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Zorluk: ZorTroubleshooting IP Addressing and DHCP Services

A network administrator is troubleshooting an issue where database servers deployed on a new broadcast domain (VLAN 104, 10.104.0.0/2210.104.0.0/22) fail to acquire dynamic IPv4 addresses and assign themselves 169.254.x.x169.254.x.x addresses instead. A packet capture on the local router subinterface `GigabitEthernet0/0.104` shows incoming broadcast DHCP DISCOVER frames from clients, but no DHCP OFFER packets return. The central DHCP server is located on VLAN 10 (10.10.10.210.10.10.2). The configuration of the local subinterface is as follows:

interface GigabitEthernet0/0.104
encapsulation dot1Q 104
ip address 10.104.0.1 255.255.252.0

Which of the following actions should the network administrator perform to resolve this issue and enable dynamic address assignment for clients in VLAN 104? (Select TWO.)

  1. Configure the command `ip helper-address 10.10.10.2` under the `GigabitEthernet0/0.104` subinterface.Cevap
  2. Ensure intermediate firewalls and router Access Control Lists (ACLs) permit UDP ports 67 and 68 traffic between the router relay interface and the central DHCP server.Cevap
  3. C
    Configure static IP address 169.254.0.1169.254.0.1 as the default gateway on the central DHCP server to allow routing to APIPA endpoints.
  4. D
    Change the `GigabitEthernet0/0.104` subinterface subnet mask to 255.255.255.0255.255.255.0 to increase the number of usable host addresses available for the scope.

Cevap

The administrator must configure the DHCP relay agent using `ip helper-address 10.10.10.2` on the client-facing router subinterface `GigabitEthernet0/0.104` and ensure that router ACLs/firewalls allow UDP ports 67 and 68 traffic between the relay agent and the central DHCP server.
When clients are located on a separate subnet or VLAN from the central DHCP server, routers block the initial client DHCP DISCOVER broadcast packets by default. To fix this, the router subinterface serving as the client gateway (`GigabitEthernet0/0.104`) must be configured with `ip helper-address 10.10.10.2` to convert local broadcasts into unicast packets directed to the DHCP server. Additionally, because DHCP relay relies on UDP ports 67 and 68, any network ACLs or firewalls along the traffic path between the router interface and the DHCP server must explicitly permit UDP 67/68 traffic.

Adım Adım Çözüm

1
Analyze client symptoms and packet capture output
Clients acquiring 169.254.x.x (APIPA) addresses indicates complete DHCP lease acquisition failure. DHCP DISCOVER broadcast packets reach the default gateway subinterface but cannot traverse Layer 3 boundaries without a relay agent.
DHCP DISCOVER messages are Layer 2/3 broadcasts (255.255.255.255) that routers drop by default.
2
Identify the missing router subinterface configuration
Configuring `ip helper-address 10.10.10.2` on `GigabitEthernet0/0.104` intercepts client broadcast requests and unicasts them to the DHCP server at 10.10.10.2.
Cross-subnet DHCP deployment requires explicit relay agent configuration on the client's gateway interface.
3
Verify network transport security and ACL rules between relay agent and server
Ensuring UDP ports 67 (DHCP server) and 68 (DHCP client) are allowed on intermediate ACLs guarantees unhindered transmission of DHCP DISCOVER/REQUEST unicast relays and server DHCP OFFER/ACK replies.
Firewalls or implicit deny statements in router ACLs blocking UDP 67/68 prevent communication between the relay agent and the DHCP server.

Anahtar Kavram

DHCP Relay (IP Helper) and Traffic Permitting across Layer 3 Boundaries
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Konular

Troubleshooting IP Addressing and DHCP Services

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