[c-nsp] dai / dhcp snooping bug

Mattias Gyllenvarg mattias at gyllenvarg.se
Tue Aug 11 07:48:42 EDT 2015


I recently solved an issue a client had with a very similar setup and the
same symptoms.

They had a very complex PBR setup and the unicasts in the renew process got
misplaced .

tis 11 aug. 2015 kl 00:40 skrev Mike <mike-cisconsplist at tiedyenetworks.com>:

> On 08/10/2015 12:37 PM, Gert Doering wrote:
> > Hi,
> >
> > On Mon, Aug 10, 2015 at 06:31:16AM -0700, Mike wrote:
> >> I've loaded SE7 and - suprise -  same problem, so it's not fixed. I have
> >> a directly connected device I can cause to refresh it's dhcp lease, and
> >> sure enough, a refresh doesn't do it, but a reboot of that device which
> >> casues a new round of dhcp discovery, does in fact work. A packet
> >> capture seems to confirm the unicast case failing - a client with an
> >> existing lease renewing will use unicast to the dhcp server, whereas a
> >> client starting up will use broadcast to find servers, and both the
> >> 'discover' and 'request' phases in that case are broadcast destination.
> >> That was painful.
> > Wild idea... put an ACL into place that will block the unicast renewal?
> >
> > gert
> I had that idea too. Another idea was to see if there might be some way
> to work with it... My dhcp model is one where the server is directly
> connected to the vlans being served, but I recently made changes in the
> direction of going to a full-on dhcp relay model instead where all
> switches are doing that instead. The open question then is, does it work
> correctly if the switch is acting as a dhcp relay? I unfortunately don't
> have the equipment on standby to set up a lab and test this out (story
> of my life), but if it worked then my problem would mostly be solved.
> Another idea would be to see if I could configure the dhcp server to
> just ignore unicast requests (easier than putting ACL's on the the
> switches).
> Mike-
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