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866 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring IP parameters – Layer 2 Switches
26
By allowing multiple subnet DHCP requests to be sent on the same wire, you can reduce the
number of router ports required to support secondary addressing as well as reduce the number of
DHCP servers required, by allowing a server to manage multiple subnet address assignments.
FIGURE 135 DHCP requests in a network without DHCP Assist on the Layer 2 Switch
In a network operating without DHCP Assist, hosts can be assigned IP addresses from the wrong
subnet range because a router with multiple subnets configured on an interface cannot distinguish
among DHCP discovery packets received from different subnets.
For example, in Figure 135, a host from each of the four subnets supported on a Layer 2 Switch
requests an IP address from the DHCP server. These requests are sent transparently to the router.
Because the router is unable to determine the origin of each packet by subnet, it assumes the
lowest IP address or the ‘primary address’ is the gateway for all ports on the Layer 2 Switch and
stamps the request with that address.
When the DHCP request is received at the server, it assigns all IP addresses within that range only.
With DHCP Assist enabled on a Layer 2 Switch, correct assignments are made because the Layer 2
Switch provides the stamping service.
Step 3:
DHCP Server generates IP
addresses for Hosts 1,2,3 and 4.
All IP address are assigned
in the 192.95.5.1 range.
DHCP requests for the other sub-nets
were not recognized by
the non-DHCP assist router causing
incorrect address assignments.
DHCP
Server
207.95.7.6
192.95.5.35
192.95.5.5
192.95.5.30
192.95.5.10
Router
Layer 2 Switch
Host 1
Host 2
Host 3
Host 4
192.95.5.x
Subnet 1
200.95.6.x
Subnet 2
202.95.1.x
Subnet 3
202.95.5.x
Subnet 4
Hub
Step 1:
DHCP IP address requests
for Hosts 1, 2, 3 and 4 in
Sub-nets 1, 2, 3 and 4.
Step 2:
Router assumes the lowest
IP address (192.95.5.1) is the
gateway address.
192.95.5.1
200.95.6.1
202.95.1.1
202.95.5.1
IP addresses configured
on the router interface.