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

A host PC on a local subnet connects to router interface GigabitEthernet0/0, which is configured with IP address 192.168.100.1/24192.168.100.1/24 and the command `ip helper-address 10.50.1.10`. When the host broadcasts a DHCP DISCOVER message, the router performs DHCP relay operations. Which source and destination IP addresses are placed in the Layer 3 IP header of the unicast packet forwarded by the router to the DHCP server?

  1. Source IP: 192.168.100.1192.168.100.1, Destination IP: 10.50.1.1010.50.1.10Answer
  2. B
    Source IP: 0.0.0.00.0.0.0, Destination IP: 255.255.255.255255.255.255.255
  3. C
    Source IP: 10.50.1.1010.50.1.10, Destination IP: 192.168.100.1192.168.100.1
  4. D
    Source IP: 192.168.100.1192.168.100.1, Destination IP: 255.255.255.255255.255.255.255

Answer

The router sets the source IP address to 192.168.100.1192.168.100.1 (the ingress interface address receiving the client broadcast) and the destination IP address to 10.50.1.1010.50.1.10 (the DHCP server address configured in the `ip helper-address` statement).
When a Cisco router interface configured with `ip helper-address 10.50.1.10` receives a broadcast DHCP DISCOVER message, it acts as a DHCP relay agent. It populates the gateway IP address (`giaddr`) field in the DHCP payload with the interface's IP address (192.168.100.1192.168.100.1) and encapsulates the payload into a unicast IP packet. The outer IP header uses the interface IP (192.168.100.1192.168.100.1) as the Source IP and the configured helper address (10.50.1.1010.50.1.10) as the Destination IP.

Step-by-Step Solution

1
Analyze the incoming packet from the client host.
The host sends a UDP broadcast DHCP DISCOVER message with source IP 0.0.0.00.0.0.0 and destination IP 255.255.255.255255.255.255.255.
Unconfigured DHCP clients cannot perform unicast routing initially and must broadcast to discover available servers.
2
Examine the router's DHCP relay mechanism triggered by `ip helper-address`.
The router intercepts the broadcast on interface GigabitEthernet0/0 (192.168.100.1192.168.100.1) and sets the gateway IP address (`giaddr`) field in the DHCP payload to 192.168.100.1192.168.100.1.
The `giaddr` field tells the remote DHCP server which subnet scope to select an IP address from.
3
Determine the outer Layer 3 IP header parameters of the relayed packet.
The router constructs a unicast IP packet with Source IP 192.168.100.1192.168.100.1 and Destination IP 10.50.1.1010.50.1.10.
The relay agent must encapsulate the request in a unicast packet directed to the configured helper address to route across intermediate networks.

Key Concept

DHCP Relay Agent IP Packet Unicast Forwarding and GIADDR Operation
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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