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Zorluk: OrtaDHCP Server, Client, and Relay Agent Operation and Configuration

A network administrator configures a Cisco IOS router interface with the `ip helper-address 10.1.1.50` command to support clients on subnets without a local DHCP server. Which two actions does the router perform upon receiving a DHCPDISCOVER broadcast message from a client host on that interface? (Select two.)

  1. The router converts the broadcast destination IP address (255.255.255.255255.255.255.255) into a unicast IP address matching the helper address (10.1.1.5010.1.1.50).Cevap
  2. The router inserts the IP address of the receiving client-facing interface into the `giaddr` (Gateway IP Address) field of the DHCP message body.Cevap
  3. C
    The router must have the `ip helper-address` command configured on its egress interface facing the DHCP server destination.
  4. D
    The router floods the original DHCPDISCOVER broadcast frame out all active interfaces except the incoming client interface.

Cevap

The router converts the destination broadcast address to the unicast address of the DHCP server (10.1.1.50) and populates the `giaddr` field with the IP address of the receiving interface.
When a Cisco router acts as a DHCP relay agent, it intercepts client DHCPDISCOVER broadcast messages on the ingress interface configured with `ip helper-address`. It modifies the packet by changing the destination IP from broadcast (255.255.255.255255.255.255.255) to the unicast IP of the DHCP server. Additionally, it inserts the IP address of the receiving ingress interface into the `giaddr` field of the DHCP payload so the server knows which subnet scope to assign an address from.

Adım Adım Çözüm

1
Analyze incoming packet type
Client sends a broadcast DHCPDISCOVER packet to destination IP 255.255.255.255255.255.255.255 and destination UDP port 67.
DHCP clients do not initially have an IP address or knowledge of the network architecture.
2
Process packet with DHCP Relay Agent logic
The router intercepts the broadcast on the interface where `ip helper-address 10.1.1.50` is configured.
The `ip helper-address` command tells the router to forward UDP port 67/68 broadcasts as unicast traffic.
3
Modify DHCP header payload and IP header
The router sets the `giaddr` field to its own receiving interface IP address and changes the packet destination IP to 10.1.1.5010.1.1.50.
The server relies on `giaddr` to identify the correct client IP pool, and unicast routing to deliver the packet across intermediate subnets.

Anahtar Kavram

DHCP Relay Agent Operation and Header Modification
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Konular

DHCP Server, Client, and Relay Agent Operation and Configuration

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