Cable Modem [really responsible engineering]

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Ted;

  I haven't been able to figure your statement above out. I
thought you were saying that given a client device MAC address, one
could determine which cable modem MAC address was servicing that
client MAC, even in the cases where the cable modem was just acting as
a bridge.

  This is embarassing, but I have been spending time on the
DOCSIS 1.0 spec (http://www.cablemodem.com/specifications.html), and I
can't seem to find where this behavior is specified as required.
Could you give me some hints as to which document it is in and which
section?

        Given the DOCSIS specifications, the two mechanisms I can
think of for yielding this information are SNMP and DHCP. In regards
to DHCP, time spend with the DOCSIS spec, your excellent book and
tcpdump says to me that it is not in the DCHPREQUEST packet, either as
'giaddr' or as any DHCP option I can find.

       Time with the RFCs and the DOCSIS spec have not yielded to me
any SNMP object in any of the DOCSIS required SNMP MIB RFCs which
would return all the the bridged MAC

  What am I missing? Your guidance is much appreciated.

thanks,
fletcher

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[ On Thursday, June 28, 2001 at 13:41:55 (-0400), Fletcher E Kittredge wrote: ]

Subject: Re: Cable Modem [really responsible engineering]

  I haven't been able to figure your statement above out. I
thought you were saying that given a client device MAC address, one
could determine which cable modem MAC address was servicing that
client MAC, even in the cases where the cable modem was just acting as
a bridge.

Luckily not much of this thread has been polluting NANOG, so I'm not
100% sure what your goal is here, but I will point out that:

The client-IP addresses active on any CPE interface of any cable modem
compliant with the DOCS-CABLE-DEVICE-MIB (rfc2669) can easily be
discovered with SNMP. From there it's very simple to find the client
MAC in your dhcpd.leases file (though I don't know why the IP#s wouldn't
be sufficient for most needs). Note too that this same table should
also act as a filter to lock static addresses to a given modem if its
entries are assigned by the operator.

The only problem of course is that you've got to SNMP-scan all your
modems to find which customer's using a given IP# (I did that a couple
of times just this morning! ;-). There are no doubt ways to cache this
information, and maybe even refresh a cache intelligently. There may
even be a better, proprietary, way do do such searches in well
implemented head-end systems (doesn't seem to be in Terayon's though).