[j-nsp] MX BNG with both local server and dhcp relay

[Dave Bell]

Hi,

I'm having issues with DHCP relay on a Juniper MX BNG, and was wondering if
anyone had an insight on what may be the cause of my issue.

I've got subscribers terminating on the MX, authenticated by RADIUS, and
then placed into a VRF to get services. In the vast majority of cases the
IP addressing information is passed back by RADIUS, and so I'm using the
local DHCP server on the MX to deal with that side of things.

In one instance I require the use of an external DHCP server. I've got the
RADIUS server providing an Access-Accept for this subscriber, and also
returning the correct VRF in which to terminate the subscriber. I've also
tried passing back the external DHCP server via RADIUS.

In the VRF, I've got the DHCP relay configured, and there is reachability
to the appropriate server

The MX however seems reluctant to actually forward DHCP requests to this
server. From the logging, I can see that the appropriate attributes are
received and correctly decoded. The session gets relocated into the correct
routing instance, but then it tries to look for a local DHCP server.

I have the feeling that my issues are due to trying to use both the local
DHCP server and DHCP relay depending on the subscriber scenario. If I
change the global configuration of DHCP from local server to DHCP relay, my
configuration works as expected though with the detriment of the scenario
where the attributes returned via RADIUS no longer work due to it not being
able to find a DHCP relay.

Since the MX decides how to authenticate the subscriber based on where the
demux interface is configured, I think ideally I would need to create a
different demux interface for these type of subscribers that I can then set
to be DHCP forwarded, thought I don't seem to be able to convince the
router to do that yet.

Has anyone come across this, and found a workable solution?

Regards,
Dave