Our network consists of two domains. Within the two domains are two different IP Schemes 10.10.1.* and 10.10.2.*. On each of these domains is a DHCP server. Thes issue I am currently having is, it seems that DHCP clients are getting half the information from one DHCP server and half from the other. For example the IP address that is given would be 10.10.1.34 but all the rest of the information in the ipconfig is from the other DHCP server. So how can it be getting the IP address from one DHCP and the rest of the scope information from the other?
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Machines Picking up DHCP infor from 2 DHCP servers
- Thread starter GeoDM
- Start date
CiscoGuy79
Technical User
is their an ip-helper address in the router configs allowing the computers on the other network to see both dhcp servers?
- Thread starter
- #3
What info is getting confused? Are these on the same layer 2 domain? Or are they seperated by a router as CiscoGuy79 suggests? If they are seperated by a router (which is preferable BTW) then there is an option to set a UDP helper (or proxy) which allows the DHCP scope to cross the router.
If they are on the same layer 2 domain then what is the difference in the scopes? Surely they should be the same except for the IP address ranges?
If they are on the same layer 2 domain then what is the difference in the scopes? Surely they should be the same except for the IP address ranges?
- Thread starter
- #5
The two domains are pretty much separate. There needs to be a Router between them. I have recently inherited the system and this definitely needs to be done especially with these types of issues.
The info that is getting confused is the Connection-specific DNS Suffix. The ip address will be different from the DNS suffix. If the suffix is domain.1 then the ip should be 10.10.1.*. Instead is domain.2 and the ip is 10.10.1.*. It’s like it gets some info from both DHCPs.
I am going to place a router between the two domains in the next few days. Is there any recommendations? The two networks are not big at all. I have been looking at the Cisco 1841. Is that too much?
The info that is getting confused is the Connection-specific DNS Suffix. The ip address will be different from the DNS suffix. If the suffix is domain.1 then the ip should be 10.10.1.*. Instead is domain.2 and the ip is 10.10.1.*. It’s like it gets some info from both DHCPs.
I am going to place a router between the two domains in the next few days. Is there any recommendations? The two networks are not big at all. I have been looking at the Cisco 1841. Is that too much?
- Thread starter
- #6
When a machine that is configured for DHCP is initialized, it will broadcast a request for a lease from ANY DHCP server (DHCPDISCOVER). ALL DHCP servers that receive the request respond to the client with a lease offer (DHCPOFFER). The DHCP client accepts the first offer it receives and broadcasts a message specifying the IP address it has selected (DHCPREQUEST). The server that sent the DHCPOFFER responds with an acknowledgment (DHCPACK) and updates its database. When an address is renewed, the client will send the DHCPREQUEST directly to the server that gave the original lease. If the address is still available (which in your case it should be) the server will send a DHCPACK back to the client. Because this process starts at 50% of the lease expiration time, if the client does not receive a response it will continue to use the IP address it has and attempt to contact the initial DHCP server. If the lease expires or the DCHPREQUEST is met with a DHCPNACK (meaning the address is no longer available) the process starts from the beginning.
So. It does not matter what subnet the client WAS on if it is looking for a new lease. If both subnets are on the same layer 2 devices, any DHCP server can answer. Are your servers seperated in such a way that there may be latency or connectivity issues at renewal time? What I mean is that if all your clients on subnet 1 are on the remote end of the radio link and both DHCP servers are on subnet 2, any server is game. Routers in this case will not solve your problem because there is no routing to be done (nothing to route on layer 2) and any helper-address statement would just add latency. BTW, do you have the DOMAIN option set up in your DHCP server configs?
So. It does not matter what subnet the client WAS on if it is looking for a new lease. If both subnets are on the same layer 2 devices, any DHCP server can answer. Are your servers seperated in such a way that there may be latency or connectivity issues at renewal time? What I mean is that if all your clients on subnet 1 are on the remote end of the radio link and both DHCP servers are on subnet 2, any server is game. Routers in this case will not solve your problem because there is no routing to be done (nothing to route on layer 2) and any helper-address statement would just add latency. BTW, do you have the DOMAIN option set up in your DHCP server configs?
Any chance of a mini-diagram of what your config is? Is one subnet a protected subnet via the PIX? And is the second subnet is the subnet outside the PIX but inside the Router? OR are the two subnets classed as inside the PIX but in different security domains? Or is one subnet inside, one outside but the router is not the default gateway?
As I say, a picture (ASCII diagram is enough) would tell us loads.
As I say, a picture (ASCII diagram is enough) would tell us loads.
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