CCNA RSE Lab: 8.1.4.4 Troubleshooting DHCPv4

CCNA RSE Lab: 8.1.4.4 Troubleshooting DHCPv4

Topology

Addressing Table

Objectives
Part 1: Build the Network and Configure Basic Device Settings
Part 2: Troubleshoot DHCPv4 Issues

Background / Scenario
The Dynamic Host Configuration Protocol (DHCP) is a network protocol that lets the network administrators manage and automate the assignment of IP addresses. Without DHCP, the administrator must manually assign and configure IP addresses, preferred DNS servers, and the default gateway. As the network grows in size, this becomes an administrative problem when devices are moved from one internal network to another.

In this scenario, the company has grown in size, and the network administrators can no longer assign IP addresses to devices manually. The R2 router has been configured as a DHCP server to assign IP addresses to the host devices on router R1 LANs. Several errors in the configuration have resulted in connectivity issues. You are asked to troubleshoot and correct the configuration errors and document your work. Ensure that the network supports the following:

  1. The router R2 should function as the DHCP server for the 192.168.0.0/25 and 192.168.1.0/24 networks connected to R1.
  2. All PCs connected to S1 and S2 should receive an IP address in the correct network via DHCP.

Note: The routers used with CCNA hands-on labs are Cisco 1 941 Integrated Services Routers (ISRs) with Cisco IOS Release 1 5.2(4)M3 (universalk9 image). The switches used are Cisco Catalyst 2960s with Cisco IOS Release 1 5.0(2) (lanbasek9 image). Other routers, switches and Cisco IOS versions can be used. Depending on the model and Cisco IOS version, the commands available and output produced might vary from what is shown in the labs. Refer to the Router Interface Summary Table at the end of this lab for the correct interface identifiers.

Note: Make sure that the routers and switches have been erased and have no startup configurations. If you are unsure, contact your instructor.

Instructor Note: Instructions for erasing the switches and routers are provided in the Lab Manual.

Required Resources

  • 3 Routers (Cisco 1941 with Cisco IOS Release 15.2(4)M3 universal image or comparable)
  • 2 Switches (Cisco 2960 with Cisco IOS Release 15.0(2) lanbasek9 image or comparable)
  • 2 PCs (Windows 7, Vista, or XP with terminal emulation program, such as Tera Term)
  • Console cables to configure the Cisco IOS devices via the console ports
  • Ethernet and serial cables as shown in the topology
Part 1: Build the Network and Configure Basic Device Settings

In Part 1, you will set up the network topology and configure the routers and switches with basic settings, such as passwords and IP addresses. You will also configure the IP settings for the PCs in the topology.

Step 1: Cable the network as shown in the topology.
Step 2: Initialize and reload the routers and switches.
Step 3: Configure basic settings for each router.
a. Disable DNS lookup.
b. Configure device name as shown in the topology.
c. Assign class as the privileged EXEC password.
d. Assign cisco as the console and vty passwords.
e. Configure logging synchronous to prevent console messages from interrupting command entry.
f. Configure the IP addresses for all the router interfaces.
g. Set clock rate to 128000 for all DCE router interfaces.
h. Configure RIP for R1 .

i. Configure RIP and a static default route on R2.

j. Configure a summary static route on ISP to the networks on R1 and R2 routers.

Step 4: Verify network connectivity between the routers.
If any pings between the routers fail, correct the errors before proceeding to the next step. Use show ip route and show ip interface brief to locate possible issues.

Step 5: Configure basic settings for each switch.
a. Disable DNS lookup.
b. Configure device name as shown in the topology.
c. Configure the IP address for the VLAN 1 interface and the default gateway for each switch.
d. Assign class as the privileged EXEC mode password.
e. Assign cisco as the console and vty passwords.
f. Configure logging synchronous for the console line.

Step 6: Verify the hosts are configured for DHCP.

Step 7: Load the initial DHCP configuration for R1 and R2.

Router R1

Router R2

Part 2: Troubleshoot DHCPv4 Issues

After configuring routers R1 and R2 with DHCPv4 settings, several errors in the DHCP configurations were introduced and resulted in connectivity issues. R2 is configured as a DHCP server. For both pools of DHCP addresses, the first nine addresses are reserved for the routers and switches. R1 relays the DHCP information to all the R1 LANs. Currently, PC-A and PC-B have no access to the network. Use the show and debug commands to determine and correct the network connectivity issues.

Step 1: Record IP settings for PC-A and PC-B.
a. For PC-A and PC-B, at the command prompt, enter ipconfig /all to display the IP and MAC addresses.

b. Record the IP and MAC addresses in the table below. The MAC address can be used to determine which PC is involved in the debug message.

IP Address/Subnet Mask

MAC Address
PC-A No IP address is assigned by DHCP. (Students may record an APIPA address starting with 169.254.x.x. This is a private local address that Microsoft OS assigns when the host cannot reach a DHCP server to obtain an IP address.) 0050:56BE:768C
PC-B No IP address is assigned by DHCP. (Students may record an APIPA address starting with 169.254.x.x. This is a private local address that Microsoft OS assigns when the host cannot reach a DHCP server to obtain an IP address.) 0050:56BE:F6DB

Step 2: Troubleshoot DHCP issues for the 192.168.1.0/24 network on router R1.
Router R1 is a DHCP relay agent for all the R1 LANs. In this step, only the DHCP process for the 192.168.1.0/24 network will be examined. The first nine addresses are reserved for other network devices, such as routers, switches, and servers.

a. Use a DHCP debug command to observe the DHCP process on R2 router.

b. On R1, display the running configuration for the G0/1 interface.

If there are any DHCP relay issues, record any commands that are necessary to correct the configurations errors.
DHCP relay was incorrectly configured for G0/1 interface. The command ip helper-address 192.168.0.254 needs to be added to the R1 router. The incorrect helper address should be removed from the configuration. The issue can be resolved using the following commands:

c. In a command prompt on PC-A, type ipconfig /renew to receive an address from the DHCP server.
Record the configured IP address, subnet mask, and default gateway for PC-A.

IP address: 192.168.1.3, subnet mask: 255.255.255.0, default gateway: 192.168.1.1

d. Observe the debug messages on R2 router for the DHCP renewal process for PC-A. The DHCP server attempted to assign 192.168.1.1/24 to PC-A. This address is already in use for G0/1 interface on R1 . The same issue occurs with IP address 192.168.1.2/24 because this address has been assigned to S1 in the initial configuration. Therefore, an IP address of 192.168.1.3/24 has been assigned to PC-A. The DHCP assignment conflict indicates there may be an issue with the excluded-address statement on the DHCP server configuration on R2.

e. Display the DHCP server configuration on R2. The first nine addresses for 192.168.1.0/24 network are not excluded from the DHCP pool.

f. At the command prompt on PC-A, type ipconfig /release to return the 192.168.1.3 address back to the DHCP pool. The process can be observed in the debug message on R2.

g. At the command prompt on PC-A, type ipconfig /renew to be assigned a new IP address from the DHCP server. Record the assigned IP address and default gateway information. IP address/subnet mask: 192.168.1.10/24 Default gateway: 192.168.1.1
The process can be observed in the debug message on R2.

h. Verify network connectivity.

Can PC-A ping the assigned default gateway? ___________ Yes
Can PC-A ping the R2 router? ___________ Yes
Can PC-A ping the ISP router? ___________ Yes

Step 3: Troubleshoot DHCP issues for 192.168.0.0/25 network on R1 .
Router R1 is a DHCP relay agent for all the R1 LANs. In this step, only the DHCP process for the 192.168.0.0/25 network is examined. The first nine addresses are reserved for other network devices.

a. Use a DHCP debug command to observe the DHCP process on R2.

R2# debug ip dhcp server events

b. Display the running configuration for the G0/0 interface on R1 to identify possible DHCP issues.

Record the issues and any commands that are necessary to correct the configurations errors. DHCP relay was not configured on the R1 G0/0 interface. The issue can be resolved using the following commands:

c. From the command prompt on PC-B, type ipconfig /renew to receive an address from the DHCP server. Record the configured IP address, subnet mask, and default gateway for PC-B. IP address: 192.168.0.10, subnet mask: 255.255.255.128, default gateway: 192.168.11 .1
d. Observe the debug messages on R2 router for the renewal process for PC-A. The DHCP server assigned 192.168.0.10/25 to PC-B.

e. Verify network connectivity.

Can PC-B ping the DHCP assigned default gateway? ___________ No
Can PC-B ping its default gateway (192.168.0.1)? ___________ Yes
Can PC-B ping the R2 router? ___________ Yes
Can PC-B ping the ISP router? ___________ Yes

f. If any issues failed in Step e, record the problems and any commands to resolve the issues.
PC-B is unable to ping the DHCP assigned default gateway. The issue can be resolved using the following commands.

g. Release and renew the IP configurations on PC-B. Repeat Step e to verify network connectivity.
h. Discontinue the debug process by using the undebug all command.

Reflection
What are the benefits of using DHCP?
Answers will vary. DHCP can prevent address conflicts caused by a previous assigned IP address still in use, supply additional configuration values, such as a DNS server, and can be used with mobile or portable computers.

Router Interface Summary Table

Router Interface Summary

Router Model Ethernet Interface #1 Ethernet Interface #2 Serial Interface #1 Serial Interface #2
1800 Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
1900 Gigabit Ethernet 0/0
(G0/0)
Gigabit Ethernet 0/1
(G0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2801 Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/1 /0 (S0/1 /0) Serial 0/1 /1 (S0/1 /1)
2811 Fast Ethernet 0/0
(F0/0)
Fast Ethernet 0/1
(F0/1)
Serial 0/0/0 (S0/0/0) Serial 0/0/1 (S0/0/1)
2900 Gigabit Ethernet 0/0 (G0/0) Gigabit Ethernet 0/1 (G0/1) Serial 0/0/0(S0/0/0) Serial 0/0/1 (S0/0/1)
Note: To find out how the router is configured, look at the interfaces to identify the type of router and how many interfaces the router has. There is no way to effectively list all the combinations of configurations for each router class. This table includes identifiers for the possible combinations of Ethernet and Serial interfaces in the device. The table does not include any other type of interface, even though a specific router may contain one. An example of this might be an ISDN BRI interface. The string in parenthesis is the legal abbreviation that can be used in Cisco IOS commands to represent the interface.

Device Configs
Router R1 (Corrected)

Router R2 (Corrected)

Router ISP

More Resources

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