DHCP Lab Config(2 VLANs) — Cisco Packet Tracer

Source: Cisco CCNA Packet Tracer Ultimate labs: CCNA Exam prep labs

Lab: Packet Tracer lab to follow along can be found here

(Note, you will first need to build DHCP Lab Config(1 VLAN) — Cisco Packet Tracer in order to build on top of this one)

Then use this guide to continue.

Topology Overview

Devices

• Router1 (ISR4321)
• Acts as the DHCP server
• Switch1 (3650 — Multilayer Switch)
• Provides SVIs and inter-VLAN routing
• PC0 — Intended VLAN 10 client
• PC1 — Intended VLAN 20 client

Connections

• PC0 → Switch1 Fa0/1
• PC1 → Switch1 Fa0/2
• Switch1 Gi0/1 → Router1

Observed Symptoms

Issue 1: PC0 (VLAN 10)

• Set to DHCP
• Fails to receive a valid IP
• ipconfig shows: No IP address (IPIPA Address) and no gateway
• Cannot ping: 10.1.10.1 or 1.1.1.1

Step 1: Verify Access Port VLAN (Layer 2)

On Switch1: show vlan brief

Confirm: PC0’s access port (Fa0/1) is assigned to VLAN 10

Step 2: Verify VLAN 10 SVI Status (Layer 3)

On Switch1: show ip interface brief

Confirm:

• vlan10 is up/up
• IP address is 10.1.10.0

Step 3: Check DHCP Relay Configuration

On Switch1: show running-config | section interface Vlan10

Verify whether the following command is present: ip helper-address 10.1.1.1

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Finding

• ip helper-address is missing on VLAN 10

Fix:

On Switch1:

• Add the DHCP relay back to VLAN 10 SVI
• Add the commands below to the Switch:
• interface vlan10
• ip address 10.1.10.1

Issue 2: PC1 (VLAN 20)

(Please resolve Issue 1 before proceeding to Issue 2.
If Issue 1 is not fixed first, the symptoms observed in Issue 2 will not accurately reflect the intended scenario.)

• Set to DHCP
• Receives an IP, but it’s from the WRONG subnet
• ipconfig shows: 10.1.10.x (VLAN 10 range)
• Gateway 10.1.10.1 (not VLAN 20 gateway)
• Cannot ping: 10.1.20.1 (expected gateway)

Step 1: Verify PC1’s IP and Gateway (Start at the endpoint)

On PC1:

• Confirm IPv4 is set to DHCP
• Run: ipconfig

Expected symptom (after Issue 1 is fixed):

• IP address is 10.1.10.x (wrong subnet)
• Default gateway is 10.1.10.1 (wrong gateway)

Key: PC1 is not actually in VLAN 20.

Step 2: Test the expected gateway

On PC1: ping 10.1.20.1

Expected: Ping Fails

Step 3: Verify VLAN membership of PC1’s switchport (Layer 2 check)

On Switch1: show vlan brief

Check:

• Which VLAN is Fa0/2 assigned to?

Expected finding:

• Fa0/2 is in VLAN 10 (incorrect)

Issue identified, wrong access VLAN on the port connected to PC1.

Fix:

On Switch1 (run the commands below):

conf t
interface fa0/2
 no switchport access vlan 20
 switchport access vlan 10

Step 4: Renew DHCP and re-test

On PC1:

• Renew DHCP (toggle DHCP off/on in Packet Tracer)
• Run: ipconfig and ping 10.1.20.1
• PC1 should now receive an IP in 10.1.20.x
• Default gateway should be 10.1.20.1
• PC1 can now ping 10.1.20.1