DHCP High Availability

DHCP High Availability:-

A highly available solution must account for all possible points of failure, including server failures, WAN link interruptions, and router failures. You can increase the fault tolerance and availability of your design by using one or more of the following, depending on your needs and hardware cost considerations:

• Split-scope configurations
• Clustered DHCP servers
• Using standby servers

Split-Scope Configurations:-

You can increase fault tolerance by splitting DHCP scopes between multiple DHCP servers. With a split-scope configuration, if one server becomes unavailable, the other server can take its place and continue to lease new IP addresses or renew existing clients.

Splitting DHCP scopes also helps to balance server loads.

When splitting the IP address pool of a scope between two servers, assign the same scope to both servers, and exclude opposite portions of the address range. You also need to make identical reservations at both DHCP servers, so that either server can assign the reserved IP address, ensuring that the intended device receives the address that is reserved for its use.

Clustered DHCP Servers:-

Windows Clustering can automatically detect the failure of an application or server and restart the application on or transfer the server role to an alternate server. Users experience only a brief break in service.

Windows Clustering creates a virtual DHCP server so that if one of the clustered nodes fails, the namespace and all of the services contained in that node are automatically transferred to a second node. No changes are visible to the client, which sees the same IP address for the clustered DHCP servers.

Standby Servers:-

A standby server and its scopes are not activated for use under normal conditions, and are activated by the administrator only when needed, such as when a DHCP server fails or is taken offline for an extended period of time. Standby servers require manual administration to ensure failover transition, and therefore might not be as effective as other failover methods, such as split scopes and clustered servers.

To use a standby configuration, configure an additional DHCP server to server as a backup if the primary server goes offline. You can either configure the standby server to be identical to your primary DHCP server or configure the standby server with unused scopes to temporarily replace the primary DHCP server.

Differences between Quick Format and Full Format

1. Quick Format: If you choose the Quick format option, format removes files from the partition, but does not scan the disk for bad sectors. Only use this option if your hard disk has been previously formatted and you are sure that your hard disk is not damaged.

2. Full format: When you choose to run a regular format on a volume, files are removed from the volume that you are formatting and the hard disk is scanned for bad sectors. The scan for bad sectors is responsible for the majority of the time that it takes to format a volume.

DHCP Rely Agent

DHCP Rely Agent:

If you have multiple subnets in your network, and do not have a DHCP server on every subnet, determine whether your current routers (Software\Hardware) relay DHCPmessages.

The DHCP relay agent relays DHCP message traffic between the DHCP-enabled clients on the local network and a remote DHCP server located on another physical network by using the IP address of the remote DHCP server.

How relay agents work

A relay agent relays DHCP messages which were broadcasted by NIC's on the network.

The following illustration shows how client C on Subnet 2 obtains a DHCP address lease from DHCP server 1 on Subnet 1.



1. Client C generated a broadcast message when it turned in the networ. It will use UDP protocal for this generation on the port 67 (the port number reserved and shared for DHCP server communication).

2. In this case DHCP relay agent willtake the broadcasted message from the network and check for the gateway in the DHCP message header. If it did not find any gateway information it will fill that with router IP and sent it to Subnet 1 where DHCP server is located.

3. When the DHCP server recives the message, it will check the gateway IP and will check the DHCP scope where the router IP has beed configured.

4. After that it will check the DHCP scope and will release an IP from the scope and will to the DHCP relay with the IP configuration.

5. Then, DHCP realy agent will send that informatino to the client and it will take DHCPREQUEST from the client and will send it to server.

6. Again, it will receive DHCPACK packet from the server and will forward that to the client.


 Configure the DHCP Relay Agent

•Add the DHCP Relay Agent
•Configure global DHCP Relay Agent properties
•Enable the DHCP Relay Agent on a router interface

Add the DHCP Relay Agent

1. Open Routing and Remote Access.
2. In the console tree, click General.
   Where?
   • Routing and Remote Access/server name/IP Routing/General
3. Right-click General, and then click New Routing Protocol.
4. In the Select Routing Protocol dialog box, click DHCP Relay Agent, and then click OK.

Configure global DHCP Relay Agent properties

1. Open Routing and Remote Access.
2. In the console tree, click DHCP Relay Agent.
   Where?
   • Routing and Remote Access/server name/IP Routing/DHCP Relay Agent
3. Right-click DHCP Relay Agent, and then click Properties.
4. On the General tab, in Server address, type the IP address of your DHCP server, and then click Add.
5. Repeat step 4 for each DHCP server you need to add, and then click OK.

Enable the DHCP Relay Agent on a router interface

1. Open Routing and Remote Access.
2. In the console tree, click DHCP Relay Agent.
   Where?
   • Routing and Remote Access/server name/IP Routing/DHCP Relay Agent
3. Right-click DHCP Relay Agent, and then click New Interface.
4. Click the interface you want to add, and then click OK.
5. In the DHCP Relay Properties dialog box, on the General tab, verify that the Relay DHCP packets check box is selected.
6. If needed, in Hop-count threshold and Boot threshold (seconds), click the arrows to modify the thresholds.