Configuring DNS and DHCP

Chapter 20

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Chapter ObjectivesAt the end of this Chapter you will be

able to:Configure, verify and troubleshoot DHCP and DNS operation on a router. What is CDP and how to use it.Describe the technological requirements for running IPv6 in conjunction with IPv4

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Dynamic Host Configuration Protocol - Basics

The Dynamic Host Configuration Protocol (DHCP) allows devices to dynamically acquire their addressing information. DHCP is actually based on BOOTP. It is built on a client/server model and defines two components:

Server: Delivering host configuration information.Client: Requesting and acquiring host configuration information.

DHCP provides the following advantages:It reduces the amount of configuration on devices.It reduces likelihood of configuration errors.It gives you more control by centralizing IP addressing information.

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When acquiring addressing information, a DHCP client goes through four steps:

A client generates a DHCPDISCOVER broadcast to discover who the DHCP servers are on the LAN segment.

All DHCP servers on the segment can respond to the client with a DHCPOFFER unicast message, which offers IP addressing information to the client. If a client receives messages from multiple servers, it chooses one (typically the first one).

Upon choosing one of the offers, the client responds to the corresponding server with a DHCPREQUEST message, telling the server that it wants to use the addressing information the server sent. If there is only one server and the server’s information conflicts with the client’s configuration, the client will respond with a DHCPDECLINE message.

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The DHCP server responds with a DHCPACK, which is an acknowledgment to the client indicating that it received the DHCPREQUEST message and that the client accepted the addressing information.

DHCP Configurationpowered by DJ 5

Here’s the configuration of Router acting as a DHCP server:

Router(config)# ip dhcp pool dhcppoolRouter(config-dhcp)# network 192.168.1.0 255.255.255.0Router(config-dhcp)# domain-name thisnetwork.comRouter(config-dhcp)# dns-server 192.168.1.2Router(config-dhcp)# default-router 192.168.1.1Router(config-dhcp)# lease 5Router(config-dhcp)# exitRouter(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.2

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DHCP Client Configuration:You can configure an interface on your IOS router to use DHCP to acquire its IP address.

Here is the command to set up a DHCP client on your router:Router(config)# interface type [slot_#/]port_#Router(config-if)# ip address dhcp

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Configure Internal DHCP Server on a Router through SDM

On the SDM configure mode window, select Additional Tasks under the Tasks column that is on the left side of the window.powered by DJ 8

On the Additional Tasks page, expand the DHCP tree and choose DHCP Pools as shown in this example. In the DHCP Pools column shown on the right side of this page, click Add to create a new DHCP pool.

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On the Add DHCP Pool page, specify the DHCP Pool Name, DHCP Pool Network, Subnet mask, Starting IP address, Ending IP address and Default Router parameters as shown in this example:And Click OK.

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Resolving HostnamesIdentifying Methods to Resolve Hostname To use a hostname rather than an IP address to connect to a remote device, the device that you are using to make the connection must be able to translate the hostname to an IP address.

There are two ways to resolve hostnames to IP addresses: Building a host table on each router.Building a Domain Name System (DNS) server, which is similar to a dynamic host table

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Cisco Discovery Protocol

What is Cisco Discovery Protocol?The Cisco Discovery Protocol (CDP) is a proprietary Layer 2 network Protocol developed by Cisco Systems which runs on most Cisco equipment and is used to share information about other directly connected Cisco equipment such as the operating System version and IP addresses.

How CDP Works?Cisco devices send CDP announcements to the multicast destination address 01-00-0c-cc-cc-cc (which is also used for other Cisco proprietary protocols such as VTP). CDP announcements (if supported and configured in IOS) are sent by default every 60 seconds on interfaces which support Sub network access Protocol (SNAP) headers, including Ethernet, Frame Relay and ATM. Each Cisco device that supports CDP stores the information received from other devices in a table which can be viewed using the show cdp neighbors command.

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Field Description

Deice ID The hostname of the device directly connected.Local Interface The port or interface on which you are receiving the

CDP packet.Holdtime T The amount of time the router will hold the

information before discarding it if no more CDP packets are received.

Capability The neighbor’s capability, such as router, switch, or repeater. The capability codes are listed at the top of the command output.

Platform The type of Cisco device. In the above output, a Cisco 2509, Cisco 2511, and Catalyst 5000 are attached to the switch. The 2509 sees only the switch and the 2501 router connected through its serial 0 interface.

Port ID The neighbor device’s port or interface on which the CDP packets are multicast.

Output of the show cdp neighbor Command

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Internet Protocol Version 6 (IPv6) Why the Need for Ipv6?IPv4 cannot address the Internet’s exponential and continual growth. Issues raised from RFC1380:

Class B network exhaustion Routing Table Explosion Limited security Need for new data flows

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Benefits of IPv6:

Scale: The new protocol will scale to support 1012 hosts (1 Trillion) and 109 individual networks (1 Billion)

Performance: Commercially available routers must be able to process/forward IPv6 traffic fast enough fully utilizing commonly available high-speed media.

Transition: The protocol must have a straightforward transition plan from IPv4.

Configuration, Administration, and Operation.

The protocol must support unicast, anycast and multicast packet transmission (broadcast is replaced by multicast)

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Service Classes: the protocol must allow the network to associate packets with particular service classes (QoS).

Mobility: the protocol must support mobile hosts (ICMPv6 and DHCPv6).

IPv6 Addressing and Expressions just as understanding how IP addresses are structured and used is critical with IPv4 addressing, it’s also vital when it comes to IPv6. You’ve already read about the fact that at 128 bits, an IPv6 address is much larger than an IPv4 address.

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IPv6 Routing Protocols

Most of the routing protocols we’ve already discussed have been upgraded for use in IPv6 networks. Also, many of the functions and configurations that we’ve already learned will be used in almost the same way as they’re used now. The routing protocols that we’ll still use in v6 got a new name and a facelift. Let’s talk about a few of them now.

RIPng

EIGRPv6

OSPFv3

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THANK YOU

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