DHCP Failover Hot-Standby Mode


In the previous blog on DHCP Failover, we discussed the DHCP failover load balance mode where both DHCP servers respond to client requests and load balance the requests between them based on an admin specified load distribution ratio.

In the other mode of a failover relationship, known as the Hot-Standby mode (Active-Passive), only one of the servers actively leases IP addresses and option configuration to clients in given subnet(s)/scope(s) while the other server (standby) is passive. The standby server services the clients, only in event of active server being down. The clients fallback to the active server once the active server becomes available again post the outage.

The Load balance mode is more suited for single site deployment where the 2 DHCP servers in a failover relationship are co-located with the subnets/scopes being served by them. As the servers are in network proximity with the clients, the clients do not experience any latency while acquiring or renewing an IP address.

Hot-Standby mode is more suited for multi-site deployment topologies. Each site would have a local DHCP server which is configured to provide the DHCP service to the clients on the local network and DHCP server at a remote site would be standby server. In a normal state of operation, computers and devices on a given site receive IP addresses and other network configuration from the DHCP server located at the same site as the clients. However, in the event of the local DHCP server being down, the DHCP server from the remote site would provide the service to the clients.

You could choose to deploy hot standby mode in a single-site deployment also if you need to.

Hot-Standby Mode Configuration

While configuring a failover relationship in Hot standby mode, there are 2 configuration parameters which are specific to the Hot Standby mode:

  • Role of the Server (Active/Standby)

The role of a server participating in hot standby failover relation can be set to Active / standby server as part of new relationship configuration.

  • Reserve Address percentage

As discussed in the blog on Load balanced mode, the free IP address pool of a scope which is part of a failover relationship in Load Balance mode, will be apportioned in the configured load distribution percentage to enable both DHCP servers to respond to client requests for new IP addresses. However, in a hot standby failover mode, free IP address pool is owned by the Active server entirely as it is serving all the client requests. In the event of an outage of the active server, the standby server needs to be able to renew existing IP address leases as well as give out new IP address leases to clients who request a new lease. For the later scenario – leasing new IP addresses to clients, the standby server needs a free IP address pool available to it from which it can give out new leases. The standby server will take over the free IP address pool of the active server only after it transitions into Partner Down state from Communicated Interrupted state and a time period of MCLT (Maximum Client Lead Time) has expired. This is as per the DHCP failover protocol. To enable the standby server to serve new IP address leases to clients during this interim period – i.e. till it transitions to Partner Down and takes over the entire free IP address pool of a scope – a percentage of free IP address pool needs to be available to standby server. This can be provided by the configuration parameter – reserve address percentage.

The percentage of addresses reserved for the hot standby server can be configured for a failover relationship configured for hot standby mode.  Free IP addresses in proportion to the percentage value configured would be assigned to the hot standby server. If address reserve percentage is set to 0, no addresses will be reserved for the hot standby server and new client leases cannot be granted by the hot standby server in case of outage of active server. The default value for reserve address percentage is 5%.  Since the reserve address percentage is meant for an interim period as discussed above, the value chosen for it can typically be small (5-10%).

Figure 1: Creating a Failover Relation in Hot Standby Mode

A new failover Relationship can be configured for Hot-standby mode and even an existing relationship in load balancing mode can be converted to hot-standby mode and vice-versa.

Operation in Hot Standby mode

Unlike the Load Balance mode, where the 2 DHCP servers compute a hash of the MAC address of the clients and decide whether to respond to the client or not, in hot standby mode, the servers do not compute hash of the MAC address of clients. The active server responds to all client requests and the standby server does not respond to any client requests at all while operating in NORMAL state. When active server goes down, the standby server transitions into Communication Interrupted state and starts responding to the clients. Once the active server is up, the standby server retreats into standby mode and stops responding to clients. This facilitates failback of the clients back to the active server.

Other Links

Team DHCP

Comments (71)

  1. teamdhcp says:

    Sam, is the active DHCP server – in the Hot standby configuration – getting the DHCP client messages ? If not, you may be missing configuration of DHCP relay agent/IP helper with IP address of the DHCP servers.

  2. Anonymous says:

    Hi Team,

    Sorry for coming late to the party. I’m scoping out using DHCP failover in hot standby in a for a multi site environment, probably in a ring or hub and spoke topology as described in another blog post.

    Struggling to find definitive answer about relaying / IP helper in this scenario which is bugging me.

    Please can you clarify….

    -Is relaying / IP Helper needed in a multi-subnet hot standby config?

    -If not, how do the DHCP request broadcasts from clients traverse the router when the primary is down?

    -If so, what is the best way to configure it. (ie, list IP addresses as in load balance mode)

    Hope this makes sense?

  3. Anonymous says:

    Amitraj, are these options configured at the scope level or the server level. Server level options are not replicated by the failover configuration wizard (or PowerShell cmdlet). If these are scope options, can you try the "Replicate Scope" action in MMC
    on the specific scope and check if these get replicated.

    Regarding your question about Cisco TFTP server, the remote site would be a different DHCP scope on the server correct ?

  4. teamdhcp says:

    Hi Chang, Hot standby configuration was designed for a deployment scenario where a remote site has a local DHCP server which is setup for failover with another DHCP server at a different location. The local server at the site would be Active/primary while the server which is remote is standby and takes over when the local server goes down.
    What I gather from your description above – the Active server and the standby server both will be at a central location servicing remote and local subnets. In this scenario, both load balance and hot standby can be deployed. If you want to balance load between servers, you can use load balance. Hot standby has an operational simplicity in the sense that the admin knows that all clients are being served out of primary server (unless there is an outage).

  5. Anonymous says:

    Hello Ben, IP helper/relaying needs to be configured. The relays/IP helper should have IP addresses of both the DHCP servers. Only the primary server will respond to the clients while it is still running. If it goes down, standby server will start responding
    to the clients.

  6. teamdhcp says:

    Hi Chang, when the link goes down, both servers will go into a failover state called “Communicated Interrupted”. In this state, both servers will renew IP address leases which are already leased. For clients trying to obtain a new IP address lease (not renew existing lease), again both DHCP servers will grant new leases but they will do so from different portion of the free IP address pool of the scope – thereby avoiding any conflict. The standby server during this period (when it is Communicated Interrupted) will give new IP address leases from the percentage of the free IP pool reserved for it (reserve address percentage). If the link continues to be down for a long period (state switch over interval), if automatic switchover is enabled, both server will move into Partner down. The situation of both servers running in partner down should be avoided.

  7. teamdhcp says:

    Hi Rimvydas, your point about standby server giving leases for only MCLT and hence, failback is not delayed that much is a fair observation. Each of the behavior choices have pros and cons. However, the standby server receiving large number of requests should not put too much extra load on the server given that the server is not responding to unicast renew and hence the processing overhead is low. Also, we have tested DHCP failover to pretty high scale so server not being able to keep up is not a concern. Is there any other blocking concern that I am missing here ?
    Appreciate your feedback on the documentation. We are working on your feedback.

  8. Anonymous says:

    Repeating incase this was missed….

    Hi Team,

    Sorry for coming late to the party. I’m scoping out using DHCP failover in hot standby in a for a multi site environment, probably in a ring or hub and spoke topology as described in another blog post.

    Struggling to find definitive answer about relaying / IP helper in this scenario which is bugging me.

    Please can you clarify….

    -Is relaying / IP Helper needed in a multi-subnet hot standby config?

    -If not, how do the DHCP request broadcasts from clients traverse the router when the primary is down?

    -If so, what is the best way to configure it. (ie, list IP addresses as in load balance mode)

    Hope this makes sense?

    Any thoughts on this one team??

  9. Anonymous says:

    You can manually change the state of a server which is running in communication interrupted to partner down using DHCP MMC or DHCP PowerShell.

    In MMC, go to IPv4->Properties, go to Failover tab, select the specific failover relationship and click edit. You will see "Change to partner down" button on the edit page. This button is enabled when the server is running in communication interrupted state.

    In PowerShell, You can run the following command -

    Set-DhcpServerv4Failover -ComputerName <servername> -Name <Failover relationship name> -PartnerDown

    Again, this will work only if the server is in communication interrupted state.

  10. teamdhcp says:

    Hi Chang, when the link goes down, both servers will go into a failover state called “Communicated Interrupted”. In this state, both servers will renew IP address leases which are already leased. For clients trying to obtain a new IP address lease (not renew existing lease), again both DHCP servers will grant new leases but they will do so from different portion of the free IP address pool of the scope – thereby avoiding any conflict. The standby server during this period (when it is Communicated Interrupted) will give new IP address leases from the percentage of the free IP pool reserved for it (reserve address percentage). If the link continues to be down for a long period (state switch over interval), if automatic switchover is enabled, both server will move into Partner down. The situation of both servers running in partner down should be avoided.

  11. teamdhcp says:

    Hi Sam, you do not need to reconfigure. You can convert from load balance mode to hot standby and vice-versa using the DHCP MMC or DHCP PowerShell cmdlet Set-DhcpServerv4Failover with -Mode parameter. If you change to Hot-standby, the standby server is designated by the system depending on from which DHCP server you had originally created the failover relationship.

  12. Anonymous says:

    Gaurav, yes you can have a single DHCP server at the central site as a standby server for each of the remote site DHCP servers. See the hub and spoke model in the blog at –

    http://blogs.technet.com/b/teamdhcp/archive/2012/09/05/multi-site-deployment-topologies-for-dhcp-failover.aspx

  13. teamdhcp says:

    Hi AmritRaj
    As stated in the blog "Hot-Standby mode is more suited for multi-site deployment topologies. Each site would have a local DHCP server which is configured to provide the DHCP service to the clients on the local network and DHCP server at a remote site would
    be standby server." Still if you want to have load balance mode then you would need a relay agent at office which would forward any DHCP Broadcast requests to the server in data centre.

    The latency will not be an issue because a load balanced scope internally takes care of load distribution hence some iP’s will be assigned by one Server and others by the partner Server. The servers will not be competing with each other in offering the ip address
    to the client since only one server will offer the IP address to the client at a given time.
    Thanks

  14. teamdhcp says:

    Hi Rimvydas, your observation is correct. The unicast renew requests will not be responded to by the standby server. The rebind requests (broadcast) will be responded to. The standby server does not respond to renew for the following reason – by not responding to renew but only to rebind, the failback of the client to the original server is sooner i.e. if the active server is up by the time the client gets into rebinding state, it will respond to the client and not the standby server. So, the client will continue to talk to the active server.

  15. teamdhcp says:

    Hi AmritRaj
    For automatic state switchover to happen from communication interrupt to partner down state, you need to enable state switchover interval. If you don’t do that then you would need to manually transition primary server to partner down mode. Also we do not have
    any particular recommendation on MCLT and state switchover interval as the choice entirely depends on your deployment. So 100 % utilized scopes I don’t think its possible to transition them to hot stand by failover mode with 5% reserved quota without deleting
    some leases and freeing up the scope.
    Thanks

  16. teamdhcp says:

    Hi Kapil, the issue you have mentioned sounds familiar to the first issue discussed in the blog at

    http://blogs.technet.com/b/teamdhcp/archive/2014/02/26/dhcp-failover-patch-to-address-a-reservation-issue-and-another-issue-related-to-failover-partner-not-accepting-state-transition-from-bad-address-gt-active-has-been-released.aspx.

    Please install KB 2919393 for Windows Server 2012 or KB 2919355 if you are running Windows Server 2012 R2. This should resolve it if its the same issue.

  17. teamdhcp says:

    Hi Ramvydas, regarding your comment “correct mistakes in examples as there are mistakes in there. For example when both servers are up clients get IP addresses with lease time equal to scope lease time and not MCLT time.”
    When both servers are up and failover relationship is in NORMAL state, a _new_ client lease is given for MCLT duration. When the client comes back for _renewing_ the lease at a time (MCLT/2), it will get the scope lease duration.

  18. teamdhcp says:

    Hi Joey, if the 2 DHCP servers are able to reach other, the failover relationship will be in NORMAL state. In NORMAL state, the client will get an IP address from the standby server provideed it is retransmitting the request messages (most clients including Windows does retransmissions). A standby server which is NORMAL failover state, will respond to 2nd retransmission of DISCOVER and REBIND protocol messages (but not RENEW). So, the short answer is yes, the client will be able to renew an existing lease or obtain a new lease provided the client is retransmitting the messages as required by DHCP RFC.

  19. teamdhcp says:

    Joey, can you please elaborate on what are the exact steps you are performing and what are your observations during those steps.

  20. teamdhcp says:

    Steve, after deactivating the scope, you need to perform "Replicate Scope" to replicate the state of the scope (deactivated) to the partner server. This will ensure that the scope is deactivates on the partner server and it will not respond to clients.

  21. teamdhcp says:

    Hi Dave, your observation is correct, if the standby server is running in COMMUNICATION INTERRUPTED state and if it runs out of IP addresses in it’s reserved IP address pool, it will not be able to give new leases though it will continue to renew existing leases. An admin should manually move the server from COMMUNICATION INTERRUPTED state to PARTNER DOWN state if he/she has not enabled automatic state switchover and the active server is going to be down for quite some time. DHCP server logs events when it transitions into COMMUNICATION INETRRUPTED state which should be picked up by a monitoring solution and alert the admin. The System Center Operations Manager has a DHCP server management pack which picks up these events and sends alerts in the SCOM console. You can download the SCOM DHCP management pack for 2012 here – http://www.microsoft.com/en-in/download/details.aspx?id=39062

  22. teamdhcp says:

    Hi PSY, As soon as primary server goes down, the partner standby server will move into COMMUNICATION INTERRUPTED state (Lost contact with partner) and will start responding to renew requests from the clients. It does not wait for MCLT to expire for renewing clients.

  23. Albert Mihai says:

    Thank you. I’ve just tested in my lab and everything checks out.

  24. teamdhcp says:

    Hi Amitraj, the 2 DHCP failover servers can be in different subnet. There is no performance impact.
    The deployment that you have mentioned is support and quite typical.

  25. teamdhcp says:

    Hi Chang, if both servers move into Partner Down, the problem is both servers take over the free IP pool of the scope (after expiry of MCLT since moving into partner down state). This can cause server 1 to give a specific free IP to a client and server 2 gives the same IP to a different client leading to IP address conflict. For this reason, both server running in partner down should be avoided. Only one server should run in Partner Down state – this server will renew as well as grant new leases to clients. You may want to increase the state switchover interval to a longer value unless
    - the scope is running at a very high utilization rate of IP addresses
    - the reserve address percentage of IP addresses given to standby would be inadequate to meet influx of _new_ lease requests.
    Let me know if your have further questions.

  26. teamdhcp says:

    DHCP Failover itself has no dependency on AD so the lease synchronization protocol itself would work fine between 2 DHCP servers in different AD domains without trust. However, you may run into issues while configuring since the user setting up failover
    needs to be member of DHCP Administrators account on both the DHCP servers for which you are setting up failover.

  27. teamdhcp says:

    Hi Chang, if both servers move into Partner Down, the problem is both servers take over the free IP pool of the scope (after expiry of MCLT since moving into partner down state). This can cause server 1 to give a specific free IP to a client and server 2 gives the same IP to a different client leading to IP address conflict. For this reason, both server running in partner down should be avoided. Only one server should run in Partner Down state – this server will renew as well as grant new leases to clients. You may want to increase the state switchover interval to a longer value unless
    - the scope is running at a very high utilization rate of IP addresses
    - the reserve address percentage of IP addresses given to standby would be inadequate to meet influx of _new_ lease requests.
    Let me know if your have further questions.

  28. teamdhcp says:

    Kenneth, DHCP failover protocol does not have any dependency on domain membership of the 2 DHCP servers. However, to configure DHCP failover, the user needs to have permissions of "DHCP administrators" on both the DHCP servers. I am not sure if that’s
    possible with 2 servers being in different forest w/o trust relationship.

  29. Albert Mihai says:

    Just to go over lease duration one more time: for BOTH Load Balance and Hot Standby modes, under NORMAL operating scenario (both servers up) then a new client requesting a new lease will get one for MCLT time. When doing the renewal (broadcast message at 7/8 of MCLT) it will get a lease as specified by the corresponding scope.

    When the relationship is no longer in NORMAL state (either COMMUNICATION INTERRUPTED or PARTNER DOWN), in BOTH Load Balance and Hot Standby mode, the client will get from the “backup” server (Standby server in Hot Standby mode – if Reserved Addresses available – or the server that doesn’t own the hash bucket for that respective client’s MAC address in Load Balance mode – after successive retransmissions) a lease of MCLT time.When doing the renewal (broadcast message at 7/8 of MCLT) it will get a lease as specified by the corresponding scope.

    Essentially we get the same values for lease times regardless of failover relationship status, either it’s a new lease or a renewal, right ?

  30. Anonymous says:

    Hi Bashar, you can install it on 2 Windows Server 2012 AD domain controllers. There is no need for dedicated servers for DHCP Failover

  31. teamdhcp says:

    Albert, most of what you described is correct. Some corrections -
    - lease renew via a unicasted message is done by the client at 1/2 of the lease period (not 7/8th)
    - if unicast renew message fails to get a response, at 7/8th of the lease period, the client starts sending the same message as a broadcast (this is called REBIND state/message).

    So, in your first para, client will get lease duration as specified on the corresponding scope at MCLT/2.

    Regarding your second para, the server in PARTNER DOWN or COMMUNICATION INTERRUPTED state always gives lease for MCLT duration and never full scope lease duration.

  32. Anonymous says:

    If you do not specify the "state switchover interval" parameter, how do you instruct a server that the partner is down? Reading the documentation:

    "Since there is no way for the server to detect the reason for loss of communication with its partner, the server will continue to remain in communication interrupted state until the administrator manually changes the state to partner down."

    I don't see an option to manually change to partner down.

    thanks!

  33. Bashar Bader says:

    Can we install this new DHCP Failover in 2 Windows server 2012 AD domain controller servers or it should be in two dedicated servers?

  34. dragan says:

    Normally it was my best solution to put it together with the DC Role, but the Best Practices Analyzer had some failures if put the roles on same machine.

    Or was this only for Srv 2008 ?  technet.microsoft.com/…/ee941109(v=ws.10).aspx

  35. teamdhcp says:

    There is no change in this regard – you can run DHCP and DC together. The BPA failure that you may be seeing is related to setting DNS credentials when running DC and DHCP together. See

    technet.microsoft.com/…/ee941181(v=ws.10).aspx

    This is to ensure that dynamic DNS updates from DHCP server work fine.

  36. Arvin says:

    Is it possible to configure One active  and Two Hot Standby servers… ?   One standby  in same site and second standby in another site ?

  37. teamdhcp says:

    Arvin, for a given scope(s) DHCP failover supports only 2 DHCP servers. If you require redundancy beyond 2 DHCP servers, you can use DHCP Failover in conjunction with Windows Failover cluster i.e. you can deploy DHCP server 1 on a Windows Failover cluster at site 1 and then configure DHCP Failover between DHCP server 1 to DHCP Server 2 at site 2 in hot standby mode.

  38. PSY says:

    Is it possible to have the Maximum Client Lead Time to 20 minutes and set State Switch over Interval to 60 minutes. From what I gather, after 60 mins the active server will be transition to a down state but since the MCLT is 20 mins… Will the standby DHCP server poll thrice in a hour before it takes over the scope. Thanks.

  39. teamdhcp says:

    PSY, You can set MCLT to 20 minutes and state switch over to 60 minutes. When a server is unable to communicate with its partner DHCP server (which could be because the partner server is down or there is a network connectivity problem between the two), it will automatically transition to COMMUNICATION INTERRUPTED state. If state switch over is enabled, it will start a timer for the specified state switch over interval (in this case 60 minutes). During this time (state switch over interval), it will try repeatedly to establish contact with the partner DHCP server – every few seconds. If the server succeeds in re-establishing communication with partner DHCP server, the server transitions back to NORMAL state of the failover relationship. If even after expiry of the state switch over time, the server is not able to establish communication with partner DHCP server, it automatically moves to PARTNER DOWN state. After moving to PARTNER DOWN state, the server will start another timer for MCLT. On expiry of this timer, if it is still in PARTNER DOWN state, it will take over the portion of the free IP pool which was owned by the partner DHCP server. Even while in PARTNER DOWN state, the server will continue to make attempts to re-establish communication with partner server every few seconds. If communication with partner is re-established, it will transition out of PARTNER DOWN state and eventually moved back into NORMAL state.

  40. Danny Stettler says:

    Is it possible to have the failover server in another non-trusted domain?

  41. PSY says:

    Thanks for the previous response on MCLT and State Switch Over. You mentioned that once the state is moved to PARTNER DOWN, the MCLT timer is kicked in and after the MCLT is lapsed then the free IP pool is seized and owned by the other server. What would happen if the client doesn't request a new IP but rather renews it. Will the standby server be able to renew the same IP address issued by the primary server before it lost communication and if the MCLT timer has not lapsed. So for example if the MCLT value is 1 hour and the primary server went down in 30 minutes, will the clients requesting renew be serviced by the standby server or will it have to wait for the time till the MCLT value expires?

  42. Rimvydas says:

    Hi DHCP Team,

    Can you clarify the following thing. When standby server moves to PARTNER DOWN state, how exactly must it respond to client requests? Because I'm seeing the following:

    Client gets an IP address from standby server. Lease duration is equal to MCLT time. Everything ok here. But when client tries to renew its IP address from standby server at 50% MCLT time by sending direct request to dhcp server, dhcp server simply refuses to respond. And only when client goes to rebinding state, only then DHCP server renews client's lease. And then the same bahavior again – dhcp standby server does not respond to direct client's renew requests and only responds when client goes to rebinding state. Is such behavior expected and normal???

  43. Chang says:

    Hi DHCP team, a quick question on the Hot Standby configuration design. If there are 2 DHCP servers configured in a hot standby mode across 2 data centers and if only the link connectivity if lost between the 2. Which DHCP server will lease out IP addresses, both the servers are active at this point in time.

  44. Chang says:

    Thank you very much for the quick response. So if the State Switch Interval has passed and still if the link is down, will both DHCP servers stop leasing out new IP address as it is in partner down state. Our SSI interval is set to 10 mins so there is a possibility that the link is down for more than 10 mins. Also I am assuming that the lease renewal will be serviced by the standby server.

  45. Chang says:

    Thanks again for the revert. We have a scenario where we have centralized DHCP Servers with only the Active DHCP server servicing clients across many subnets . We are planning to have a Hot Standby configuration instead of Load balance. In this scenario which would be the best design recommendation… Thanks.

  46. Rimvydas says:

    Hi teamdhcp,

    Thank you for your clarifications. But I still think that there will be no bad things for standby server to respond to direct renew requests. Clients get IPs from standby with lease time equal to MCLT which is short so I thing there will be no huge delays for the clients to get back to primary server. To be honest I see no difference. When primary server is up, standby will stop to respond and we'll have the same bahavior with almost the same times. As for now, because standby server does not respond to direct renew requests – it is simply bombarded by huge ammount of direct clients requests. I think this is not good thing.
    And please, expand your docs and include what exactly standby server does when it is in partner down state. There is mentioned nothing what it does at such time.
    And please, correct mistaked in examples as there are mistakes in there. For example when both servers are up clients get IP addresses with lease time equal to scope lease time and not MCLT time.

  47. JoeyM says:

    Hi DHCP team, If the DHCP servers are configured in a Hot standby and if the client loses connectivity only with the primary server, will it receive the IP from the standby server?

  48. JoeyM says:

    Hi DHCP team, Thanks for the clarification about the standby server leasing out the IP address. My understanding of it was the same. However in our lab the client is unable to renew/lease IP address from the standby server if it loses connectivity with the Primary DHCP server. We have configured a scope in Hot Standby mode with enough free IP add reserve on the standby server. Am i missing something? thanks!

  49. Kapil says:

    We have 2 DHCP 2012 implemented at my site and both are in cluster with hotstandby mode (one is active and another is standby). We face issue that the IPs which are reserved earlier are getting deleted and the same IP is getting assigned to some other machine which is not reserved. Require guidance for resolving the issue.

  50. Dave says:

    I'm a little confused about something not explicitly mentioned in the documentation (at least, not that I can find). The way I understand things, if the standby server is in the COMMUNICATIONS INTERRUPTED state and has available addresses in its reserved pool, it can lease those addresses to clients that don't already have an existing lease.

    If the state switchover interval isn't enabled, the standby server won't automatically transition to PARTNER DOWN, so what happens if its reserved pool is exhausted? Does it simply stop giving out new leases? Assuming clients can't contact the active server, either because it's down or the link between it and the clients is severed, are they just out of luck until something gets fixed or someone transitions the standby server to the PARTNER DOWN state?

  51. Sam says:

    How do you convert failover from load balance mode to hot standby, do you have to Deconfigure existing failover first? I think "Yes", but just want to make sure.

  52. Sam says:

    Its strange that in hot standby setup in my test lab none of my client are getting ip address. Works perfectly fine in load balancing mode. Tried using both the DHCP servers to be primary in case of standby.

  53. Sam says:

    Please ignore my previous comment with Standby mode issue. It turned out that my VMWARE workstation ran into some issues and fix was to restart all the DHCP and DC servers.. Thanks for your help :-)

  54. Amitraj Kulkarni says:

    For a Load Balancing mode, Do we need to place both servers on a same subnet ? or we can place it on a diffrent subnet , is there any performance impact?

    We are planning to place one server at our Datacenter and another in our office and we have 1GB point to point network connection between Datacenter and Office, please advice..

    regards
    Amitraj

  55. Amitraj says:

    Thanks for the clarification

    can you post series of testing scenarios for both Load sharing mode and hot standby mode, as i have built lab and want to perform these tests before deploying this on production environment.

  56. Amitraj Kulkarni says:

    For Load Balancing mode:

    As explained previously, We are planning to place one DHCP server at our Data center and another in our office and we have 1GB point to point network connection between Data center and Office,

    1.since both servers will be on a separate sites how clients from office will get IP's from both the servers.

    will the DHCP server located in office gets overloaded? as client is close to the DHCP server in office.

    or how the clients requests will go to DHCP server located in Data center ( as no of hops are more)

    2. If the N/w latency of DHCP server located in Data center is more compared to the DHCP server located at office. how the clients will manage to get IP's from both DHCP servers ( will the DHCP request will be balanced between both servers)

    3. Does latency mismatch between DHCP servers to client will make any difference in serving the DHCP requests?

  57. Amitraj kulkarni says:

    I have recently migrated windows 2003 r2 and windows 2008 R2 DHCP servers to windows server 2012 r2 and configured a hot standby failover between centralized hot standby failover server and these two servers.
    All the scopes have lease duration of 8 hours and my question is what recommendation from MS is on
    • Maximum Client Lead Time ?
    • State Switch over Interval ?
    For automatic failover to happen back and forth from primary partner server and hot standby server?
    Do we need to manually transition primary server into partner down mode

  58. Amitraj Kulkarni says:

    Hi

    we have many scopes , which are already utilized 100% lease quota.
    for such scopes how do we transition to hotstnadby failover mode? as i understand we need to have 5% of free address.

  59. Steve says:

    Hi
    We are currently in the process of testing the Hot Stand by mode in preparation for an upgrade. During the testing process we are trying to deactivate a single scope on the primary DHCP server and seeing if the secondary DHCP server will respond. Is this even
    possible ?

  60. Kenneth says:

    Hi,
    Can the Hot Standby be a member server in another Active Directory domain, not the same forest and not With any domain trust configured?

  61. Gaurav says:

    Situation : 3 Remote sites and One Central Site
    Is it possible to have one Active DHCP Server in each Remote site and using the 4th Central DHCP Server for Hot-Stand By Relationship for all the 3 remote DHCP Servers, OR is it actually required to have a separate Stand By Server for each remote site.

  62. Amitraj Kulkarni says:

    We have hotstandby failover configured in Hub and spoke model.
    Single central standby failover server and other remote servers have failover established.

    when i configure failover relation, i don't get all the scope options reflected on central failover server.
    for example for wireless , we need to have option 043 on primary server with 10.141.x.x IP but when i go and check on failover server scope options for this subnet it shows blank (even though 043 option enabled )

    There are some other options on the scope that also are not showng up on failover partner like, TFT server IP and 066 Boot server hostname and 067 Bootfilename

    due this wireles wasn't working from failover partner.

    Most of the non default dhcp options are with emptyvalue on failover partner.

    Please advice how to replicate scopes options with actual value.

  63. amitraj kulkarni says:

    and also we have diffrent cisco TFTP servers for remote site, how can we make sure it is reflected on failover partner

  64. Jeff Jones says:

    We have an issue where we have multiple sites and one DHCP Hot Standby (working good) issue we have run into is the DHCP Vendor Classes have different display names for the same option (ie one is named Cisco 3502i Series AP service Provider and the other
    is Cisco 3502i Series AP) when you try to add the server in with hot standby it fails. We have found that if we remove it from one and re-add it with the same name it will add. Is there an easy way to rename the Vendor Classes so that I can script something
    to rename them before we add to our hot standby? I have quite a few out there that where not named in the standard naming convention we use.

    Thanks and great article and responses!

  65. teamdhcp says:

    Jeff, Yes you could script this using DHCP PowerShell cmdlets. Here are the steps with CPE-OCPhone as an example. I will try and post a script to do this.

    /* Remove any option value assigned for vendor class specific option – in this case Vlanid (option id 10) */
    Remove-DhcpServerv4OptionValue -ScopeId -VendorClass " CPE-OCPhone " -OptionId 10

    /* Remove the vendor class specific option definition – vlanid (option id 10)*/
    Remove-DhcpServerv4OptionDefinition -VendorClass "CPE-OCPhone" -OptionId 10

    /*Remove the vendor class definition – CPE-OCPhone*/
    Remove-DhcpServerv4Class -Name " CPE-OCPhone" -Type Vendor

    /*Add the vendor class definition with correct case – CPEOCPHONE*/
    Add-DhcpServerv4Class -Name "CPE-OCPHONE" -Type Vendor -Data "CPE-OCPHONE"

    /*Add the vendor class specific option definition – vlanid (option id 10) */
    Add-DhcpServerv4OptionDefinition -VendorClass "CPE-OCPHONE" -OptionId 10 -Name "VLAN ID" -Type "Word"

    /* Re-add the option value assigned for vendor class specific option which was removed in the first step – in this case Vlanid (option id 10) */
    Set-DhcpServerv4OptionValue -ScopeId 10.20.0.0 -VendorClass "CPE-OCPHONE" -OptionId 10 -Value 10

  66. Gosa says:

    We have hub spoke model AD with multiple remote sites and each remote site there is one DC with DHCP server. Is it reasonable to make hot standby model between two remote site DHCP servers ?

  67. Gosa says:

    What firewall ports must be open between dhcp servers ?

  68. teamdhcp says:

    Gosa, all failover communication between the DHCP failover servers is over TCP port 647. Firewall rules for this are installed by default on the Windows Server where DHCP server role is installed.

  69. teamdhcp says:

    Gosa, regarding your question about doing hot standby model between two remote site DHCP servers – yes, this is a supported deployment. Please see the blog:

    http://blogs.technet.com/b/teamdhcp/archive/2012/09/05/multi-site-deployment-topologies-for-dhcp-failover.aspx

  70. Mitch says:

    For a project on school i have a (gateway HQ, branch network an child network in each network i have a dhcp server, so i have 3 i would like to configure the one in the hq network as a hot spare for the branch one and load sharing for the branch and child,
    is this possible? And could you explain a bit about the configuration of tese? Thanks in advance.

  71. teamdhcp says:

    Mitch, yes you can configure the DHCP server HQ network as a hot standby for the branch network and load sharing between DHCP servers at branch and child network. You can setup these by:
    – Configure a failover relationship from the DHCP server at branch network. Select the HQ DHCP server as partner and select the mode as hot standby. This will provide high availability for the DHCP scopes of branch network.

    – next, Configure failover relationship from the DHCP server at child network. Select the DHCP server at the branch network as the partner and select load balancing mode. This will provide high availability for the DHCP scopes of the child network.