Posts Tagged ‘import’


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.
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