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Difficulty: EasyDHCP Architecture and Relay Services

A network router interface configured with an IP helper address receives an incoming DHCPDISCOVER broadcast packet from a local host. Which of the following actions does the router perform as part of its DHCP relay operation? (Select TWO.)

  1. Replaces the client broadcast destination IP address with a unicast IP address directed to the specified DHCP server.Answer
  2. Populates the gateway IP address (giaddrgiaddr) field in the DHCP payload with its own receiving interface IP address.Answer
  3. C
    Encapsulates the DHCP request into a TCP stream on port 67 to establish a reliable connection with the DHCP server.
  4. D
    Floods the broadcast packet out of all router interfaces to locate an available DHCP server across connected VLANs.

Answer

When a router acts as a DHCP relay agent, it converts the client's local broadcast packet into a unicast packet directed toward the centralized DHCP server and populates the gateway IP address (giaddrgiaddr) field with the receiving interface's IP address.
A DHCP relay agent processes local client broadcast messages by converting the destination IP address to the unicast IP of the remote DHCP server and writing the receiving interface's IP address into the gateway IP address (giaddrgiaddr) field of the DHCP payload so the server knows which subnet scope to select.

Step-by-Step Solution

1
Identify the primary role of a DHCP relay agent upon receiving a broadcast request.
The router intercepts the broadcast DHCPDISCOVER packet on the client subnet interface.
DHCP clients initially send broadcast requests because they do not know the network configuration or server address.
2
Determine how the router modifies the IP header for relaying.
The destination address changes from a broadcast address to the configured unicast address of the DHCP server.
Routers do not forward broadcast packets across network boundaries; unicast routing delivers the request across Layer 3 subnets.
3
Determine how the DHCP server identifies the correct scope.
The router inserts its own receiving interface IP address into the giaddrgiaddr header field.
The DHCP server requires the giaddrgiaddr information to select an IP address from the pool corresponding to the client's subnet.

Key Concept

DHCP Relay Unicast Forwarding and Gateway IP Address (giaddrgiaddr) Insertion
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Topics

DHCP Architecture and Relay Services

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