RE: DHCP Question
- From: "alexk" <alexk@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 2 Feb 2006 14:07:27 -0800
KJK,
Yes you are right- DHCP is a broadcast based protocol at layer 2 and layer 3
of the OSI model. In the first DHCP packet (Discover) the layer 4
information is addressed using an UDP packet with a destination port 67
(DHCP). The source port of the packet is UDP 68.
My question to you is this: Are both DHCP servers located in the same VLAN?
(My guess is yes they are)- then this was probably done for a fault tolerant
sitiation. If this is the case, then there is no really easy way to
guarantee which DHCP server will service the client first. (Realistically-
the first DCHP offer packet received from the client will determine which
DHCP server is selected) If one of the DHCP server is performing gratitious
ARPs before sending the offer, or if it is just plain older slower, then this
may account for the one DHCP server in your network having all of its
addresses leased. To stop this hassle and provide fault tolerance still- I
assume you have a second VLAN (there would be no need for one without the
second).
Place one of the DHCP servers in each of the VLANs. On each server
configure a scope that includes the local network ID the DHCP server will be
servicing. When a discover packet comes in- the GIADDR field (one of the
DHCP Discover fields) is set to 0.0.0.0 (without the relay agent). When this
packet comes in, the DHCP server will examine the field and look at the IP
address of the adapter that the Discover packet come in from. It will select
the scope network ID - different than the range!!!! (if one exists) that
includes the IP address of this adapter. Make sure you have a scope that
includes the IP address of the adapter and list 80% of the network ID as the
range. Create a second scope that has a network ID for the scope the other
VLAN. When your Relay Agent forwards the packet to the DHCP server - the
GIADDR field will be update according to the IP address of the Relay Agent.
This address must be in then network ID of the second scope. Configure 20%
or less of the DHCP range for the other VLAN. Dont forget to install the
Relay Agent in each VLAN with the heartbeat set to four pointing to the DHCP
server in the other VLAN.
Dont forget RANGES are not the Network IDs i am referring to!!! if you need
more info respond or check out this link:
http://www.microsoft.com/windows2000/techinfo/reskit/deploymentscenarios/scenarios/dhcp_dhcp_config_mul_sub_env.asp
or look at RFC 2131
alex k
"KJK" wrote:
I have a interesting question about DHCP that I am having a tough time.
figuring out.
Basically my network that I inherited has 2 different DHCP servers with the
scopes split in half on each server. Several users for whatever reason are
picked up on one of the servers while the other server gets the majority of
leases (99%).
The users are in the same VLAN so it cannot be a dhcp relay thing. I am also
aware that DHCP is a broadcasting protocol so whatver server responds first
ives the lease.....
Somewhat confused though becuase the several computers that get the lease
there is nothing special.
I also tried to create a reservation on the not as much used dhcp server and
it did not work.....
Any insight would be appreciated.
KJK
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