Configuring a LAN with DHCP and VLANs

The Cisco 819 Integrated Services Routers (ISRs) support clients on both physical LANs and virtual LANs (VLANs). The routers can use the Dynamic Host Configuration Protocol (DHCP) to enable automatic assignment of IP configurations for nodes on these networks.

Figure 12-1 shows a typical deployment scenario with two physical LANs connected by the router and two VLANs.

Figure 12-1 Physical and Virtual LANs with DHCP Configured on the Cisco Router

 

 

1

Fast Ethernet LAN (with multiple networked devices)
2

Router and DHCP server—Cisco 819 ISR—connected to the Internet
3

VLAN 1
4

VLAN 2

DHCP

DHCP, which is described in RFC 2131, uses a client/server model for address allocation. As an administrator, you can configure your Cisco 800 series router to act as a DHCP server, providing IP address assignment and other TCP/IP-oriented configuration information to your workstations. DHCP frees you from having to manually assign an IP address to each client.

When you configure a DHCP server, you must configure the server properties, policies, and DHCP options.

Note Whenever you change server properties, you must reload the server with the configuration data from the Network Registrar database.

VLANs

The Cisco 819 routers support four Fast Ethernet ports on which you can configure VLANs.

VLANs enable networks to be segmented and formed into logical groups of users, regardless of the user’s physical location or LAN connection.

Configuration Tasks

Perform the following tasks to configure this network scenario:

Note The procedures in this chapter assume you have already configured basic router features, as well as PPPoE or PPPoA with NAT. If you have not performed these configurations tasks, see the “Basic Router Configuration” section and “Configuring a VPN Using Easy VPN and an IPSec Tunnel” section as appropriate for your router.

Configure DHCP

Perform these steps to configure your router for DHCP operation, beginning in global configuration mode:

SUMMARY STEPS

1. ip domain name name

2. ip name-server server-address1 [s erver-address2...server-address6 ]

3. ip dhcp excluded-address low-address [ high-address ]

4. ip dhcp pool name

5. network network-number [ mask | prefix-length ]

6. import all

7. default-router address [ address2...address8 ]

8. dns-server address [ address2...address8 ]

9. domain-name domain

10. exit

DETAILED STEPS

 

Command
Purpose

Step 1

ip domain name name

Example:

Router(config)# ip domain name smallbiz.com
Router(config)#
 

Identifies the default domain that the router uses to complete unqualified hostnames (names without a dotted-decimal domain name).

Step 2

ip name-server server-address1 [server-address2...server-address6]

Example:

Router(config)# ip name-server 192.168.11.12
Router(config)#
 

Specifies the address of one or more Domain Name System (DNS) servers to use for name and address resolution.

Step 3

ip dhcp excluded-address low-address [high-address]

Example:

Router(config)# ip dhcp excluded-address 192.168.9.0
 

Specifies IP addresses that the DHCP server should not assign to DHCP clients. In this example, we are excluding the router address.

Step 4

ip dhcp pool name

Example:

Router(config)# ip dhcp pool dpool1
Router(config-dhcp)#
 

Creates a DHCP address pool on the router and enters DHCP pool configuration mode. The name argument can be a string or an integer.

Step 5

network network-number [mask | prefix-length]

Example:

Router(config-dhcp)# network 10.10.0.0 255.255.255.0
Router(config-dhcp)#
 

Defines subnet number (IP) address for the DHCP address pool, optionally including the mask.

Step 6

import all

Example:

Router(config-dhcp)# import all
Router(config-dhcp)#
 

Imports DHCP option parameters into the DHCP portion of the router database.

Step 7

default-router address [address2...address8]

Example:

Router(config-dhcp)# default-router 10.10.10.10
Router(config-dhcp)#
 

Specifies up to eight default routers for a DHCP client.

Step 8

dns-server address [address2...address8]

Example:

Router(config-dhcp)# dns-server 192.168.35.2
Router(config-dhcp)#
 

Specifies up to eight DNS servers available to a DHCP client.

Step 9

domain-name domain

Example:

Router(config-dhcp)# domain-name cisco.com
Router(config-dhcp)#
 

Specifies the domain name for a DHCP client.

Step 10

exit

Example:

Router(config-dhcp)# exit
Router(config)#
 

Exits DHCP configuration mode and enters global configuration mode.

Configuration Example

The following configuration example shows a portion of the configuration file for the DCHP configuration described in this chapter:

ip dhcp excluded-address 192.168.9.0
!
ip dhcp pool dpool1
import all
network 10.10.0.0 255.255.255.0
default-router 10.10.10.10
dns-server 192.168.35.2
domain-name cisco.com
!
ip domain name smallbiz.com
ip name-server 192.168.11.12
 

Verify Your DHCP Configuration

Use the following commands to view your DHCP configuration.

  • show ip dhcp import —Displays the optional parameters imported into the DHCP server database.
  • show ip dhcp pool —Displays information about the DHCP address pools.
  • show ip dhcp server statistics —Displays the DHCP server statistics, such as the number of address pools, bindings, and so forth.
 
Router# show ip dhcp import
Address Pool Name: dpool1
 
Router# show ip dhcp pool
Pool dpool1 :
Utilization mark (high/low) : 100 / 0
Subnet size (first/next) : 0 / 0
Total addresses : 254
Leased addresses : 0
Pending event : none
1 subnet is currently in the pool :
Current index IP address range Leased addresses
10.10.0.1 10.10.0.1 - 10.10.0.254 0
 
Router# show ip dhcp server statistics
Memory usage 15419
Address pools 1
Database agents 0
Automatic bindings 0
Manual bindings 0
Expired bindings 0
Malformed messages 0
Secure arp entries 0
 
Message Received
BOOTREQUEST 0
DHCPDISCOVER 0
DHCPREQUEST 0
DHCPDECLINE 0
DHCPRELEASE 0
DHCPINFORM 0
 
Message Sent
BOOTREPLY 0
DHCPOFFER 0
DHCPACK 0
DHCPNAK 0
Router#
 

Configure VLANs

Perform these steps to configure VLANs on your router, beginning in global configuration mode:

SUMMARY STEPS

1. vlan ?

2. ISL VLAN ID

3. exit

DETAILED STEPS

 

Command
Purpose

Step 1

vlan ?

Example:

Router# config t
Router(config)# vlan database?
 
WORD ISL VLAN IDs 1-4094
accounting VLAN accounting configuration
ifdescr VLAN subinterface ifDescr
 
Router(config)# vlan 2

Enters VLAN configuration mode.

Step 2

ISL VLAN ID

Example:

Router(config)# vlan 2
Router(config- vlan)#

Adds VLANs, with identifiers ranging from
1 to 4094.

For details about this command and additional parameters that can be set, see Cisco IOS Switching Services Command Reference.

Step 3

exit

Example:

Router(config- vlan)# exit
Router(config)#

Updates the VLAN database, propagates it throughout the administrative domain, and returns to global configuration mode.

Assign a Switch Port to a VLAN

Perform these steps to assign a switch port to a VLAN, beginning in global configuration mode:

SUMMARY STEPS

1. interface switch port id

2. switchport access vlan vlan-id

3. end

DETAILED STEPS

 

Command
Purpose

Step 1

interface switch port id

Example:

Router(config)# interface FastEthernet 2
Router(config-if)#
 

Specifies the switch port that you want to assign to the VLAN.

Step 2

switchport access vlan vlan-id

Example:

Router(config-if)# switchport access vlan 2
Router(config-if)#
 

Assigns a port to the VLAN.

Step 3

end

Example:

Router(config-if)# end
Router#
 

Exits interface mode and returns to privileged EXEC mode.

Verify Your VLAN Configuration

Use the following commands to view your VLAN configuration.

  • show —Entered from VLAN database mode. Displays summary configuration information for all configured VLANs.
  • show vlan-switch—Entered from privileged EXEC mode. Displays detailed configuration information for all configured VLANs.
 
Router# vlan database
Router(vlan)# show
 
VLAN ISL Id: 1
Name: default
Media Type: Ethernet
VLAN 802.10 Id: 100001
State: Operational
MTU: 1500
Translational Bridged VLAN: 1002
Translational Bridged VLAN: 1003
 
VLAN ISL Id: 2
Name: VLAN0002
Media Type: Ethernet
VLAN 802.10 Id: 100002
State: Operational
MTU: 1500
 
VLAN ISL Id: 3
Name: red-vlan
Media Type: Ethernet
VLAN 802.10 Id: 100003
State: Operational
MTU: 1500
 
VLAN ISL Id: 1002
Name: fddi-default
Media Type: FDDI
VLAN 802.10 Id: 101002
State: Operational
MTU: 1500
Bridge Type: SRB
Translational Bridged VLAN: 1
Translational Bridged VLAN: 1003
 
VLAN ISL Id: 1003
Name: token-ring-default
Media Type: Token Ring
VLAN 802.10 Id: 101003
State: Operational
MTU: 1500
Bridge Type: SRB
Ring Number: 0
Bridge Number: 1
Parent VLAN: 1005
Maximum ARE Hop Count: 7
Maximum STE Hop Count: 7
Backup CRF Mode: Disabled
Translational Bridged VLAN: 1
Translational Bridged VLAN: 1002
 
VLAN ISL Id: 1004
Name: fddinet-default
Media Type: FDDI Net
VLAN 802.10 Id: 101004
State: Operational
MTU: 1500
Bridge Type: SRB
Bridge Number: 1
STP Type: IBM
 
VLAN ISL Id: 1005
Name: trnet-default
Media Type: Token Ring Net
VLAN 802.10 Id: 101005
State: Operational
MTU: 1500
Bridge Type: SRB
Bridge Number: 1
STP Type: IBM
 
Router# show vlan-switch
 
VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0, Fa1, Fa3
2 VLAN0002 active Fa2
1002 fddi-default active
1003 token-ring-default active
1004 fddinet-default active
1005 trnet-default active
 
VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 1002 1003
2 enet 100002 1500 - - - - - 0 0
1002 fddi 101002 1500 - - - - - 1 1003
1003 tr 101003 1500 1005 0 - - srb 1 1002
1004 fdnet 101004 1500 - - 1 ibm - 0 0
1005 trnet 101005 1500 - - 1 ibm - 0 0