A network administrator configures a Layer 3 switch interface serving the Finance subnet () with `ip helper-address 10.10.10.254` to direct DHCP traffic to a centralized DHCP server on the Management subnet (). When a client host on the Finance subnet broadcasts a DHCPDISCOVER packet, how does the Layer 3 switch process this packet before transmitting it to the DHCP server?
- It populates the gateway IP address (`giaddr`) field with the IP address of its receiving interface and forwards the packet as a unicast UDP message to destination port 67.Answer
- BIt forwards the client's broadcast packet unchanged to 10.10.10.254 using destination UDP port 68.
- CIt encapsulates the DHCPDISCOVER frame into a Layer 2 Ethernet broadcast frame and floods it out all active switch ports.
- DIt converts the packet payload into a TCP segment to guarantee delivery across subnets to 10.10.10.254.
Answer
The Layer 3 switch populates the gateway IP address (`giaddr`) field with the receiving interface's IP address on the client subnet and forwards the packet as a unicast UDP message to the DHCP server at 10.10.10.254 on destination port 67.
When an `ip helper-address` (DHCP relay agent) is configured on a router or Layer 3 switch interface, it intercepts local DHCP broadcast messages (DHCPDISCOVER/DHCPREQUEST). The relay agent inserts the IP address of the interface that received the broadcast into the `giaddr` (gateway IP address) field of the DHCP message header. This informs the remote DHCP server which IP subnet scope to assign an address from. The relay agent then forwards the modified packet directly to the configured DHCP server IP as a unicast UDP packet on destination port 67.
Step-by-Step Solution
Key Concept
DHCP Relay Agent Behavior and `giaddr` Header Insertion
Estimated Time:1m 0s