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DHCP Configuration and Troubleshooting

Jun 30, 2025

Overview

This lecture covers configuring DHCP (Dynamic Host Configuration Protocol) on Cisco devices, including DHCP server, client, and relay agent setup, and demonstrates troubleshooting DHCP issues using Boson NetSim.

DHCP Configuration on Cisco Devices

  • R2 is configured as a DHCP server with three pools: 192.168.1.0/24, 192.168.2.0/24, and 203.0.113.0/30.
  • The first 10 addresses in the first two subnets and R2’s .1 address in the third subnet are excluded from DHCP assignment.
  • DHCP pool settings include network address, DNS server, domain name, and default gateway.

DHCP Client and Relay Configuration

  • R1’s G0/0 interface is set as a DHCP client using the ip address dhcp command.
  • R1 is configured as a DHCP relay agent on G0/1 interface with the ip helper-address command pointing to the DHCP server.

Verifying DHCP Operation

  • PC2 successfully receives an IP from R2, confirming correct DHCP pool and options.
  • R1 successfully receives a /30 address from R2 as a DHCP client.
  • PC1 receives an IP from R2 through R1 as a relay agent, confirming relay configuration.

Troubleshooting DHCP in Boson NetSim

  • In the NetSim lab, DHCP clients on VLAN100, 200, and 300 could not obtain addresses.
  • Verification using ipconfig and show ip dhcp binding confirms no DHCP bindings.
  • Physical connectivity for switch ports is confirmed UP/UP.
  • The likely cause is the no service dhcp command on Router1; DHCP service must be enabled with service dhcp.
  • Once enabled, clients in VLAN200 and VLAN300 receive addresses, but VLAN100 still has an issue to be resolved.

Key Terms & Definitions

  • DHCP (Dynamic Host Configuration Protocol) — Automatically assigns IP configuration to network devices.
  • DHCP Server — Device that provides IP addresses and configuration to clients.
  • DHCP Client — Device that receives IP addressing from a DHCP server.
  • DHCP Relay Agent — Forwards DHCP messages between clients and servers on different subnets.
  • Excluded Address — IP addresses not assigned by DHCP, often reserved for static setup.

Action Items / Next Steps

  • Practice configuring and troubleshooting DHCP server, client, and relay agent on Cisco devices.
  • Complete the unused task(s) from the NetSim bonus lab, especially resolving VLAN100’s DHCP issue.
  • Review relevant commands: ip dhcp excluded-address, ip dhcp pool, ip helper-address, service dhcp, and verification tools (show ip dhcp binding, ipconfig).

Full transcript