CCNA 2 v6.0 Study Material – Chapter 8: DHCP

Chapter 8 – Sections & Objectives

8.1 DHCPv4
- Explain how DHCPv4 operates in a small- to medium-sized business network.
- Configure a router as a DHCPv4 server.
- Configure a router as a DHCPv4 client.
- Troubleshoot a DHCP configuration for IPv4 in a switched network.
8.2 DHCPv6
- Explain the operation of DHCPv6.
- Configure stateless DHCPv6 for a small to medium-sized business.
- Configure stateful DHCPv6 for a small to medium-sized business.
- Troubleshoot a DHCP configuration for IPv6 in a switched network.

DHCPv4:
- assigns IPv4 addresses and other network configuration information dynamically
- useful and timesaving tool for network administrators
- dynamically assigns, or leases, an IPv4 address from a pool of addresses
A Cisco router can be configured to provide DHCPv4 services.
Administrators configure DHCPv4 servers so that leases expire. Then the client must ask for another address, although the client is typically reassigned the same address.

A Cisco router running the Cisco IOS software can be configured to act as a DHCPv4 server. To set up DHCP:
- Exclude addresses from the pool
- Set up the DHCP pool name.
- Define the range of addresses and subnet mask. Use the default-router command for the default gateway. Optional parameters that can be included in the pool – dns server, domain-name.
To disable DHCP, use the no service dhcp command.

Commands to verify DHCP:
- show running-config | section dhcp
- show ip dhcp binding
- show ip dhcp server statistics
On the PC, issue the ipconfig /all command.

Using an IP helper address enables a router to forward DHCPv4 broadcasts to the DHCPv4 server. Acting as a relay.

The figure shows an extended ACL permitting only packets with UDP destination ports of 67 or 68. These are the typical ports used by DHCPv4 clients and servers when sending DHCPv4 messages. The extended ACL is used with the debug ip packetcommand to display only DHCPv4 messages.

SLAAC uses ICMPv6 Router Solicitation and Router Advertisement messages to provide addressing and other configuration information that would normally be provided by a DHCP server:

A router must have IPv6 routing enabled before it can send RA messages: Router(config)# ipv6 unicast-routing

SLAAC is the default option on Cisco routers. Both the M flag and the O flag are set to 0 in the RA, as shown in the figure.

To modify the RA message sent on the interface of a router to indicate stateless DHCPv6, use the following command: Router(config-if)#ipv6 nd other-config-flag

This option is the most similar to DHCPv4. In this case, the RA message informs the client not to use the information in the RA message. All addressing information and configuration information must be obtained from a stateful DHCPv6 server. Router(config-if)# ipv6 nd managed-config-flag