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Zorluk: ZorTroubleshooting IP Addressing and DHCP Services

A network engineer provisions a new subinterface `GigabitEthernet0/0.40` on a branch router to serve an isolated IoT subnet (172.16.40.0/24172.16.40.0/24). Devices connected to VLAN 40 are unable to reach external resources and auto-assign link-local IPv4 addresses in the 169.254.x.x169.254.x.x range. A terminal capture on the router shows the following configuration:

text
interface GigabitEthernet0/0.10
description Corporate_Users
encapsulation dot1Q 10
ip address 172.16.10.1 255.255.255.0
ip helper-address 10.10.10.50
!
interface GigabitEthernet0/0.40
description IoT_Devices
encapsulation dot1Q 40
ip address 172.16.40.1 255.255.255.0

Existing client workstations on `GigabitEthernet0/0.10` receive leases dynamically from the central DHCP server at 10.10.10.5010.10.10.50 without issue. Which configuration change on the branch router will resolve the IP addressing failure for VLAN 40?

  1. Add the command `ip helper-address 10.10.10.50` under the `GigabitEthernet0/0.40` subinterface interface configuration mode.Cevap
  2. B
    Reconfigure `GigabitEthernet0/0.40` to use `encapsulation dot1Q 10` so it can share the existing DHCP relay agent setup.
  3. C
    Configure the central DHCP server to enable APIPA relaying for unconfigured subinterfaces.
  4. D
    Increase the usable host capacity on `GigabitEthernet0/0.10` to include the 172.16.40.0/24172.16.40.0/24 network range.

Cevap

Add the command `ip helper-address 10.10.10.50` under the `GigabitEthernet0/0.40` subinterface interface configuration mode.
DHCP broadcasts (UDP ports 67 and 68) are confined to their local Layer 2 broadcast domain and do not pass through Layer 3 boundaries. Subinterface `GigabitEthernet0/0.40` acts as the default gateway for VLAN 40. Without the `ip helper-address` command on this specific subinterface, the router drops the client DHCPDISCOVER broadcast frames. Adding `ip helper-address 10.10.10.50` configures the router to act as a DHCP relay agent for VLAN 40, converting incoming client broadcasts into unicast UDP packets sent directly to the central DHCP server at 10.10.10.5010.10.10.50.

Adım Adım Çözüm

1
Analyze client symptoms and subinterface configuration.
IoT devices on VLAN 40 receive 169.254.x.x addresses, indicating APIPA assignment caused by a failure to complete the DHCP DORA process.
DHCP client broadcast requests (DHCPDISCOVER) are stopped by default at Layer 3 router interfaces unless a relay agent is configured.
2
Compare working subinterface `GigabitEthernet0/0.10` with non-working subinterface `GigabitEthernet0/0.40`.
`GigabitEthernet0/0.10` has `ip helper-address 10.10.10.50` configured, while `GigabitEthernet0/0.40` is missing any relay statement.
DHCP relay agent directives must be explicitly applied to each routed logical interface/subinterface that receives client broadcasts.
3
Identify the required remediation step.
Applying `ip helper-address 10.10.10.50` to subinterface `GigabitEthernet0/0.40` allows the router to convert client UDP broadcasts into unicast packets routed to the centralized server.
This establishes the necessary Layer 3 relay mechanism for VLAN 40.

Anahtar Kavram

DHCP Relay Agent and IP Helper Placement on VLAN Subinterfaces
Tahmini Süre:2m 0s
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Troubleshooting IP Addressing and DHCP Services

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