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Zorluk: ZorIP Addressing Services

A network administrator is deploying core IP addressing services in a multi-subnet corporate office. Workstations are located on VLAN 10 (10.10.10.0/2410.10.10.0/24), while the centralized DHCP server resides on VLAN 50 (10.10.50.0/2410.10.50.0/24). Users on VLAN 10 report that their systems are failing to receive dynamic IP address assignments. Upon reviewing the configuration on the Layer 3 switch bridging these subnets, the administrator discovers that the command `ip helper-address 10.10.50.5` was placed exclusively under the VLAN 50 interface rather than the VLAN 10 Switch Virtual Interface (SVI). Which of the following best explains why clients on VLAN 10 are unable to obtain IP addresses?

  1. The Layer 3 switch does not intercept client DHCP broadcast requests unless the relay command is enabled on the client-facing gateway interface.Cevap
  2. B
    The DHCP server requires a corresponding DNS PTR record on VLAN 50 before issuing IP lease configurations to remote subnets.
  3. C
    DHCPDISCOVER broadcast traffic is dropped because IP helper functions process relay requests over TCP port 67 instead of UDP port 67.
  4. D
    DHCP relay agents must be positioned on the server-side interface to strip Layer 2 MAC headers before forwarding frames across subnets.

Cevap

The Layer 3 switch does not intercept client DHCP broadcast requests unless the relay command is enabled on the client-facing gateway interface.
DHCP clients broadcast their initial lease requests (DHCPDISCOVER) onto their local subnet (VLAN 10). Because Layer 3 routers do not forward broadcasts, a DHCP relay agent (`ip helper-address`) must be configured on the ingress interface facing the clients (VLAN 10 SVI). The relay agent converts the broadcast into a unicast packet addressed directly to the DHCP server (10.10.50.510.10.50.5). When configured only on VLAN 50, the router never intercepts the broadcasts generated by hosts on VLAN 10.

Adım Adım Çözüm

1
Analyze how DHCP discovery messages travel across Layer 3 boundaries.
Workstations send DHCPDISCOVER packets as local Layer 2/3 broadcasts (255.255.255.255255.255.255.255). Routers block broadcast traffic by default.
Broadcast traffic cannot cross subnets without an intermediary service.
2
Identify where the DHCP relay agent must listen.
The `ip helper-address` directive configures the router interface to listen for incoming UDP port 67 broadcasts on that specific interface.
Because the broadcast originates on VLAN 10, the configuration must exist on the VLAN 10 interface to catch incoming client requests.
3
Evaluate the current configuration placement.
Placing `ip helper-address` on VLAN 50 only listens for broadcasts originating on VLAN 50, leaving VLAN 10 broadcasts unhandled and dropped at the gateway.
Relaying requires listening on the ingress interface facing the requesting clients.

Anahtar Kavram

DHCP Relay / IP Helper Address Interface Placement
Tahmini Süre:2m 0s
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IP Addressing Services

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