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In this lab we'll start off with the simple topology of 2 routers

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router 1 and router 2

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what we'll do here, is configure router 2 as the DHCP server

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and we'll configure router 1 as the DHCP client in this topology.

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No configuration has been done on this routers.

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At the moment as an example, on router 2, sh ip int brief

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shows me that all the interfaces are currently down

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no IP addresses has been configured.

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So the first thing I’m gonna do is go on to the FastEthernet interface

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and configure an IP address of 10.1.1.2/24 mask.

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I'll no shut that interface and now I'll configure the router as the DHCP server.

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So to configure the DHCP server, the first thing you need to do is type ip dhcp

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that gives you a range of options but the one that we're looking for is pool

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so pool, and then we specify a name such as clients, as an example.

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This is just the name, to uniquely identify the pool that you're creating

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and that’s because you can have multiple pools configured on a router.

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Network, we need to specify the network that devices will be allocated addresses from

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now it's important that the IP address that DHCP requests are being received on.

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Which in this example a F0/0 be in the same subnet as the network.

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Otherwise the router is not gonna allocate addresses from the pool.

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So this interface is configured with an IP address of 10.1.1.2

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and we are configuring a network of 10.1.1.0/24

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there are many options available in the pool.

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We’ve already configured the network and mask.

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Next option, you'll probably want to configure is default gateway or default router.

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I’m gonna set that to router 2.

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You'd also want to specify options such as DNS server.

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In this example, I'll configure the router as DNS server

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but typically what your going to do here

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is configure a real DNS server somewhere in your topology.

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So I'll just configure a single DNS server being the local router.

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You can specify how long addresses are allocated to hosts, the default is 1 day

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I’m gonna specify lease time of 7 days.

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Now for devices such as IP phones, you can specify different DHCP option codes.

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150 is TFTP and I’m gonna specify an IP address of a TFTP server.

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Now in the real implementation, your IP telephones will get their configuration

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typically from a Cisco Unified Communications Manager

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or from a Cisco Unified Communications Manager's express router.

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So this would be the IP address of the Cisco communications manager.

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In our example once again, I’m just gonna set that to the local router.

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Now to stop conflicts, an allocation of addresses that belong to servers and routers

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Use the command   ip dhcp excluded-address

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and you specify a range of addresses to exclude from allocation to devices.

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Now my router has an IP address of 10.1.1.2

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I don’t want the router to allocate its own IP address to host devices.

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So I’m going to set the exclusion range to include the router's IP address

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and some extra IP addresses here, that would typically be determined

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by how many servers or other devices are going to have IP addresses statically configured.

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sh run    will show me my configuration.

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So it’s quite a simple config, I’ve got my exclusion range

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I’ve created a pool called clients.

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Clients will be allocated addresses in this IP address subnet.

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Default router is router 2, the local router.

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DNS server is set to that and option 150 is set to that as well.

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Once again typically, this will be set to servers in your live network.

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I’ve set the lease to 7 days, the default is 1 day.

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show ip dhcp   gives me different options, including pool.

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This can be useful because sometimes you can make a typing mistake

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and you may exclude all of your addresses.

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But notice here, total addresses is 254, lease addresses is 0

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no addresses have been leased, address range is in this range.

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Let’s look at bindings, of any address has been allocated

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at the moment no addresses has been allocated.

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So let’s go to the client

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I’m gonna change the MAC address of this client, to something very simple

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so that we can read it in the output

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so MAC address is gonna be A,B,C

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IP address is gonna be set to DHCP

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so the PC will request an IP address from the server

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when I type no shut   show debug

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lets run a debug so we can see what’s happening

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debug ip dhcp server    and we have different options

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I’m gonna go for packet, which shows me lots of detail.

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So debug ip dhcp server packet    now when I hit no shut

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we should start seeing requests from the client, to the DHCP server and there we go

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we've got a DHCP discover message, received from the client on interface FastEthernet 0/0

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The DHCP server will allocate an address without class information

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we didn’t configure a name or class for this device.

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The DHCP servers offering an address to the client, offering it this IP address

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and then sending the reply to the MAC address of the PC that we configured, so that MAC address.

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The client then requests that IP address from the DHCP server

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DHCP server sents back an acknowledgement.

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So sh ip int brief FastEthernet 0/0 on router 1

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has been allocated to this address through DHCP

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sh ip route   the router has learnt about the gateway of last resort

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because we've configured a default gateway or default router in the DHCP pool.

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So router 1 has learnt about that network

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and a static default router has been added to the routing table.

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It’s a simple as that to configure a basic DHCP server on the Cisco router

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and allocate addresses to client devices.

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to actually view the messages sent between the client and the server.