Category: DHCP

One of the readers sent me an interesting question: he’d like to know the IP address of his home router (to be able to connect to it from the office), but its IP address is assigned through DHCP and changes occasionally.

I wanted to solve the problem by hooking an EEM applet onto the DHCP-6-ADDRESS_ASSIGN syslog message. No good; as it turns out, Cisco IOS generates the logging message only when a DHCP-acquired IP address is assigned to an interface without one. If the IP address is changed via DHCP, the change is not logged.

You would think that an expired DHCP lease is not a big deal for a DHCP client. Although the interface IP address is lost, you can always try to get a new address from the DHCP server.

IOS has a different opinion: when the DHCP lease expires on a router configured with ip address dhcp interface configuration command, the interface is administratively shut down and re-enabled. Here’s a sample printout taken from a router running 15.6(1)T software:

ip dhcp pool fedi
   host 192.168.200.206 255.255.255.240
   hardware-address 000f.fe83.bca9
   dns-server 208.67.220.220 208.67.222.222

This article is part of You've asked for it series.

1. The router and the cable modem are power-cycled.
2. The router starts to bridge between all LAN interfaces, effectively connecting inside workstations directly to the cable modem.
3. One of the workstations could detect a LAN failure (due to router reload) and restart the DHCP process (a Windows XP host would definitely do that).
4. The DHCP requests from the workstation are bridged straight to the cable modem which caches the workstation's MAC address and forwards the DHCP request.
5. The workstation is assigned a public IP address (at this time, the workstation is connected directly to Internet and thus vulnerable).
6. The router loads Cisco IOS and reinitializes the Ethernet interfaces. Bridging between internal and external interfaces is stopped.
7. The router sends DHCP request on the outside interface, but the modem ignores it, as the MAC address of the DHCP request differs from the previously cached one.

In most cases, the cable modem has to be power-cycled to lose the cached MAC address.

This behavior can be observed only if the router and the cable modem are reset at the same time and the cable provider doesn't care much about MAC security and allows the modem to learn the MAC address. If you reset only the cable modem, the router is not bridging (no problem); if you reset just the router, the cable modem still caches the router's MAC address and ignores the DHCP request from the inside workstation(s).

Cisco has published a series of documents describing how you can connect a SOHO site to two ISPs.

Their configuration also includes a nice trick: the ip dhcp client route track number command is a convenient replacement for a static default route with the track option if one of the upstream interfaces uses DHCP and the router generates the default route based on DHCP replies.

• I've added a Linux box to my home network;
• It used my Cisco router to get a dynamic DHCP address;
• I've inspected the DHCP bindings on the Cisco router to find the new MAC address and configured a host DHCP pool as I'm using the Linux box as a server;
• Even after multiple configuration changes, the IOS would fail to use the host DHCP pool.

The only solution I've found was to restart the IOS DHCP server with the no service dhcp followed by service dhcp configuration commands. Obviously, you lose all DHCP bindings when you restart the DHCP server (which could be a problem if you use conflict logging) unless you've configured the router to store them in an external file.

In a response to my post Redundant DHCP Server I've speculated that a Cisco router should coexist with a Windows-based DHCP server if you configure them with non-overlapping address ranges. I was wrong, Edgar Cahuana discovered that Microsoft's DHCP server wants to have complete control over the LAN it's serving and shuts down if it detects another DHCP server on the same LAN.

To make the two DHCP servers coexist, you have to disable rogue DHCP server detection in Windows DHCP server.

Using DHCP to assign server IP addresses is usually not a wise decision. To start with, you have to define static DHCP mappings, which rely on client-id attribute in the DHCP request (usually the MAC address of the client). For me, the easiest way to find the correct client ID is as follows:

• Use DHCP to assign the IP address to the server
• Note the newly assigned IP address
• Use the show ip dhcp bindings | include ip-address command to display the client-id to IP address binding.
• Create a static DHCP mapping (for example, by configuring a host DHCP pool on the router) and release/renew IP address on the server

If you'd like to implement persistent DHCP bindings on Cisco IOS, but cannot store them on an external server, you could always use the on-board NVRAM. Simply configure ip dhcp database nvram:dhcp.txt. Later on, you can examine the contents of the dhcp.txt file with more nvram:dhcp.txt command.

This post was written in 2007, when a lot of low-end Cisco routers still shipped with flash formatted in the “old” Cisco format and the flash was not really usable to store ever-changing files. For more details on storing DHCP bindings in onboard flash, read the Flash-based DHCP Database blog post.

The on-line configuration help for the ip dhcp conflict logging configuration command (logging: Record address conflicts in a log file) is one of the more misleading texts I've found in Cisco IOS (and the CCO documentation is not much better). Here's how it actually works ...