8.1.4.4 Lab – Troubleshooting DHCPv4 Answers

8.1.4.4 Lab – Troubleshooting DHCPv4 (Instructor Version – Optional Lab)

Instructor Note: Red font color or gray highlights indicate text that appears in the instructor copy only. Optional activities are designed to enhance understanding and/or to provide additional practice.

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 1941 Integrated Services Routers (ISRs) with Cisco IOS Release 15.2(4)M3 (universalk9 image). The switches used are Cisco Catalyst 2960s with Cisco IOS Release 15.0(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.

R1(config)# router rip
R1(config-router)# version 2
R1(config-router)# network 192.168.0.0
R1(config-router)# network 192.168.1.0
R1(config-router)# no auto-summary
R1(config-router)# exit

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

R2(config)# router rip
R2(config-router)# version 2
R2(config-router)# network 192.168.0.0
R2(config-router)# default-information originate
R2(config-router)# no auto-summary
R2(config-router)# exit
R2(config)# ip route 0.0.0.0 0.0.0.0 209.165.200.225

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

ISP(config)# ip route 192.168.0.0 255.255.254.0 209.165.200.226

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

!interface GigabitEthernet0/0
! ip helper-address 192.168.0.254
interface GigabitEthernet0/1
 ip helper-address 192.168.0.253
! ip helper-address 192.168.0.254

Router R2

ip dhcp excluded-address 192.168.11.1 192.168.11.9
!ip dhcp excluded-address 192.168.1.1 192.168.1.9
ip dhcp excluded-address 192.168.0.1 192.168.0.9
ip dhcp pool R1G1
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
ip dhcp pool R1G0
network 192.168.0.0 255.255.255.128
default-router 192.168.11.1
!default-router 192.168.0.1

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.

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.

R2# debug ip dhcp server events

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

R1# show run interface g0/1
interface GigabitEthernet0/1
 ip address 192.168.1.1 255.255.255.0
 ip helper-address 192.168.0.253
 duplex auto
 speed auto

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:

R1(config)# interface g0/1
R1(config-if)# no ip helper-address 192.168.0.253
R1(config-if)# ip helper-address 192.168.0.254

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.

*Mar 5 06:32:16.939: DHCPD: Sending notification of DISCOVER:
*Mar 5 06:32:16.939: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:32:16.939: DHCPD: circuit id 00000000
*Mar 5 06:32:16.939: DHCPD: Seeing if there is an internally specified pool class:
*Mar 5 06:32:16.939: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:32:16.939: DHCPD: circuit id 00000000
*Mar 5 06:32:16.943: DHCPD: Allocated binding 2944C764
*Mar 5 06:32:16.943: DHCPD: Adding binding to radix tree (192.168.1.1)
*Mar 5 06:32:16.943: DHCPD: Adding binding to hash tree
*Mar 5 06:32:16.943: DHCPD: assigned IP address 192.168.1.1 to client
0100.5056.be76.8c.
*Mar 5 06:32:16.951: %DHCPD-4-PING_CONFLICT: DHCP address conflict: server pinged
192.168.1.1.
*Mar 5 06:32:16.951: DHCPD: returned 192.168.1.1 to address pool R1G1.
*Mar 5 06:32:16.951: DHCPD: Sending notification of DISCOVER:
*Mar 5 06:32:16.951: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:32:16.951: DHCPD: circuit id 00000000
*Mar 5 06:32:1
R2#6.951: DHCPD: Seeing if there is an internally specified pool class:
*Mar 5 06:32:16.951: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:32:16.951: DHCPD: circuit id 00000000
*Mar 5 06:32:16.951: DHCPD: Allocated binding 31DC93C8
*Mar 5 06:32:16.951: DHCPD: Adding binding to radix tree (192.168.1.2)
*Mar 5 06:32:16.951: DHCPD: Adding binding to hash tree
*Mar 5 06:32:16.951: DHCPD: assigned IP address 192.168.1.2 to client
0100.5056.be76.8c.
*Mar 5 06:32:18.383: %DHCPD-4-PING_CONFLICT: DHCP address conflict: server pinged
192.168.1.2.
*Mar 5 06:32:18.383: DHCPD: returned 192.168.1.2 to address pool R1G1.
*Mar 5 06:32:18.383: DHCPD: Sending notification of DISCOVER:
*Mar 5 06:32:18.383: DHCPD: htype 1 chaddr 0050.56be.6c89
*Mar 5 06:32:18.383: DHCPD: circuit id 00000000
*Mar 5 06:32:18.383: DHCPD: Seeing if there is an internally specified pool class:
*Mar 5 06:32:18.383: DHCPD: htype 1 chaddr 0050.56be.6c89
*Mar 5 06:32:18.383: DHCPD: circuit id 00000000
*Mar 5 06:32:18.383: DHCPD: Allocated binding 2A40E074
*Mar 5 06:32:18.383: DHCPD: Adding binding to radix tree (192.168.1.3)
*Mar 5 06:32:18.383: DHCPD: Adding binding to hash tree
*Mar 5 06:32:18.383: DHCPD: assigned IP address 192.168.1.3 to client
0100.5056.be76.8c.
<output omitted>

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.

R2# show run | section dhcp
ip dhcp excluded-address 192.168.11.1 192.168.11.9
ip dhcp excluded-address 192.168.0.1 192.168.0.9
ip dhcp pool R1G1
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.1
ip dhcp pool R1G0
 network 192.168.0.0 255.255.255.128
 default-router 192.168.1.1

Record the commands to resolve the issue on R2.
______________________________________________________

R2(config)# no ip dhcp excluded-address 192.168.11.1 192.168.11.9
R2(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.9

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.

*Mar 5 06:49:59.563: DHCPD: Sending notification of TERMINATION:
*Mar 5 06:49:59.563: DHCPD: address 192.168.1.3 mask 255.255.255.0
*Mar 5 06:49:59.563: DHCPD: reason flags: RELEASE
*Mar 5 06:49:59.563: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:49:59.563: DHCPD: lease time remaining (secs) = 85340
*Mar 5 06:49:59.563: DHCPD: returned 192.168.1.3 to address pool R1G1.

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.

*Mar 5 06:50:11.863: DHCPD: Sending notification of DISCOVER:
*Mar 5 06:50:11.863: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:50:11.863: DHCPD: circuit id 00000000
*Mar 5 06:50:11.863: DHCPD: Seeing if there is an internally specified pool class:
*Mar 5 06:50:11.863: DHCPD: htype 1 chaddr 0050.56be.768c
*Mar 5 06:50:11.863: DHCPD: circuit id 00000000
*Mar 5 06:50:11.863: DHCPD: requested address 192.168.1.3 has already been assigned.
*Mar 5 06:50:11.863: DHCPD: Allocated binding 3003018C
*Mar 5 06:50:11.863: DHCPD: Adding binding to radix tree (192.168.1.10)
*Mar 5 06:50:11.863: DHCPD: Adding binding to hash tree
*Mar 5 06:50:11.863: DHCPD: assigned IP address 192.168.1.10 to client
0100.5056.be76.8c.
<output omitted>

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.

R1# show run interface g0/0
interface GigabitEthernet0/0
 ip address 192.168.0.1 255.255.255.128
 duplex auto
 speed auto

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:

R1(config)# interface g0/0
R1(config-if)# ip helper-address 192.168.0.254

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.

*Mar 5 07:15:09.663: DHCPD: Sending notification of DISCOVER:
*Mar 5 07:15:09.663: DHCPD: htype 1 chaddr 0050.56be.f6db
*Mar 5 07:15:09.663: DHCPD: circuit id 00000000
*Mar 5 07:15:09.663: DHCPD: Seeing if there is an internally specified pool class:
*Mar 5 07:15:09.663: DHCPD: htype 1 chaddr 0050.56be.f6db
*Mar 5 07:15:09.663: DHCPD: circuit id 00000000
*Mar 5 07:15:09.707: DHCPD: Sending notification of ASSIGNMENT:
*Mar 5 07:15:09.707: DHCPD: address 192.168.0.10 mask 255.255.255.128
*Mar 5 07:15:09.707: DHCPD: htype 1 chaddr 0050.56be.f6db
*Mar 5 07:15:09.707: DHCPD: lease time remaining (secs) = 86400

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.

R2(config)# ip dhcp pool R1G0
R2(dhcp-config)# default-router 192.168.0.1

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.

R2# undebug all
All possible debugging has been turned off

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

Device Configs

Router R1 (Corrected)

R1# show run
Building configuration...
Current configuration : 1419 bytes
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R1
!
boot-start-marker
boot-end-marker
!
!
enable secret 4 06YFDUHH61wAE/kLkDq9BGho1QM5EnRtoyr8cHAUg.2
!
no aaa new-model
memory-size iomem 15
!
!
!
!
!
!
!
no ip domain lookup
ip cef
no ipv6 cef
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
interface Embedded-Service-Engine0/0
no ip address
shutdown
!
interface GigabitEthernet0/0
ip address 192.168.0.1 255.255.255.128
ip helper-address 192.168.0.254
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 192.168.1.1 255.255.255.0
ip helper-address 192.168.0.254
duplex auto
speed auto
!
interface Serial0/0/0
ip address 192.168.0.253 255.255.255.252
clock rate 128000
!
interface Serial0/0/1
no ip address
shutdown
!
!
router rip
version 2
network 192.168.0.0
network 192.168.1.0
no auto-summary
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
!
!
!
!
control-plane
!
!
!
line con 0
password cisco
logging synchronous
login
line aux 0
line 2
no activation-character
no exec
transport preferred none
transport input all
transport output pad telnet rlogin lapb-ta mop udptn v120 ssh
stopbits 1
line vty 0 4
password cisco
login
transport input all
!
scheduler allocate 20000 1000
!
end

Router R2 (Corrected)

R2# show run
Building configuration...
Current configuration : 1552 bytes
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname R2
!
boot-start-marker
boot-end-marker
!
!
enable secret 4 06YFDUHH61wAE/kLkDq9BGho1QM5EnRtoyr8cHAUg.2
!
no aaa new-model
memory-size iomem 15
!
!
!
!
ip dhcp excluded-address 192.168.0.1 192.168.0.9
ip dhcp excluded-address 192.168.1.1 192.168.1.9
!
ip dhcp pool R1G1
network 192.168.1.0 255.255.255.0
default-router 192.168.1.1
!
ip dhcp pool R1G0
network 192.168.0.0 255.255.255.128
default-router 192.168.0.1
!
!
!
no ip domain lookup
ip cef
no ipv6 cef
multilink bundle-name authenticated
!
!
!
license udi pid CISCO1941/K9 sn FTX163283R9
license accept end user agreement
!
!
!
!
!
!
!
!
interface Embedded-Service-Engine0/0
no ip address
shutdown
!
interface GigabitEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface GigabitEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0/0
ip address 192.168.0.254 255.255.255.252
!
interface Serial0/0/1
ip address 209.165.200.226 255.255.255.252
clock rate 128000
!
!
router rip
version 2
network 192.168.0.0
default-information originate
no auto-summary
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
ip route 0.0.0.0 0.0.0.0 209.165.200.225
!
!
!
!
control-plane
!
!
!
line con 0
password cisco
logging synchronous
login
line aux 0
line 2
no activation-character
no exec
transport preferred none
transport input all
transport output pad telnet rlogin lapb-ta mop udptn v120 ssh
stopbits 1
line vty 0 4
password cisco
login
transport input all
!
scheduler allocate 20000 1000
!
end

Router ISP 

ISP#show run
Building configuration...
Current configuration : 1247 bytes
!
version 15.2
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption
!
hostname ISP
!
boot-start-marker
boot-end-marker
!
!
enable secret 4 06YFDUHH61wAE/kLkDq9BGho1QM5EnRtoyr8cHAUg.2
!
no aaa new-model
memory-size iomem 10
!
!
!
!
!
!
!
no ip domain lookup
ip cef
no ipv6 cef
multilink bundle-name authenticated
!
!
!
!
!
!
!
!
!
!
!
interface Embedded-Service-Engine0/0
no ip address
shutdown
!
interface GigabitEthernet0/0
no ip address
shutdown
duplex auto
speed auto
!
interface GigabitEthernet0/1
no ip address
shutdown
duplex auto
speed auto
!
interface Serial0/0/0
no ip address
shutdown
clock rate 2000000
!
interface Serial0/0/1
ip address 209.165.200.225 255.255.255.252
!
ip forward-protocol nd
!
no ip http server
no ip http secure-server
!
ip route 192.168.0.0 255.255.254.0 209.165.200.226
!
!
!
!
control-plane
!
!
!
line con 0
password cisco
logging synchronous
login
line aux 0
line 2
no activation-character
no exec
transport preferred none
transport input all
transport output pad telnet rlogin lapb-ta mop udptn v120 ssh
stopbits 1
line vty 0 4
password cisco
login
transport input all
!
scheduler allocate 20000 1000
!
end