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Difficulty: HardDHCP Architecture and Relay Services

A network engineer is configuring a Layer 3 switch to relay DHCP requests from client workstations on VLAN 35 (172.16.35.0/24172.16.35.0/24) to two redundant central DHCP servers located on VLAN 100 (10.100.0.1010.100.0.10 and 10.100.0.1110.100.0.11). Which of the following statements correctly describe the behavior and operational mechanics of the DHCP relay agent when processing these client requests? (Select TWO options.)

  1. The relay agent inserts the IP address of its local ingress interface (VLAN 35 SVI) into the `giaddr` field of the DHCP payload before forwarding the message.Answer
  2. The relay agent duplicates and forwards the original broadcast `DHCPDISCOVER` frame as separate unicast IP packets addressed to each configured DHCP server.Answer
  3. C
    The relay agent encapsulates the client request inside a connection-oriented TCP segment targeted at destination port 67 to guarantee delivery across the routed network.
  4. D
    The `ip helper-address` commands must be configured in global configuration mode to allow automatic relay across all active SVIs on the switch.

Answer

The correct statements are that the relay agent populates the `giaddr` field with the local ingress SVI IP address to identify the client subnet, and that it replicates incoming broadcast discovery messages into individual unicast IP packets directed to each defined target server.
The statements confirming that the local ingress interface IP address is populated into the `giaddr` field and that the broadcast `DHCPDISCOVER` is duplicated into unicast UDP packets sent to each configured destination server are both correct. The `giaddr` field is critical for cross-subnet DHCP operations because central DHCP servers inspect this field to determine which IP pool scope to draw addresses from. Additionally, when redundant helper addresses are configured on a Layer 3 interface, the relay agent creates separate unicast UDP messages targeting each server address specified.

Step-by-Step Solution

1
Analyze incoming broadcast reception at the Layer 3 interface
The Layer 3 switch receives a client broadcast `DHCPDISCOVER` frame on the VLAN 35 interface.
DHCP clients initially issue Layer 2 broadcast packets (255.255.255.255255.255.255.255) because they do not possess an IP address or knowledge of the server location.
2
Evaluate relay payload modifications (`giaddr` insertion)
The relay agent writes the IP address of its VLAN 35 SVI into the `giaddr` (Gateway IP Address) field in the DHCP header.
The central DHCP server requires the `giaddr` field value to select the matching address pool (172.16.35.0/24172.16.35.0/24) and route the reply back.
3
Evaluate packet duplication and transport protocol usage
The relay agent creates two separate unicast UDP packets (destination UDP port 67) directed to 10.100.0.1010.100.0.10 and 10.100.0.1110.100.0.11.
DHCP operates entirely using UDP; when multiple relay targets exist, the agent unicasts a copy of the request to each configured helper destination address.

Key Concept

DHCP Relay Unicast Forwarding and GIADDR Field Modification
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Topics

DHCP Architecture and Relay Services

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