DHCP Fail-over Implementation Windows Server 2012R2
This article is about DHCP Fail-over implementation on
Windows Server 2012R2, by the time I publish this blog Microsoft launched
windows server 2016 and 2019 Technical preview but I would like to share this
for those who are still with 2012R2 and planning for DHCP Fail-over implementation.
Microsoft introduced this feature in windows 2012 to make
the DHCP service reliable and redundant.
I will take you all through the best practices to be
followed and the detailed implementation plan, also i would like to flag the
limitations in DHCP Fail-over scope option replication in W2K12 and its solution
provided by Microsoft.
Document Scope
This document is focusing more on
the limitation of DHCP Failover replication and its solution in Windows server
2012 platform; however an overview of the failover feature and its installation
and configuration which has been performed on a demo site is covered here in
the first half of this document.
Recommended to read the reference
links provided in this document before installing it in production environment
as the content may amend time to time by Microsoft.
Introduction
DHCP is one of the critical
components of an IT environment today. Ensuring its continuous availability is
one of the top priorities of any IT administration. In Windows Server 2012,
DHCP server can be configured to provide high availability by pairing two DHCP
servers in a failover relationship. Two DHCP servers in a failover relationship
synchronize the IP address lease information on a continual basis there by
keeping their respective databases up-to-date with client information and in
sync with each other
DHCP Server Failover Feature
The DHCP failover feature can be
used in two relationship modes:
Load Balance
(Active-Active): Two independent DHCP servers share the
responsibility for servicing clients in a scope or a set of scopes as per a
configured load balance ratio. In case anyone of the servers fails the other
assumes the complete responsibility for servicing the clients.
Load balancing in a single site with a single subnet
Load balancing in a single site with multiple subnets
Hot Standby
(Active-Passive): A DHCP server can be designated as a
standby server for a primary DHCP server. The standby server assumes the load
in case primary server goes down.
Hot Standby in a multi-Site infrastructure
Hot Standby in a single site infrastructure
Both these modes increase the
redundancy of DHCP service in the network and make it more fault-tolerant. This
fail-over feature can be used in different topologies like hub and spoke
topology or ring topology.
Note: But if the user makes any changes in any
property/configuration (e.g. add/remove option values, reservation) of a
failover scope, he/she needs to ensure that it is replicated to the failover
server.
Due to these limitations in DHCP
Fail-over replication, it is preferred to use Hot stand-by mode in both single
and multi-site environment.
Solution: DHCP Failover Auto Config Sync (DFACS) which
will be explained later in this document.
The DHCP Failover feature has been explained in greater detail on http://technet.microsoft.com/en-us/library/hh831385.aspx
DHCP Fail-over Implementation
Pre-requisites
1) Ensure
that a valid recent full backup of the server is available.
2) Take a
backup of DHCP Configurations on the existing server.
3) Identify a
server to configure DHCP standby partner server.
4) Check the
WDS roll is installed on the proposed DHCP standby server (If yes make sure that the port 67 is checked
in the WDS
5) Ensure
that the TCP port 647 is open in the firewall of both servers in the DHCP
failover relationship.
If TCP port 647 is blocked, create an inbound and outbound
firewall rule to allow the connection. (If it is a multisite environment make
sure that TCP port 647 is opened in layer3 devices/Firewalls)
6) Ensure
that the clocks on both servers are synchronized to within one minute (Check
NTP server’s info)
7) Check
existing DHCP server will update dynamic DNS records, if so new and existing
DHCP server should use the same credentials
Ex: Service
account – “DHCPConfig” (Verify the password)
8) If DHCP
Failover partner is configure in hot-standby mode ensure each scope is having
enough IP addresses to reserve 5-10% for the partner server.
Open DHCP manager à
Navigate to the scopes configured under ipv4 à
Right click on the scopes which are going to add to the failover à click “Display
Statistics”.
The output will be shown as below
a) Scope – Example scope 1
IP Range – 10.20.0.0/22 (255.255.252.0)
Description,Details
Total Addresses819
In Use,479 (58%)
Available,340
(41%)
b) Scope – Example scope 2
Description,Details
Total Addresses,244
In Use,42(17%)
Available,202(82%)
09) The maximum number of fail-over relationships for either
DHCP server
The local/partner server already has 31 (maximum allowed)
fail-over relationships. A server cannot have more than 31 fail-over
relationships.
10) The operating system on the fail-over partner server
The version of specified DHCP server does not support
fail-over.
11) Are scopes already present on the fail-over partner
server
Following scopes already exist on the specified partner
server. These scope(s) will need to be deleted on the partner server before
configuring fail-over
For detailed
considerations and pre-requisites, please refer the links below
https://sjohnonline.blogspot.com/2018/12/in-this-article-i-am-sharing-detailed.html
https://sjohnonline.blogspot.com/2018/12/in-this-article-i-am-sharing-detailed.html
Implementation Plan
DHCP Role
installation on partner server
1) Login to
server (Proposed DHCP standby server) with domain admin credentials
2) Install
DHCP service role on the proposed DHCP standby server but not installed with
WDS role.
3) On Server
Manager à under Configure
this local server à click Add
Roles and Features.
4) In the
Add Roles and Features Wizard à click Next
three times, and then on the Select server roles page select the DHCP Server
checkbox.
When you are prompted to add required features àclick Add Features.
5) Click
next three times and then click Install.
6) Wait for
the installation process to complete.
7) On the
Authorization screen of the DHCP Post-Install Configuration wizard, select the
appropriate option for selecting a user with domain admin rights. This is
required to authorize the server. When done, click Commit.
Configure DHCP
failover
1) Open the
DHCP console from Existing DHCP Server
Expand IPv4
Right-click on IPV4 from the left column and select
Configure Failover
Select all scopes
2) Click Add
Server to add the partner server and add the Proposed DHCP Standby server as
DHCP partner server, then click next
3) Give a
valid failover relation name “servername1
– servername2 Failover “
4) Maximum
client lead time set it as 1 hour (Default value)
5) Select
the failover mode as "Hot Standby", then specify the role of partner
server as "Standby"
6) Enter the
address reservation for standby server (As per the total address availability
in each scope)
7) State
Switchover interval set to 60 minutes (Default)
8) Enable
message authentication and enter the shared secret, then click next and finish
9) Check if
all the jobs are completed successfully
Configure IP Helper
(On a Layer 3 device)
As the routers block broadcast by default and DHCP Uses broadcast for the IP leasing mechanism, If your infrastructure is having multiple subnet and each subnet that does not have its own DHCP server then will need to be configured with a IP helper address on the router or layer 3 device to direct DHCP broadcast request to the DHCP Server.
1) Login to
the core switch “IP addresss” by
entering the admin credentials
2) Take the
backup of switch
3) Add
standby DHCP Server’s IP address as secondary IP helper address to the Vlan’s
which are already configured with IP Helper address.
For HP Layer 3
switches (Follow the below commands)
#vlan 105
#name "example1"
#untagged A12
#ip helper-address 10.20.0.10 ! Primary DHCP Server
#ip helper-address 10.20.0.11 ! Standby DHCP Server
#ip address 10.20.4.1 255.255.255.0
#tagged
A1,A3,A5,A24,B18-B20,C1-C2,C4,C6,C13-C24,D1,D13-D24
#ipigmp
Exit
For Cisco Switches
interface vlan123
descsvi for vl123 dhcp relay example
ip address 10.20.4.1
ip helper-address 10.20.0.10 ! Primary DHCP server
ip helper-address 10.20.0.11 ! Secondary DHCP server
4) Repeat
step 3 for all the other Vlan’s
vlan 110 - name "example2"
vlan 115 - name "example3"
vlan 160 - name
"exampl34"
5) Copy the
changes to startup config
6) Take the
backup of Running & startup config and save it in a secure location.
#Copy running configtftp
#Copy startup configtftp
If you are using any other layer 3 device, use your vendor guide and check how to configure IP Helper.
Post Implementation Checks
1) Right-click
on Scope on the left column and select Properties, then open Failover tab
Review the failover configuration status of the main server
- Role of this server should be "Active"
2) Repeat
the operation on the partner server
Review the failover configuration status of the partner
server - Role of this server should be "Standby"
3) Failover
the server in out of office hours and check whether the clients are getting new
IP address.
Rollback Plan
1) Go to the primary server.
Right click IPV4 hit properties.
Go to failover tab.
Select server pair and hit delete.
2) Remove the DHCP role from replica server
3) Revert the IP helper configuration.
