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AdvancedPublished 2026-07-02

DHCP Troubleshooting Capstone: Branch Relay

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Advanced CCNA troubleshooting capstone for centralized DHCP across a relay. A centralized IOS-XE router (DHCP-SRV) serves the branch LAN behind BR-RTR via ip helper-address. The lab imports in a deliberately broken state: the DHCP pool scope and gateway are misconfigured on the server, and the relay configuration is missing on BR-RTR. Learners must diagnose using show outputs and Linux tools, fix all three discrepancies, and verify that two Alpine clients dynamically receive usable leases and can reach DHCP-SRV.

Track
Est. time
65 minutes
Difficulty
Advanced
Topics

Learning objectives

  • Trace DHCP across a routed relay path and identify where DISCOVER frames stop
  • Diagnose DHCP server pool mismatches using show ip dhcp pool and show ip dhcp binding
  • Correct an IOS DHCP pool: network/mask, default-router, dns-server, domain-name, lease, and exclusions
  • Enable a working DHCP relay with ip helper-address on the correct interface toward the centralized server
  • Verify end-to-end success from Linux clients with udhcpc, ip addr, and ping

Troubleshooting focus

  • Verify the relay path delivers broadcast requests: check for ip helper-address on the client-facing router interface
  • Confirm the server has a pool whose network matches the client LAN; an off-by-one or wrong subnet prevents address assignment
  • Ensure the default-router option equals the actual gateway interface IP of the client subnet
  • Use show ip dhcp pool and show ip dhcp binding to corroborate pool utilization and active leases
  • On clients, re-run udhcpc and confirm an in-scope address and correct default route

Topology

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How this lab is graded
  • Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with access-class, are stated in the guide and do have to match.
  • Addresses, modes and protocol keywords are exact. An IP address, a subnet mask, switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide.
  • Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the running-config — a change that only exists in a terminal session never reaches the grader.
  • You can submit as many times as you like. Your best score stands, and each attempt tells you which checks passed so you can work the gaps.
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