Using the DHCP Import statement

A question on GroupStudy gave me an idea for the short post dedicated to explaining the use of DHCP “import all” command. The command first appeared in IOS 12.2T. It allows importing certain DHCP information learned from some external source, such as another DHCP server. This is helpful in reducing the amount of configuration needed in large hub-and-spoke networks, where spokes use centralized servers (e.g. WINS, DNS, TFTP). Instead of configuring the repetitive settings in every spoke router, you may import them by requesting an IP address for the router via DHCP. More than that, any change in central configuration could be easily imported in the remote routers, using DHCP address refresh. Here is how it works:

1) The router requests an IP address on its WAN interface via DHCP. In addition to the IP/subnet information, the router also learns other DHCP information, such as various DHCP options (DNS, WIN, TFTP IP addresses). This is store with the local DHCP client configuration.
2) The is a local pool configured in the router, with the subnet corresponding to the local Ethernet interface (say office network). This pool is configured with the statement “import all”.
3) By the virtue of “import all” statement and the default “origin dhcp” setting, the local pool imports the information learned by the router’s DHCP client. The imported information does not preempt the local subnet and mask, but instead add missing information.
4) Every time the DHCP lease expires, the router will re-request it, thus re-learning all other information as well.

As an alternative to using the DHCP it is possible to use IPCP for information import, if the WAN link uses PPP protocol (e.g. PPPoE). You simply need the statement “ip address negotiated” on the PPP link plus configured “origin ipcp” under the DHCP pool. Notice that the amount of IPCP options is much smaller than that of DHCP. However, you may still send WINS and DNS servers IP addresses, and even the netmask, using the command “ppp ipcp mask”. See the post The myster of “PPP IPCP mask request command” for more information on this command.

Here is a sample configuration.

R1 is the DHCP client to R3. The server has a DCHP pool for subnet 10.0.0.0/24 configured, with TFTP, DNS and WINS servers set. All servers belong to the Ethernet segment connected to R3. R1 imports those settings from R3. Notice the use of the Dialer interface in R1 - you cannot configure "ip address dhcp" on the physical interface, as this feature was intended for use with PPPoE/PPPoA interfaces. Also, pay attention to the "ip unnumbered" command on R3's PPP interface. It is important to have this command, so that incoming DHCP REQ packets are matched against R3's DHCP pool.

R1:

interface Serial0/1

 no ip address

 encapsulation ppp

 dialer in-band

 dialer pool-member 1

!

interface Dialer1

 ip address dhcp

 dialer pool 1

!

ip dhcp pool LOCAL

   import all

   network 20.0.0.0 255.255.255.0

   default-router 20.0.0.1

!

interface FastEthernet0/0

 ip address 20.0.0.1 255.255.255.0

R3:

ip dhcp pool POOL

   network 10.0.0.0 255.255.255.0

   dns-server 10.0.0.100 10.0.0.200

   netbios-name-server 10.0.0.100

   option 150 ip 10.0.0.150

   default-router 10.0.0.3

!

interface FastEthernet0/0

 ip address 10.0.0.3 255.255.255.0

!

interface Serial1/2

 ip unnumbered FastEthernet0/0

 encapsulation ppp

 clock rate 128000

And now some quick verifications:

R1#show ip dhcp pool

Pool LOCAL :
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
20.0.0.1 20.0.0.1 - 20.0.0.254 0

R1#show ip dhcp import

Address Pool Name: LOCAL
Domain Name Server(s): 10.0.0.100 10.0.0.200
NetBIOS Name Server(s): 10.0.0.100
TFTP Server Address Option: 10.0.0.150

R3#show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address Client-ID/ Lease expiration Type
Hardware address/
User name
10.0.0.2 0063.6973.636f.2d30. May 04 2009 10:21 AM Automatic
3030.642e.6564.6338.
2e34.6636.302d.4469.
31

And last, but not least. It is possible to import the DHCP pool information from a RADIUS server, which sounds pretty attractive. However, we do not provide an example in this post. Maybe next time :)