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Zorluk: ZorTroubleshooting IP Addressing and DHCP Services

A network administrator provisions a new VLAN (VLAN 80, subnet 10.80.4.0/2210.80.4.0/22) on a core multilayer switch. The central enterprise DHCP server resides on VLAN 10 at IP address 10.10.0.1010.10.0.10. Workstations connected to VLAN 80 fail to receive IP configuration parameters dynamically, resulting in clients auto-configuring 169.254.x.x169.254.x.x IPv4 addresses. A packet capture on the VLAN 80 switchport confirms that workstations are successfully broadcasting `DHCPDISCOVER` packets, but no `DHCPOFFER` packets return. Which of the following interface configuration omissions on the VLAN 80 Layer 3 gateway interface is the root cause of this connectivity issue?

  1. The Layer 3 gateway interface for VLAN 80 is missing an `ip helper-address 10.10.0.10` directive to relay broadcast DHCP requests to the remote DHCP server.Cevap
  2. B
    The client workstations have experienced a default gateway misconfiguration, preventing them from acknowledging incoming lease offers from the server.
  3. C
    The `ip helper-address` directive was incorrectly applied to the enterprise DHCP server's local gateway interface on VLAN 10 instead of the client gateway interface.
  4. D
    The defined DHCP scope mask of /22/22 restricts the pool size to a maximum of 510 usable host IP addresses, causing immediate scope exhaustion.

Cevap

The Layer 3 gateway interface for VLAN 80 is missing an `ip helper-address 10.10.0.10` directive to relay broadcast DHCP requests to the remote DHCP server.
The correct answer correctly identifies that routers break broadcast domains. When client hosts and the DHCP server reside on different VLANs/subnets, the client gateway interface (VLAN 80) must be configured with an IP helper address (`ip helper-address 10.10.0.10`). This encapsulates client `DHCPDISCOVER` broadcasts into unicast packets routed directly to the DHCP server on VLAN 10.

Adım Adım Çözüm

1
Analyze client symptoms and diagnostic packet capture output.
Workstations self-assign APIPA addresses (169.254.x.x169.254.x.x) and send broadcast `DHCPDISCOVER` frames, but receive zero `DHCPOFFER` responses.
This confirms Layer 2 local connectivity is operational, but Layer 3 DHCP traffic is failing to reach the cross-subnet DHCP server.
2
Evaluate router/switch Layer 3 boundary behavior for broadcast traffic.
Routers block MAC/IP broadcast packets (255.255.255.255255.255.255.255) by default.
Because the DHCP server resides on VLAN 10 (10.10.0.1010.10.0.10) and clients are on VLAN 80 (10.80.4.0/2210.80.4.0/22), the router interface must act as a DHCP Relay Agent.
3
Identify the mandatory configuration required to relay DHCP traffic.
Configuring `ip helper-address 10.10.0.10` under the VLAN 80 SVI/subinterface enables the gateway to forward broadcast `DHCPDISCOVER` messages as unicast packets directly to the DHCP server.
Without this directive, the DHCP requests never reach VLAN 10.

Anahtar Kavram

DHCP Relay / IP Helper Address Configuration in Multi-VLAN Environments
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Troubleshooting IP Addressing and DHCP Services

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