Redundancy is important for coping with each deliberate and unplanned outages, and that features having redundant dynamic host-configuration protocol (DHCP) servers to permit uninterrupted dynamic project of IP addresses.
For these working in Home windows environments, there are at the moment two choices for organising redundant DHCP servers: a failover situation with a essential server paired with one other in scorching standby; and a load-balancing situation by which two DHCP servers actively deal with shopper requests.
The new-standby mode requires a major server that delivers DHCP companies for all of the purchasers in a selected IP-address vary (scope) and a standby server that takes over solely when the primary server is unavailable for a interval that’s configurable within the setup.
This feature is good for situations the place the standby DHCP server is at a distinct bodily location from the primary server, making it much less probably that each will fail on the identical time as a consequence of energy failure or {hardware} failure. The standby may also be a digital machine operating on separate {hardware} and powered from a distinct electrical supply.
The load-balancing mode additionally calls for 2 servers, however each are energetic on a regular basis and share dealing with DHCP requests.
Server necessities
To arrange both possibility in a Home windows surroundings, the servers should meet two standards:
- Each servers will need to have the DHCP function enabled and configured by means of Home windows Server Supervisor or PowerShell. Normally, there’ll already be an energetic DHCP server within the community, so organising a second machine with the DHCP function enabled have to be licensed. To do that, connect with the second server utilizing the Microsoft Administration Console (MMC) DHCP snap-in, right-click on the server object and choose “Authorize”.
- For the relationships to work, settings for each servers have to be set to deal with the identical IP tackle ranges and have the identical scope/server choices. In the event that they don’t match, the failover relationship can’t be established. The best approach to do that is by copying these particulars from one server to the opposite with a backup on the prevailing DHCP server and a restore on the brand new one. To export from the prevailing DHCP server that you just need to backup, connect with it utilizing the MMC snap-in, develop the server object, right-click the server’s identify, and choose “Backup”, then choose the backup location on the searching dialog field that seems.
To determine the failover relationship, connect with the preliminary DHCP server utilizing the DHCP MMC snap-in once more, develop the server object, right-click on the “IPv4” icon or a particular scope and choose “Configure Failover”. (When configuring the failover on the “IPv4” icon fairly than on a person scope, a number of scopes may be added to the failover relationship on the identical time.) Within the dialog field that seems, choose the scopes that you really want a failover relationship created for, or verify “Choose all” within the part “Obtainable scopes” to have the entire obtainable scopes chosen. Click on “Subsequent” and specify the server you want to create the connection with by typing the absolutely certified area identify (FQDN) of the opposite server or searching for it in energetic listing utilizing the “Add Server” button.
Defining the properties of the failover requires some configuring. Some settings apply to each the load-balancing and hot-standby modes; others apply simply to 1 or the opposite. Settings for each modes embody:
- Relationship identify: By default the connection identify would be the two names of the servers being added within the format of server1-server2, however these may be modified to be extra descriptive if wanted.
- Most shopper lead time (MCLT): This units the utmost period of time both of the servers can lengthen a DHCP lease past what the accomplice server already is aware of about. Microsoft recommends the MCLT be set to 1 minute.
- State switchover interval: This units the period of time that one of many servers is unavailable earlier than it’s switched to a partner-down state. The default setting is 60 minutes throughout which the standby server will tackle the complete duty of the failover scope till its accomplice modifications state once more.
- Allow message authentication: That is an elective function to permit for a shared secret for the 2 servers to show to one another that they’re who they are saying they’re. If this setting is checked a shared secret have to be supplied which is able to use the Safe Hash Algorithm 2 (SHA-2).
For simply the load-balancing mode:
- Load Stability Proportion: This units the chances of what number of DHCP requests will likely be dealt with by every server.
For simply the hot-standby mode:
- Sizzling Standby Configuration: This setting has two elements. The primary is Function of accomplice Server which ought to be set to “Standby” if configuring from the primary server. The second “Addresses reserved for standby server” is the share of every scope that’s reserved for the standby server to make use of within the occasion the primary server is unavailable. The default is 5%. Be aware that when the primary server is unavailable, the standby server will proceed to resume IP tackle of current leases for the period of time designated by the MCLT setting. It would solely use the reserved standby addresses for brand new DHCP lease requests till the primary server is again on-line or is designated as “Associate Down”.
As soon as these settings are made, a abstract dialog field and last progress dialog field will present whether or not the connection was efficiently created. If it wasn’t, be sure that the entire scopes and scope/server choices are current on each DHCP servers then run it once more.
Sync and replication with new scopes
Any modifications to the scopes laid out in these relationships don’t sync or replicate mechanically. That have to be finished by going again to the MMC DHCP snap-in, proper clicking on the scope on the first server and selecting “Replicate scope”. Or it may be finished through PowerShell utilizing this command:
Invoke-DHCPServerv4FailoverReplication -computername DHCP-server.instance.com
A brand new scope created on one server after the connection between the servers has been established won’t mechanically be created on the opposite server. This requires the identical steps as when organising the unique relationship, however there’s a time-saving possibility. Choose the prevailing failover relationship from throughout the wizard by checking “reuse current failover relationships configured with this server (if any exist)”. This seems within the first dialog field within the wizard.
Lastly, networks generally embody a DHCP relay—usually the perform of a core change—that factors to the energetic DHCP server for VLANs and subnets. After organising failover-server relationships, be sure you add the static IP tackle of the second server to the listing of DHCP relays so community site visitors will likely be despatched to it when there’s a want for failover. That is utilized in another way relying on the place the relay is situated and the way it’s arrange, however the failover ought to be examined after it’s configurated and the relay is up to date.
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