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Zorluk: OrtaDHCP Architecture and Relay Services

An organization is deploying a new VoIP phone deployment on VLAN 30 (192.168.30.0/24192.168.30.0/24). While existing desktop computers on VLAN 10 (192.168.10.0/24192.168.10.0/24) successfully acquire dynamic IPv4 addresses from a centralized DHCP server at 10.1.1.5010.1.1.50, the newly installed VoIP phones on VLAN 30 fail to receive an IP address lease. Both VLANs terminate on a central Layer 3 core switch. Which of the following configuration changes on the Layer 3 core switch will resolve the IP addressing issue for the phones on VLAN 30?

  1. Configure `ip helper-address 10.1.1.50` under the VLAN 30 switch virtual interface (SVI).Cevap
  2. B
    Configure `ip helper-address 10.1.1.50` under the physical interface connected directly to the DHCP server.
  3. C
    Configure static route port forwarding for TCP port 67 on the core router.
  4. D
    Configure the DHCP server address as the default gateway on the VLAN 10 SVI.

Cevap

Configure `ip helper-address 10.1.1.50` under the VLAN 30 switch virtual interface (SVI).
The correct action is to configure `ip helper-address 10.1.1.50` under the VLAN 30 SVI. DHCP client discovery packets are sent as local Layer 2 broadcasts. Because routers and Layer 3 switch interfaces drop broadcast packets by default, a DHCP relay agent (configured via `ip helper-address` in Cisco-style syntax) must be enabled on the interface directly connected to the client subnet. This allows the switch to intercept incoming broadcast DHCPDISCOVER traffic on UDP port 67, encapsulate it as a unicast packet with the gateway IP address (`giaddr`) set to the VLAN 30 interface address, and send it to the DHCP server at 10.1.1.5010.1.1.50.

Adım Adım Çözüm

1
Analyze why clients on VLAN 30 fail to acquire IPv4 address leases from the remote DHCP server.
Initial DHCPDISCOVER messages are Layer 2 broadcasts (255.255.255.255255.255.255.255) that cannot traverse Layer 3 boundaries (routers/SVIs) by default.
Without a relay mechanism, broadcast requests from VLAN 30 are dropped at the SVI boundary.
2
Identify the proper location and command to enable DHCP relay functionality.
Apply `ip helper-address 10.1.1.50` on the ingress Layer 3 interface facing the client broadcast domain (VLAN 30 SVI).
The relay agent intercepts client broadcasts on port 67, adds its own IP address to the `giaddr` field, and forwards the request as a unicast packet to the target server.

Anahtar Kavram

DHCP Relay Agent Placement and Operation
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Konular

DHCP Architecture and Relay Services

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