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

A Cisco IOS router with an IP helper address configured receives a DHCP DISCOVER broadcast message from a client host. Place the operational steps performed by the relay agent in the exact sequential order from receiving the initial request to forwarding it to the remote DHCP server.

  1. 1Receives a Layer 2 Ethernet broadcast frame containing a UDP DHCP DISCOVER message on the client-facing ingress interface.
  2. 2Inspects the DHCP payload and verifies that the Gateway IP Address (giaddr) field is set to 0.0.0.0.
  3. 3Inserts the IP address of its local receiving ingress interface into the Gateway IP Address (giaddr) field.
  4. 4Replaces the destination broadcast IP address 255.255.255.255 with the unicast IP address configured in the ip helper-address command.
  5. 5Encapsulates the modified packet into a unicast Layer 2 frame and forwards it out the egress interface toward the DHCP server.

Answer

The correct sequence of DHCP Relay operations is: 1) Receive the Layer 2 broadcast frame on the client-facing ingress interface, 2) Inspect the DHCP payload to verify that giaddr is 0.0.0.0, 3) Insert the receiving ingress interface IP address into the giaddr field, 4) Replace the broadcast destination IP address (255.255.255.255) with the unicast IP address specified by the ip helper-address command, and 5) Encapsulate and route the unicast packet out the egress interface toward the DHCP server.
When a Cisco IOS router acts as a DHCP Relay Agent, it intercepts local client DHCP DISCOVER broadcast frames on the ingress interface. It inspects the DHCP payload to verify that giaddr is 0.0.0.0, inserts the ingress interface's IP address into the giaddr field, rewrites the destination IP from 255.255.255.255 to the unicast IP defined in the helper address command, and forwards the frame out the egress interface via normal unicast routing.

Step-by-Step Solution

1
Receive client broadcast request
The router interface configured with `ip helper-address` receives an incoming broadcast frame carrying a DHCP DISCOVER message (UDP port 67).
DHCP clients use local Layer 2 and Layer 3 broadcasts because they do not yet possess an IP address or know the DHCP server address.
2
Inspect the DHCP giaddr field
The router verifies that the `giaddr` field inside the DHCP message body contains 0.0.0.00.0.0.0.
A 0.0.0.00.0.0.0 value indicates the frame came directly from a client and has not already been processed by another relay agent.
3
Populate the giaddr field
The router writes the IPv4 address of the ingress interface into the `giaddr` field of the DHCP packet payload.
The remote DHCP server uses the `giaddr` IP address to select the appropriate scope/pool matching the client's local subnet and to route the DHCP OFFER back.
4
Rewrite destination IP addressing
The router changes the Layer 3 destination IP from the broadcast address 255.255.255.255255.255.255.255 to the unicast address configured with `ip helper-address`.
Routers drop Layer 3 IP broadcasts by default; converting the packet to unicast allows it to cross routed network boundaries.
5
Forward the unicast frame
The router encapsulates the unicast packet with a new Layer 2 Ethernet header and forwards it out the egress interface toward the DHCP server.
Standard IP routing forwards the unicast packet through the routed infrastructure to the destination server address.

Key Concept

DHCP Relay Agent Operational Packet Sequence and Header Rewrite Mechanics
Estimated Time:2m 0s
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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