multiple relay agents between a client and an Option 82 server

[Ingen Schenau, Jeroen van (ICTS)]

> >    I'm trying to configure network with multiple relay agents between a
> >    client and an Option 82 server. (I'm using 2 identical switches)
> >    In the documentation on my switch HP ProCurve 2900 it is written
> >    Note: In networks with multiple relay agents between a client and an
> >    Option 82 server, append can be used only if the server supports
> >    multiple Option 82 fields in a client request. [...]
> 
> I recently ran into this issue.   If you are using tagged VLANs between
> your switches, you can tell the HP Procurve to not do DHCP snooping and
> Option 82 insertion on packets received on tagged ports, by removing the
> "dhcp-snooping vlan NN" statments from your config..  The switch
> always does DHCP snooping on untagged ports in the same VLAN that has
> an "ip helper" address configured, which in most cases is enough.

Do you have a reference to some documentation on this topic? I run a
fairly sized network with ProCurve switches and I'm pretty sure that
enabling an IP Helper doesn't do anything with DHCP Snooping. DHCP
Snooping is a security feature to prevent rogue servers and it can add
Option 82 info to the packets it sees, in order to mark on which edge
port a DHCP packet entered the network. Configuring an IP helper enables
relaying a unicast copy of select broadcast packets (such as DHCP
Discover) to a pre-configured destination.


> I was using the Option 82 "keep" settings in a test network, and was
> getting duplicate DHCP requests from each intermediate switch until
> I removed the "dhcp-snooping vlan ..." statement.

Sounds more like you had several switches in the same broadcast domain
with IP helper enabled. A broadcasted DHCP Discover for example would be
seen by every switch in the VLAN, so each switch with IP helper config
would relay a unicast copy of the packet.


Regards,

Jeroen van Ingen
ICT Service Centre
University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands