Re: dhcp / giaddr / configuration for giaddr update

On Sep 24, 7:17 am, sirspammedalot...@xxxxxxxxx wrote:
thanks for the helpful reply. this has actually raised another
question for me. what happens in the case of a dhcp request being
forwarded through several routers before reaching the dhcp server,
assuming this kind of situation is supported? im going to assume the
original receiving interface gets written a single field of the packet
(giaddr). given that, then after going through additional routers,
this value would get over-written and as a result, there wouldn't be a
known return path for the response beyond the nearest router.

how would the packet find its way back to the client in this scenario?

On Sep 24, 6:15 am, Trendkill <jpma...@xxxxxxxxx> wrote:

On Sep 24, 5:13 am, sirspammedalot...@xxxxxxxxx wrote:

hi group. this may be not strictly be a cisco question and if this is
off topic, i'm sorry. it does however pertain to configuration of
cisco routers.

if a dhcp server is connected to a router that connects many different
subnets... how does the dhcp server know which subnet a dhcp request
came from? does this information come from the giaddr field of the
dhcp request packet? if yes, is the value of this field set to the
address of the of the router interface that originally received the
packet or the router interface that the packet is forwarded through?

and does the giaddr update itself require any configuration of the
router device or does the router know to update it automatically? if
it requires configuration, how is it done? thanks for reading

No sure on the field, but the value is definitely the address of the
receiving router interface, which then becomes the source of the
request. In a situation like mine where I have core routers for
hundreds of vlans, the dhcp server would never know which is which and
what to assign without knowing the subnet for which the request is
coming from. As far as I know, yes, the dhcp server must have the
correct information for the router in each network, but on the router
itself, no there is no additional configuration except for ip helper-
address on each interface it is required and a known path to whatever
ip the dhcp server is.

Only the original receiving router re-writes the packet and forwards
on. You have to remember that the main reason this is done is because
the client does not have an IP address and therefore the packet cannot
route back without a source address. The router then proxies this
request for the client using its own interface, but knows the original
boxes MAC to forward the reply when it comes. As soon as it proxies
the request, its own IP is used, which should be fully routable, and
no other routers need to interfere. To other routers, this is just
another packet, from the one interface on the one router, to the DHCP
server. Business as usual...

.

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