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

A network administrator is troubleshooting IP address allocation for clients on subnet 10.20.30.0/2410.20.30.0/24. Clients connected to router interface GigabitEthernet0/0 (10.20.30.1/2410.20.30.1/24) fail to acquire dynamic IP addresses from a centralized DHCP server at 172.16.50.10172.16.50.10. An audit of the router configuration shows that `ip helper-address 172.16.50.10` was configured on interface GigabitEthernet0/1 (the egress interface facing the DHCP server), while GigabitEthernet0/0 has no helper address configured. Which statement correctly explains why the clients on GigabitEthernet0/0 fail to receive IP leases?

  1. The broadcast DHCP DISCOVER messages received on GigabitEthernet0/0 are dropped by default because the `ip helper-address` command must be applied on the client-facing ingress interface to convert local broadcasts into targeted unicast packets.Answer
  2. B
    The router relays the DHCP DISCOVER message but populates the gateway IP address (giaddr) field with GigabitEthernet0/1's IP address, causing the DHCP server to select an incorrect address pool.
  3. C
    Routers automatically flood Layer 2 DHCP broadcasts across all operational interfaces by default, but the egress interface GigabitEthernet0/1 drops the frame because Option 82 metadata is missing.
  4. D
    The client DHCP DISCOVER broadcast reaches the server via default routing, but the server drops the request because the source IP address of the broadcast packet is unroutable.

Answer

The broadcast DHCP DISCOVER messages received on GigabitEthernet0/0 are dropped by default because the `ip helper-address` command must be applied on the client-facing ingress interface to convert local broadcasts into targeted unicast packets.
DHCP clients issue local broadcast messages to discover servers. Because routers do not forward broadcasts, the `ip helper-address` command must be applied directly to the client-facing interface (GigabitEthernet0/0). This instructs the router to intercept client broadcasts, set the gateway IP address (giaddr) field to GigabitEthernet0/0's IP address, and unicast the DHCP requests to the remote DHCP server.

Step-by-Step Solution

1
Analyze client packet behavior at the gateway interface
Clients broadcast DHCP DISCOVER packets with source 0.0.0.0 and destination 255.255.255.255 on GigabitEthernet0/0.
DHCP clients do not yet have an IP address and must use local subnetwork broadcasts to find a server.
2
Evaluate router default broadcast handling
Router interface GigabitEthernet0/0 drops the 255.255.255.255 broadcast packet because no relay mechanism is active on that interface.
Cisco IOS routers break broadcast domains and drop limited broadcast frames by default.
3
Verify correct placement of the `ip helper-address` command
Applying `ip helper-address 172.16.50.10` under `interface GigabitEthernet0/0` enables the DHCP relay agent process on incoming client broadcasts.
The relay agent must intercept broadcasts on the ingress interface, insert its own interface IP into the giaddr field, and unicast the request to the DHCP server.

Key Concept

DHCP Relay Agent Interface Placement and Broadcast Handling
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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