Persistent DHCP bindings stored in NVRAM

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.

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9 comments:

  1. Anonymous 04 September 2007 14:05
    Would it be more wise to put file in flash?
  2. Ivan Pepelnjak 04 September 2007 18:17
    You might have a problem with flash file if you don't have disk-like flash memory (the show file systems should display your flash: as a disk filesystem).
  3. Anonymous 04 September 2007 22:16
    Can you elaborate on the use of non-disk-like flash types?
  4. Ivan Pepelnjak 05 September 2007 12:38
    If you have a “traditional” flash file system, the files never disappear, they are just marked as deleted (and you have to squeeze it manually). As you have to store the DHCP bindings in a file whenever something changes (after a short delay, of course), you could have a lot of old versions left in the flash after a while … not something I would want to have in a production network.
  5. Anonymous 05 September 2007 19:48
    Makes sense. On that note if you're running DHCP on an ISR you shouldn't have that issue since the flash is formatted Class C, correct?
  6. Ivan Pepelnjak 05 September 2007 22:31
    Well, new ISR routers come with class-C formatted flash, older ones are still formatted as class-B (but can be reformatted into class-C).
  7. Anonymous 07 December 2007 01:44
    how can I format a flash in C format?
  8. Ivan Pepelnjak 07 December 2007 11:27
    It's described here. I knew I've been writing about it already, so I've entered "format flash" into the "Search blog" input field (top left) :))
  9. Anonymous 22 September 2008 11:01
    I noticed today that writing the DHCP binding to nvram has one drawback (at least on the 2801): writing to NVRAM seems to be a very CPU intensive operation, for whatever reason, so, using the default values and some DHCP activity your CPU usage will spike to 100% every 5 minutes.

    Writing to flash seems to be much better in this regard.
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