on port 67 because `ip helper-address` is enabled on GigabitEthernet0/1.
The helper command enables the router to act as a Relay Agent on the incoming client-facing interface.
3
Modify the DHCP header fields and packet format
The router sets the giaddr field to GigabitEthernet0/1's IP address and changes the packet destination to unicast (192.168.10.5).
The server relies on giaddr to pick the correct IP pool, and unicast routing allows the packet to traverse routed boundaries to reach the DHCP server.

Key Concept

DHCP Relay Agent Operation and giaddr insertion
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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