Re: ip helper / dhcp / source address

On 29 May, 18:11, Merv <merv.hr...@xxxxxxxxxx> wrote:
On May 29, 10:52 am, Trendkill <jpma...@xxxxxxxxx> wrote:



On May 29, 10:49 am, GT <ipco...@xxxxxxxxxxxxx> wrote:

On May 29, 3:09 pm, Trendkill <jpma...@xxxxxxxxx> wrote:

On May 29, 10:04 am, GT <ipco...@xxxxxxxxxxxxx> wrote:

Dear all, i just wanted to check what the source address of the IP
packet that is generated by a router configured with 'ip helper
A.B.C.D' on a local interface would be.

i need to know this as the router is connected via an IPSEC VPN
(crypto enabled on the dialer0 interface) to a central location on
which the DHCP server is connected, and i need to be sure that the
forwarded dhcp request will generate crytpo.

Thanks

It should be the IP of the SVI that received the DHCP request.  To put
it more clearly, if a node broadcasts a request for an IP, whatever
router interface received that broadcast will be used as the source IP
of the request so that the DHCP server knows which subnet to give an
IP from.  That is my understanding, but have never really sniffed the
dhcp server side of the request.

Thanks for post reply.

one (poss dum qu !) - what does SVI stand for ?

Switched virtual interface.  If you are running a layer 3 switch or
layer 2 with MSFC, its the vlan interface.  Its basically the layer 3
interface in a network that generally acts as an entry/exit point to
the subnet/vlan.  If you are running a router on a stick (router to a
switch), then the f0/1 or g0/1 would be the SVI, although in this
situation its usually just referred to as an interface.- Hide quoted text -

I am not in any sense complaining by the way - just wanted
to mention ways to find out.

Run wireshark/tcpdump/windump/other on a test "DHCPserver"
that is not accessed via VPN. Note that the machine does not
actually have to be a DHCP server, any computer that will let
you see the incoming packets will do. You don't need
to see what it sends back. Could probably do it all on
the local subnet but that would not produce such a
convincing result.

on router,
- there MAY be a debug for UDP forwarding
- debug ip packet <access-list>






.

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