An enterprise network engineer configures a DHCP relay service on a Layer 3 router interface servicing a remote VoIP VLAN (). When an IP phone boots up and sends a broadcast DHCPDISCOVER frame, the router encapsulates this message into a unicast IPv4 packet targeted directly at the centralized DHCP server (). Which field in the DHCP message header does the relay agent populate with its own interface IP address () so that the DHCP server selects the appropriate address pool?
- giaddr (Gateway IP Address)Answer
- Bciaddr (Client IP Address)
- Cyiaddr (Your IP Address)
- Dchaddr (Client Hardware Address)
Answer
The giaddr (Gateway IP Address) field is populated by the DHCP relay agent with its own interface IP address to tell the server which IP scope to allocate from.
When a DHCP relay agent intercepts a broadcast DHCPDISCOVER request, it modifies the packet by inserting the IP address of the local receiving interface into the giaddr (Gateway IP Address) field. It then forwards the packet via unicast to the remote DHCP server. Upon receipt, the DHCP server evaluates the giaddr value to match it against configured scopes, ensuring the client receives an IP address valid for its local IP subnet.
Step-by-Step Solution
Key Concept
DHCP Relay Agent and giaddr Header Field Processing