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 126.96.36.199 255.255.255.0 default-router 188.8.131.52 ! interface FastEthernet0/0 ip address 184.108.40.206 255.255.255.0
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 220.127.116.11 18.104.22.168 - 22.214.171.124 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
About Petr Lapukhov, 4xCCIE/CCDE:
Petr Lapukhov's career in IT begain in 1988 with a focus on computer programming, and progressed into networking with his first exposure to Novell NetWare in 1991. Initially involved with Kazan State University's campus network support and UNIX system administration, he went through the path of becoming a networking consultant, taking part in many network deployment projects. Petr currently has over 12 years of experience working in the Cisco networking field, and is the only person in the world to have obtained four CCIEs in under two years, passing each on his first attempt. Petr is an exceptional case in that he has been working with all of the technologies covered in his four CCIE tracks (R&S, Security, SP, and Voice) on a daily basis for many years. When not actively teaching classes, developing self-paced products, studying for the CCDE Practical & the CCIE Storage Lab Exam, and completing his PhD in Applied Mathematics.
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