A network technician is deploying IP VoIP phones on VLAN 30 (). The centralized DHCP server is located on VLAN 10 (). While devices assigned static IP addresses on VLAN 30 communicate across subnets normally, all phones set to acquire addresses automatically receive IPs in the subnet and fail to register. Inspecting the Layer 3 switch shows that inter-VLAN routing is active and operational. Which of the following is the most likely cause of this problem?
- The interface for VLAN 30 on the Layer 3 switch is missing an IP helper address pointing to the DHCP server.Answer
- BThe client phones are receiving an APIPA address because their network cards are experiencing a physical link speed mismatch with the switch.
- CThe DHCP scope for VLAN 30 has exhausted its available IP address pool due to an invalid subnet host count calculation.
- DThe default gateway parameter in the DHCP pool options is set to an address residing on a different IP subnet.
Answer
The interface for VLAN 30 on the Layer 3 switch is missing an IP helper address pointing to the DHCP server.
DHCP discovery messages are sent as Layer 2 and Layer 3 broadcasts, which routers and Layer 3 switches do not forward across subnets by default. Because the DHCP server is on VLAN 10 and the clients are on VLAN 30, the Layer 3 switch interface serving VLAN 30 requires an IP helper address configured to relay DHCP broadcast requests as unicast traffic to the server.
Step-by-Step Solution
Key Concept
DHCP Relay Agent and IP Helper Address Placement across Subnets
Estimated Time:1m 30s