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Difficulty: MediumDHCP Architecture and Relay Services

A network administrator configures a DHCP relay agent on a Layer 3 switch interface to support host workstations on VLAN 25 (10.25.0.0/2410.25.0.0/24, default gateway 10.25.0.110.25.0.1). The centralized DHCP server resides on VLAN 100 (10.100.0.510.100.0.5). When an unconfigured client on VLAN 25 broadcasts a DHCPDISCOVER packet, which TWO of the following actions does the DHCP relay agent execute to ensure the DHCP server correctly processes the request?

  1. It converts the incoming client broadcast packet into a Layer 3 unicast packet destined for 10.100.0.510.100.0.5.Answer
  2. It populates the Gateway IP Address (`giaddr`) field in the DHCP header with 10.25.0.110.25.0.1.Answer
  3. C
    It changes the destination port from UDP port 67 to UDP port 68 before forwarding the message across the router.
  4. D
    It floods the broadcast DHCPDISCOVER frame out all active switch ports across all configured VLANs.

Answer

The DHCP relay agent converts the host broadcast request into a unicast packet directed to the DHCP server (10.100.0.510.100.0.5) and populates the `giaddr` field with the receiving gateway interface IP address (10.25.0.110.25.0.1).
When a DHCP relay agent processes a client's broadcast request, it encapsulates the message into a Layer 3 unicast packet destined for the centralized DHCP server's IP address (10.100.0.510.100.0.5). To inform the DHCP server which subnet scope must be used for address assignment, the relay agent places the IP address of its receiving interface (10.25.0.110.25.0.1) into the Gateway IP Address (`giaddr`) field of the DHCP header.

Step-by-Step Solution

1
Analyze client broadcast message reception
The host on VLAN 25 broadcasts a DHCPDISCOVER message using source IP 0.0.0.00.0.0.0, destination IP 255.255.255.255255.255.255.255, UDP source port 68, and UDP destination port 67.
Unconfigured DHCP clients cannot route traffic outside their local broadcast domain.
2
Examine DHCP relay agent packet processing
The Layer 3 switch interface intercepts the broadcast, writes its own interface IP (10.25.0.110.25.0.1) into the `giaddr` header field, and converts the destination IP to the unicast address of the remote DHCP server (10.100.0.510.100.0.5).
The server requires the `giaddr` value to select the correct address pool (10.25.0.0/2410.25.0.0/24) and unicast addressing to cross routed boundaries.

Key Concept

DHCP Relay Agent and GIADDR Operation
Estimated Time:1m 30s
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Topics

DHCP Architecture and Relay Services

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