Configuring DHCP Relay, DHCP Server, and PAT

This chapter provides descriptions, application notes, and example configurations for configuring Dynamic Host Configuration Protocol (DHCP) relay, DHCP server, and Port Address Translation (PAT) on the Cisco 700 series router. It contains the following sections:

DHCP Description

DHCP is a client-server protocol that allows devices on an IP network (the DHCP clients) to request configuration information from a DHCP server. DHCP allocates network addresses from a central pool on an as-needed basis. DHCP is useful for assigning IP addresses to hosts connected to the network temporarily or for sharing a limited pool of IP addresses among a group of hosts that do not need permanent IP addresses.

DHCP allows for increased automation and fewer network administration problems by:

Eliminating the need for the manual configuration of individual computers, printers, and shared file systems.
Preventing the simultaneous use of the same IP address by two clients.
Allowing configuration from a central site.

DHCP Server Application Notes

The following are application notes for DHCP server:

DHCP relay and DHCP server are mutually exclusive.
When DHCP server is initialized, default addresses are used if no LAN or internal address exists. The Cisco 700 series router picks up the DHCP client's default gateway, netmask, and starting DHCP addresses by using the LAN IP address, if one exists. If a LAN address does not exist, the router uses the internal IP address. If neither exists, it uses the default settings: 10.0.0.1 as the LAN IP address (default gateway for DHCP clients), 255.0.0.0 as the subnet mask, and 10.0.0.2 as the starting DHCP client addresses.
For the DHCP values to be automatically generated based on the LAN or internal IP address, each DHCP value must be set to 0.0.0.0 or none, for the new values to take effect.

DHCP Relay Application Notes

The following are application notes for DHCP relay:

The IP address in the Internal profile must be on the same network as that of the DHCP clients.
A firewall configuration (where there is an Internal profile and LAN IP address) does not work with DHCP relay.
A configuration where PAT is on and DHCP relay is enabled is not valid. DHCP relay will attempt to cross from a public to a private domain. PAT prevents access to the private domain. DHCP relay fails because it must reference the router's private address.
DHCP relay and DHCP server are mutually exclusive. The Cisco 700 series router can function as one or the other, but not both.

PAT Description

Cisco 700 series routers provide PAT, enabling local hosts on a private IP network to communicate externally.

Packets destined for an external address have their private IP address plus port number translated to the router's external IP address before the IP packet is forwarded to the WAN. IP packets returning to the router have their external IP addresses (plus port number) translated back to the private IP addresses, and the packets are forwarded to the LAN.

When PAT is enabled, the transmission of RIP packets is automatically disabled to prevent a broadcast of the private IP addresses externally.

PAT Application Notes

A configuration where PAT is on and DHCP relay is enabled is not valid. DHCP relay will attempt to cross from a public to a private domain. PAT prevents access to the private domain. DHCP relay fails because it must reference the router's private address.

IPCP Description

If you are using IP routing, Cisco 700 series routers must be configured for IPCP to connect to routers running Cisco IOS software. Use the set ip framing command in profile mode to enable IPCP for user-defined WAN profiles by setting IP framing to none.

The following example illustrates IPCP enabled by disabling IP framing:

766:2503> set ip framing none

Note Do not set the ip framing or the ipx framing commands to none when configuring the permanent LAN profile.

IPCP Address Negotiation Application Notes

The following are application notes for IPCP address negotiation:

Cisco 700 series routers require a user-defined profile, configured with IP routing on, to receive an IPCP addresses.
A router running Cisco IOS software must be configured to hand off IPCP addresses.
If a manually configured IP address exists on the Internal profile of a Cisco 700 series router, the IPCP address is assigned to the WAN profile.
If a manually configured IP address exists on the LAN profile, the IPCP address is assigned to the Internal profile.

PPP IPCP Negotiation Example

This section describes how to configure a remote Cisco 700 series router for PPP IPCP negotiation to a central site router, such as a Cisco 4500 series router, that is running Cisco IOS software.

Figure 5-1 is an illustration of the configuration used in this example.

Figure 5-1: PPP IPCP Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router for point-to-point IPCP negotiation with a Cisco 4500 router:

set switch 5ess
set 1 dir directory_number
set 2 dir directory_number
set system 765
set user 4500
set ppp secret client
set ppp password client
set active 
set 1 number phone_number 
set 2 number phone_number 
set ip routing on
set ip route destination 0.0.0.0/0 gateway 0.0.0.0
set ip rip version 2
set ip rip receive v2
set ip rip update linkup
reboot

For detailed information regarding the commands listed here, refer to the Cisco700 Series Router Command Reference.

Verify the Configuration

You can use the show ip configuration all and show ip route all commands to see the IPCP negotiation address.

DHCP Relay with IPCP Negotiation Example

This section describes how to configure a remote Cisco 700 series router for DHCP relay with IPCP negotiation to a central-site router running Cisco IOS software.

Figure 5-2 is an illustration of the configuration used in this example.

Figure 5-2: DHCP Relay with IPCP Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router for DHCP Relay with IPCP Negotiation with a Cisco 4500 router:

set system 765
set dhcp relay 172.168.100.2
set user 4500
set 1 number phone_number 
set 2 number phone_number 
set ip routing on
set ip rip version 2
set ip rip receive v2
set ip rip update linkup
reboot

For detailed information regarding the commands listed here, refer to the Cisco700 Series Router Command Reference.

Verify the Configuration

You can use the show ip configuration and show ip route commands to see the IPCP negotiation address.

You can use the show dhcp configuration command to see the IP address returned by DHCP.

DHCP Server with IPCP Negotiation Example

This section describes how to configure a remote Cisco 700 series router as a DHCP server with IPCP negotiation to a central-site router running Cisco IOS software.

Figure 5-3 is an illustration of the configuration used in this example.

Figure 5-3: DHCP Server with IPCP Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router as a DHCP server with IPCP negotiation with a Cisco 4500 router:

set switch NI-1
set 1 dir 5551211
set 2 dir 5551212
set 1 spid 88855512110101
set 2 spid 88855512120101
set system 765
set dhcp server
set dhcp address 172.168.1.2 128
set dhcp netmask 255.255.255.0
set dhcp gateway primary 172.168.1.1
cd lan
set ip address 172.168.1.1
set ip netmask 255.255.255.0
set ip routing on
set user 4500
set ip routing on
set active
set 1 number phone_number 
set 2 number phone_number 
set ip routing on
set ip rip version 2
set ip rip receive v2
set ppp address negotiation local on
set ip rip update periodic
reboot

For detailed information regarding the commands listed here, refer to the Cisco700 Series Router Command Reference.

Verify the Configuration

When a call is made, you can use the show ip configuration all and show ip route all commands to see the IPCP negotiation address.

You can use the show dhcp configuration command to see the IP address returned by DHCP.

PAT with IPCP Single-Destination Negotiation Example

This section describes how to configure a remote Cisco 700 series router for PAT with IPCP single-destination negotiation to a central-site router running Cisco IOS software.

Figure 5-4 is an illustration of the configuration used in this example.

Figure 5-4: PAT with IPCP Single-Destination Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router for PAT with IPCP single-destination negotiation with a Cisco 4500 router:

set system 765
set ip pat po ftp 10.0.0.2
cd lan
set ip address 10.0.0.1
set ip netmask 255.0.0.0
set ip routing on
set user 4500
set active
set 1 number phone_number 
set 2 number phone_number 
set ip routing on
set ip rip version 2
set ip rip receive v2
set ip pat on
reboot

For detailed information regarding the commands listed here, refer to the Cisco700 Series Router Command Reference.

Verify the Configuration

When a demand call is made, you can use the show ip config all and show ip route all commands to see the IPCP negotiation address.

You can use the show ip pat command to see the services returned by PAT.

DHCP Server and PAT with IPCP Single-Destination Negotiation Example

This section describes how to configure a remote Cisco 700 series router for DHCP server with PAT and IPCP single-destination negotiation to a Cisco 4500 series router running Cisco IOS software at a central site.

Figure 5-5 is an illustration of the configuration used in this example.

Figure 5-5: DHCP Server with PAT and IPCP Single-Destination Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router for PAT with IPCP single-destination negotiation with a Cisco 4500 router:

set system 765
set ppp secret client
set ppp password client
set dhcp server
set dhcp address 172.168.99.2 128
set dhcp netmask 255.255.255.0
set dhcp gateway primary 172.168.99.1
set ip pat porthandler ftp 172.168.99.3
cd lan
set ip routing on
set ip address 172.168.99.1
set ip netmask 255.255.255.0
set user 4500
set 1 number phone_number
set 2 number phone_number
set ip routing on
set ip route destination 0.0.0.0/0 gateway 0.0.0.0
set ip address 0.0.0.0
set ip netmask 0.0.0.0
set ip pat on
set active

For detailed information regarding the commands listed here, refer to the Cisco700 Series Router Command Reference.

Verify the Configuration

When a demand call is made, you can use the show ip config all and show ip route all commands to see the IPCP negotiation address. You can also use the show dhcp config and show ip pat commands to verify the configuration.

DHCP Server with PAT and IPCP Dual-Destination PPP Negotiation Example

This section describes how to configure a remote Cisco 700 series router for DHCP server with PAT and IPCP dual-destination PPP negotiation to two routers running Cisco IOS software.

Figure 5-6 is an illustration of the configuration used in this example.

Figure 5-6: DHCP Server with PAT and IPCP and Dual-Destination PPP Negotiation

Cisco 765 Series Router Commands

Following are the commands for configuring the remote Cisco 765 series router as a DHCP server with PAT and IPCP multilink PPP to two routers running Cisco IOS software:

set system 765
set ppp secret client
set ppp password client
set dhcp server
set dhcp address 172.168.99.2 128
set dhcp netmask 255.255.255.0
set dhcp gateway primary 172.168.99.1
set ip pat porthandler ftp 172.168.99.3
cd lan
set ip routing on
set ip address 172.168.99.1
set ip netmask 255.255.255.0
set user 2500
set ppp clientname 2500
set 1 number phone_number
set 2 number phone_number
set ip routing on
set ip route destination 0.0.0.0/0 gateway 30.169.100.2
set ip address 30.169.100.1
set ip netmask 255.255.255.0
set ip pat on
set active
set user 4500
set ppp clientname 4500
set 1 number phone_number
set 2 number phone_number
set ip routing on
set ip route destination 120.50.40.0/0 gateway 173.100.10.2
set ip address 173.100.10.1
set ip netmask 255.255.255.0
set ip pat on
set active

Note If you create a multiple destination configuration with PAT enabled in both profiles, the IP route destination must be specified, using the set ip route destination command.

Verify the Configuration

When a demand call is made, you can use the show ip configuration all command to see the IPCP negotiation address.

You can use the show dhcp connections command to see the IP addresses returned by DHCP.

Table of Contents

Configuring DHCP Relay, DHCP Server, and PAT