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Zorluk: OrtaDHCP Architecture and Relay Services

A network engineer is configuring a Layer 3 gateway router to forward broadcast DHCP discovery messages from client devices in VLAN 40 (192.168.40.0/24) to a centralized DHCP server located at 10.200.10.50. On which interface must the helper address command (`ip helper-address 10.200.10.50`) be applied for the router to intercept client broadcasts and forward them as unicast packets?

  1. The inbound Layer 3 interface (SVI or subinterface) serving as the default gateway for VLAN 40Cevap
  2. B
    The outbound Layer 3 interface directly connected to the DHCP server's destination subnet
  3. C
    The Layer 2 trunk interface connecting the gateway router to the access layer switches
  4. D
    The loopback interface configured to forward incoming unicast traffic on destination UDP port 68

Cevap

The helper address must be configured on the inbound Layer 3 interface (SVI or subinterface) serving as the default gateway for VLAN 40.
DHCP client requests (DISCOVER and REQUEST) are sent as Layer 2 broadcast frames to the local default gateway. To relay these messages across router boundaries to a centralized server, the DHCP relay command (such as `ip helper-address`) must be placed directly on the client-facing Layer 3 interface (SVI or router subinterface). When configured here, the gateway intercepts the broadcast, inserts its own interface IP into the gateway IP address (giaddr) field, and unicasts the request to the central server.

Adım Adım Çözüm

1
Identify the ingress gateway interface for client broadcast traffic
DHCP Discover packets originate on VLAN 40 as local Layer 2 broadcasts targeting UDP port 67.
Routers drop broadcast packets by default unless a relay agent is configured on the client-facing gateway interface.
2
Configure the DHCP relay agent on the client-facing Layer 3 interface
The router intercepts local DHCP broadcasts on VLAN 40 and encapsulates them into unicast IP packets addressed to 10.200.10.50.
The gateway interface injects its own IP address into the relay agent IP address (giaddr) field of the DHCP packet header so the DHCP server knows which scope to allocate from.

Anahtar Kavram

DHCP Relay Agent Interface Placement and Broadcast-to-Unicast Forwarding
Tahmini Süre:1m 30s
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Konular

DHCP Architecture and Relay Services

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