You can split a scope between two or three servers so that a network can more easily handle DHCP traffic floods. In addition, stopped servers will not noticeably affect the network. An 80/20 split offers the optimum benefit.
For example, consider a Class B scope 132.255.0.0 with an address range from 132.255.0.1 through 132.255.255.255 and a subnet mask of 255.255.0.0. You could set up two servers (SRV1 and SRV2) to distribute the load as follows:
SRV1 has a scope of 132.255.0.1 through 132.255.255.255 with a subnet mask of 255.255.0.0. The exclusion range for this scope is 132.255.128.0 through 132.255.255.255.
SRV2 has a scope of 132.255.0.1 through 132.255.255.255 with a subnet mask of 255.255.0.0. The exclusion range for this scope is 132.255.0.1 through 132.127.255.255.
Similarly, you can divide a scope between three servers.
Avoid Deactivating Scopes
Do not deactivate a scope until you are ready to remove it (and its included range of addresses) permanently from use on the network. After a scope is deactivated, the DHCP server no longer accepts the scope addresses as valid, which can cause unwanted DHCP negative acknowledgement messages (DHCPNAKs). You can temporarily deactivate scope addresses by modifying exclusion ranges in an active scope to achieve the intended result without unwanted messages.
Use Proper Superscope Implementation
Although superscopes can ease DHCP management, they are not required just because a DHCP server is handling more than one scope (subnet ID). A single DHCP server can serve two or more physically different subnets separated by a router, where BOOTP/DHCP relay agents are configured to provide relay of client requests for scopes that serve subnets located away from the DHCP server. Relay agents are typically included with your routers and, where used, must be configured with IP addresses for your DHCP servers.
Use Multiple DHCP Servers for the Same Superscope
When using more than one DHCP server to serve a superscope segment, the superscope for each DHCP server should include all subnets, using placeholder scopes for the subnets to which it does not supply addresses but must recognize as valid.
For example, consider a segment running four logical IP subnets: 192.168.1.0, 192.168.2.0, 192.168.3.0, and 192.168.4.0, all with a subnet mask of 255.255.255.0. Two servers running DHCP support this segment, each configured with a superscope covering half of the subnets (the SRV1 superscope contains only subnets 192.168.1.0 and 192.168.2.0; and the SRV2 superscope contains only subnets 192.168.3.0 and 192.168.4.0). As DHCP requests arrive from clients, addresses can come from either of the superscopes. However, a problem might arise if a client gets an IP address from SRV1, and then SRV2 receives its renewal request. SRV2 does not recognize the client’s address as belonging to that subnet and responds to the client by sending a DHCPNACK.
You can avoid this problem by configuring both SRV1 and SRV2 with all logical IP subnets and using exclusions to prevent the servers from overlapping address ranges. SRV1 should have a superscope containing all four subnets and excluding all the addresses of the last two subnets. SRV2 should also have a superscope containing all four subnets but excluding all the addresses of the first two subnets.
BOOTP Relay Configuration
The correct deployment of DHCP servers prevents BOOTP relay agents from generating duplicate packets, which can cause the DHCP server to receive several copies of the same Discover or Request message. For example, Figures 4 and 5 show two BOOTP relay designs that have the same number of networks, servers, and routers. The Figure 4 design causes eight packets to reach the DHCP servers for every DHCP message sent by a client. The network design in Figure 5 eliminates duplicate packets while providing enough fault-tolerant redundancy such that any single part of the network can fail, but clients continue to received leases.
Figure 4. Inefficient Network Relay Design
Figure 5. A Network Design that Eliminates Duplicate Packets and Provides Fault-Tolerance
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