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Difficulty: Very hardDHCP Architecture and Relay Services

A client workstation connected to VLAN 45 (10.45.0.0/2410.45.0.0/24) issues a Layer 2 broadcast `DHCPDISCOVER` packet (255.255.255.255255.255.255.255) to obtain an IP configuration. The default gateway interface for VLAN 45 on the Layer 3 switch is configured with the IP address 10.45.0.110.45.0.1 and an IP helper address pointing to a centralized DHCP server at 10.200.10.2510.200.10.25. Which set of header and payload modifications will the Layer 3 switch perform on this packet before forwarding it across the routed core to the DHCP server?

  1. It sets the Gateway IP Address (`giaddr`) field in the DHCP payload to 10.45.0.110.45.0.1, sets the source IP address to 10.45.0.110.45.0.1, sets the destination IP address to 10.200.10.2510.200.10.25, and forwards the unicast packet using UDP destination port 67.Answer
  2. B
    It sets the Gateway IP Address (`giaddr`) field in the DHCP payload to 10.200.10.2510.200.10.25, retains the source IP address as 0.0.0.00.0.0.0, sets the destination IP address to 10.45.0.110.45.0.1, and forwards the packet using UDP destination port 68.
  3. C
    It leaves the Gateway IP Address (`giaddr`) field as 0.0.0.00.0.0.0, sets the source IP address to 10.45.0.110.45.0.1, sets the destination IP address to 255.255.255.255255.255.255.255, and converts the transport layer header to TCP port 67.
  4. D
    It preserves the original Layer 2 broadcast payload unchanged and encapsulates the frame into a VLAN-wide Layer 2 switchport broadcast transmitted across all trunk links using UDP port 68.

Answer

The Layer 3 switch populates the Gateway IP Address (`giaddr`) field in the DHCP payload with 10.45.0.110.45.0.1, rewrites the source IP address as 10.45.0.110.45.0.1, sets the destination IP to 10.200.10.2510.200.10.25, and forwards the unicast packet using UDP port 67.
The correct answer accurately describes the dual modification executed by a DHCP relay agent (IP helper): it writes the receiving Layer 3 interface's IP address (10.45.0.110.45.0.1) into the DHCP payload's `giaddr` (gateway IP address) field, and encapsulates the request into a standard unicast IP packet (Source: 10.45.0.110.45.0.1, Destination: 10.200.10.2510.200.10.25, UDP Port 67) so it can be routed across network subnets.

Step-by-Step Solution

1
Analyze incoming broadcast packet parameters
The client issues a `DHCPDISCOVER` frame with Source IP 0.0.0.00.0.0.0, Destination IP 255.255.255.255255.255.255.255, Source MAC = Client MAC, Destination MAC = `FF:FF:FF:FF:FF:FF`, Source Port = UDP 68, Destination Port = UDP 67.
Unconfigured clients must broadcast their initial DHCP discovery requests.
2
Identify DHCP Relay Agent (IP Helper) processing
The Layer 3 interface receiving the broadcast (10.45.0.110.45.0.1) intercepts the frame, inspects the `giaddr` field in the DHCP payload, and writes its own interface IP (10.45.0.110.45.0.1) into `giaddr`.
The centralized DHCP server uses the `giaddr` field to determine which address pool/scope (10.45.0.0/2410.45.0.0/24) to allocate an IP address from.
3
Examine IP and UDP header modifications for unicast transit
The switch constructs a unicast IP packet: Source IP = 10.45.0.110.45.0.1, Destination IP = 10.200.10.2510.200.10.25, Source Port = UDP 67, Destination Port = UDP 67.
Routers cannot route 255.255.255.255255.255.255.255 broadcasts. Converting the request to unicast allows it to traverse standard Layer 3 routed hops to reach the remote server.

Key Concept

DHCP Relay Agent Payload and Header Modification (`giaddr` Insertion & Broadcast-to-Unicast Conversion)
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Topics

DHCP Architecture and Relay Services

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