A client workstation on VLAN 10 () fails to receive an IPv4 address lease from a centralized DHCP server located on VLAN 20 (). Which TWO of the following configurations or conditions on the intervening Layer 3 router would directly prevent the client from obtaining a DHCP lease? (Select TWO.)
- The router interface attached to VLAN 10 lacks an IP helper address (DHCP relay agent) configured to forward broadcast request packets to .Answer
- An inbound Access Control List (ACL) applied to the VLAN 10 router interface is blocking UDP traffic targeted at destination ports 67 and 68.Answer
- CThe DHCP server's scope for VLAN 10 has auto-assigned an APIPA address of to the client due to default gateway unreachable errors.
- DThe router's default gateway address for VLAN 20 resides on a different subnet than the DHCP server's IP address.
Answer
The client fails to obtain an IP lease because the router interface on VLAN 10 is missing a DHCP relay (IP helper address) to forward broadcast traffic across subnets, and/or an Access Control List on the router is blocking UDP ports 67 and 68 used by DHCP.
DHCP Discover packets are sent as broadcasts (). Routers do not forward broadcasts between subnets unless a DHCP relay agent (IP helper address) is explicitly configured on the receiving interface. Additionally, DHCP relies on UDP ports 67 and 68, so blocking these ports with an ACL on the router interface prevents relay processing.
Step-by-Step Solution
Key Concept
DHCP Relay and Port Requirements for Cross-Subnet IP Addressing Services