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CCNA – DHCP Client And Relay

This lab will cover the topics 5.3.b DHCP Relay and 5.3.c Client from the Cisco Certified Network Associate (CCNA) blueprint. It will test your understanding and knowledge of configure DHCP Servers on Cisco IOS devices. Please use the initial configurations as a template for your lab utilizing whatever console means you have (GNS, Physical Gear, VIRL, etc).

CCNADHCPClientAndRelay


In this lab you configuration will need to be added to routers 2 and 3 to establish their correct IP addressing.

R2 – Ethernet0/1
Address: 12.1.2.2/24

R2- Ethernet0/0
Address 23.2.3.2/24

R3 Ethernet0/0
Address 23.2.3.3/24

Add Default routing on R3 to establish connectivity back to R2’s Ethernet 0/1 segment.

Configure a DHCP scope on R3 to provide IP addresses for R2s E0/1 segment excluding R2s address and using it as a default gateway.

Configure R2 to relay DHCP requests to R3 for address lease obtainment. R1’s Ethernet 0/1 interface should be set to use DHCP.

To begin we will configure R2’s ethernet interfaces with the indicated IP addresses.

Next we will configure R3’s Ethernet 0/0 interface and create the default routing towards R2.

As indicated in the directions we will exclude R2’s Ethernet 0/1 address from any created DHCP Pools and create a pool to supply addresses to the 12.1.2.0/24 subnet utilizing 12.1.2.2 as the default gateway. This configuration will be done on R3

With this configuration in place we will debug R1 with debug DHCP detail which will enable DHCP client messages. On R3 we will debug DHCP server packets to verify it’s DHCP pool functionality. After debugs are enable we will enable the R1 Ethernet 0/1 as a DHCP client.

I have copied the output of a single attempt from R1’s debug to obtain a DHCP lease. There is no output from R3 indicating that it never received a request for an address from a DHCP pool.

Not fix this issue we will add the necessary command for relaying DHCP requests to R2. This command is applied on the incoming interface for DHCP discovery messages. In the case of R2 it will be on the Ethernet0/1 interface.

Now that we have the helper-address in place we will bring up the R1 interface again with debugs running on all three routers.

R1’s debug is show below. We can see the router issues DHCP discover messages out it’s interface ultimately coming up with and address of 12.1.2.4 with a default gateway of 12.1.2.2.

On R2 with DHCP server debugs on we can see R2 setting the GIADDR value to the interface the DHCP Discovery came in on. This is relayed to the address listed in the helper-address configuration and is used to help identify the correct DHCP pool to pick address from.

On R3 we can see the DHCP Discover message being received and it is indicated that it came in through the relay address of R2’s interface. The DHCP server utilizing this information to select an address from a pool and send it back to the relaying router as unicast. R2 then sends it to the appropriate client and this process repeats through the DORA operation.

We can now use R1 to verify our configuration is successful. With everything in place we should see a default route achieved from the default-route command in the DHCP pool pointing to R2’s interface on Ethernet0/1. We can also ping R3’s 23.2.3.3 address.

There are no initial configurations for this topology. All you need is a blank router with two interfaces in two different LAN segments.

Matt Ouellette is a certified information technology professional residing in Southwest Michigan. His technology findings and advice can be found on his PacketPilot blog. Mr. Ouellette spent 4 years as an I.T. Technician before stepping into a Network Engineer role at Bronson Health Group. Since completing his Associates Degree in Network Administration Matt has taken a head on approach to career enrichment through obtaining credentials such as CCNP, CCNA Voice, MCSA: Server 2008, and VCP5. This passion for continued learning allows him to deliver up to date quality technical solutions.

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