How to configure DHCP on a Cisco router
How to configure DHCP on a Cisco router
The DHCP (Dynamic Host Configuration
Protocol) protocol is a feature to provide machines that require a full IP,
IPv4 address, subnet mask, default gateway, dns server... This article is about How to configure DHCP on a Cisco router
Topology for how to configure DHCP on a Cisco router
Topology for how to configure DHCP on a Cisco router
 |
| DHCP Topology |
Here, we will
perform the configuration of R1, access to
network internet
and DHCP router,
router acting as DHCP server.
Course DHCP services could be configured on R1, but
for didactic reasons I chose
to separate this functionality. understand you why later in
the article.
Initial configuration
In order to
avoid finishing with a several
page long article, I will
put here only
the necessary configuration. The default settings are not included.
On
ISP
hostname
ISP
!
interface
Loopback0
ip address 8.8.8.8 255.255.255.255
!
interface
Serial0/0
ip address 80.100.200.1 255.255.255.252
clock rate 2000000
On R1
interface FastEthernet0/0
ip address 192.168.0.1 255.255.255.0
ip nat inside
ip virtual-reassembly
duplex auto
speed auto
!
interface Serial0/0
ip address 80.100.200.2 255.255.255.252
ip nat outside
ip virtual-reassembly
clock rate 2000000
!
interface FastEthernet0/1
ip address 192.168.1.1 255.255.255.0
ip nat inside
ip virtual-reassembly
duplex auto
speed auto
!
ip route 0.0.0.0 0.0.0.0 Serial0/0
!
ip nat inside source list 1 interface Serial0/0 overload
!
access-list 1 permit any
On DHCP
hostname DHCP
!
interface FastEthernet0/0
ip address 192.168.1.5 255.255.255.0
duplex auto
speed auto
!
ip route 0.0.0.0 0.0.0.0 192.168.1.1
Attention, the default route on DHCP
is required for the rest of the Setup, I'll put in a word more far.
Reminder
The DHCP protocol
is conveyed directly in a
frame. More machine that emits a DHCP
request, with still no identity
(address IPv4 etc) has another
opportunity to communicate through
broadcast. This means that the
DHCP request from this machine
is a broadcast
frame.
This may seem unimportant, but do not forget that a broadcast is confined in his broadcast domain, in other words, it does not exceed a router. Out, here, between the DHCP server and the LAN1 there are R1... a router!
We will have to eventually ensure that R1 is configured to relay the DHCP request to the DHCP server.
This may seem unimportant, but do not forget that a broadcast is confined in his broadcast domain, in other words, it does not exceed a router. Out, here, between the DHCP server and the LAN1 there are R1... a router!
We will have to eventually ensure that R1 is configured to relay the DHCP request to the DHCP server.
Configuration method
So that DHCP
can hand out
addresses both the LAN1 and
LAN2 the we
have to configure
the following
- Exclude addresses that we desire not to distribute (that of the gateway, or of machines with fixed addresses)
- for each LAN configure a DHCP pool in which we will specify the options (gateway, DNS server, network address, …)
- Configure the DHCP Relay for LAN1, on R1.
The configuration
Good practice is to configure the
DHCP server in order that may seem illogical at first. What the question is
that once the DHCP pool is configured, the router will begin to assign
addresses. So it is advisable to first configure options (addresses to exclude
the pool settings... all before you set the address range to take in charge),
otherwise the first received by the machine configurations may incomplete.
Let's start with the router DHCP...
Exclusion of necessary addresses (address of gateways, DHCP server...) here we will exclude the first ten of each LAN (arbitrary choice).
Let's start with the router DHCP...
Exclusion of necessary addresses (address of gateways, DHCP server...) here we will exclude the first ten of each LAN (arbitrary choice).
DHCP(config)#ip dhcp excluded-address 192.168.0.1 192.168.0.10
DHCP(config)#ip dhcp excluded-address 192.168.1.1 192.168.1.10
Attention in the definition of the
addresses to be excluded, the first address is the start of the range and the
second end of the range to exclude. If you don't want that to exclude an
address, simply do not give end address.
Now let's configure the DHCP for the LAN1 pool...
Now let's configure the DHCP for the LAN1 pool...
DHCP(config)#ip dhcp pool LAN1
DHCP(dhcp-config)#default-router 192.168.0.1
DHCP(dhcp-config)#dns-server 8.8.8.8
DHCP(dhcp-config)#network 192.168.0.0 255.255.255.0
DHCP(dhcp-config)#exit
So they created here a DHCP pool
named LAN1, in which machines will receive as default gateway 192.168.0.1
(address of R1 in the LAN1), a DNS 8.8.8.8 (completely fictional here, but it
seemed essential to show how to configure)... and this configuration will be
valid for the network 192.168.0.0/24 (address range to be distributed to
subtract previously excluded addresses).
Do the same for the LAN2...
DHCP(config)#ip dhcp pool LAN2
DHCP(dhcp-config)#default-router 192.168.1.1
DHCP(dhcp-config)#dns-server 8.8.8.8
DHCP(dhcp-config)#network 192.168.1.0 255.255.255.0
DHCP(dhcp-config)#exit
Now the machines of the LAN2 must
properly receive their configuration because the DHCP server is in the same
domain that they broadcast.
Check on C2...
Check on C2...
VPCS[2]> ip dhcp
DDORA IP 192.168.1.11/24 GW 192.168.1.1
The DHCP server should also have
kept this DHCP lease in memory...
DHCP#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.1.11 0100.5079.6668.01 Mar 02 2002 12:34 AM Automatic
DHCP#
On the other hand... impossible for
C1 to receive its configuration...
VPCS[1]> ip dhcp
DDD
Can't find dhcp server
Indeed, when R1 receives the plot of
broadcast containing the query DHCP of C1, because he cannot do anything, he
"throws it' purely and simply.
We need to configure the relay of the DHCP frames to the DHCP server. For this we need to explicitly tell R1 to relay these frames to 192.168.1.5 (DHCP server) when they enter the Fa0/0 interface. Here is the manipulation:
We need to configure the relay of the DHCP frames to the DHCP server. For this we need to explicitly tell R1 to relay these frames to 192.168.1.5 (DHCP server) when they enter the Fa0/0 interface. Here is the manipulation:
R1(config)#interface fastEthernet 0/0
R1(config-if)#ip helper-address 192.168.1.5
Note that the command 'ip
helper-address' has more utility than the simple relay DHCP requests. It is
used to relay the broadcasts several protocols (DHCP, NTP, Netbios...) UDP.
C1 can now also get its configuration by DHCP
C1 can now also get its configuration by DHCP
VPCS[1]> ip dhcp
DDORA IP 192.168.0.11/24 GW 192.168.0.1
DHCP must also have withheld
information about the DHCP lease...
DHCP#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
192.168.0.11 0100.5079.6668.00 Mar 02 2002 01:27 AM Automatic
192.168.1.11 0100.5079.6668.01 Mar 02 2002 12:34 AM Automatic
DHCP#
Some further explanation...
How Does DHCP to know in what pool
of address it must draw in response to a query from the LAN1 or LAN2?
In the case of the LAN2, the request arrives
in the form of a broadcast, on the Ethernet interface of DHCP, it is sufficient
to provide a configuration for a machine inthe same broadcast domain that him
therefore the pool LAN2.
By contrast, in the case of the LAN1, the
request arrives to DHCP in the form of unicast, because it is relayed by R1. R1
has in reality, built a unicast packet, with as the source address that of its
interface in the LAN1 and as the destination of the DHCP server.
Since then when DHCP receives
the unicast request, it offers
an address from the pool
which the address range corresponds to that configured on interface Fa0/0 of R1.
A few settings and more...
Of course the DHCP configuration is
not limited to an address, mask, gateway, and a DNS server. Here are a few
other configurable parameters...
Set the domain name that will be associated with the interface receiving the configuration...
Set the domain name that will be associated with the interface receiving the configuration...
DHCP(config)#ip dhcp pool LAN1
DHCP(dhcp-config)#domain-name test.lab
Configure the DHCP lease...
DHCP(dhcp-config)#lease 5 10 30
The format is "lease . You can
also set an unlimited lease by using "infinite lease.
It is also possible to configure options 'raw '. These are specific parameters with no name. But the syntax varies depending on the data to be provided.
For example, should fill a TFTP server address so that a machine can pick up a boot image (this is the case in with deployment network systems), must indicate the option 150, with as parameter the ip address of the TFTP server.
DHCP(dhcp-config) #option 150 ip 192.168.1.50
It is also possible to configure options 'raw '. These are specific parameters with no name. But the syntax varies depending on the data to be provided.
For example, should fill a TFTP server address so that a machine can pick up a boot image (this is the case in with deployment network systems), must indicate the option 150, with as parameter the ip address of the TFTP server.
DHCP(dhcp-config) #option 150 ip 192.168.1.50
Yet a check...
We have seen the 'show ip dhcp
binding' command that displays a list of the current leases. It is also
possible to display the DHCP pools and their statistics...
DHCP#sh ip dhcp pool LAN1
Pool LAN1 :
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 254
Leased addresses : 1
Pending event : none
1 subnet is currently in the pool :
Current index IP address range Leased addresses
192.168.0.12 192.168.0.1 - 192.168.0.254 1
DHCP#
That's what set up a functional DHCP
server.
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