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PowerConnect B-Series FCX Configuration Guide 867
53-1002266-01
Configuring IP parameters – Layer 2 Switches
26
How DHCP Assist works
Upon initiation of a DHCP session, the client sends out a DHCP discovery packet for an address
from the DHCP server as seen in Figure 136. When the DHCP discovery packet is received at a
Layer 2 Switch with the DHCP Assist feature enabled, the gateway address configured on the
receiving interface is inserted into the packet. This address insertion is also referred to as
stamping.
FIGURE 136 DHCP requests in a network with DHCP Assist operating on a PowerConnect Switch
When the stamped DHCP discovery packet is then received at the router, it is forwarded to the
DHCP server. The DHCP server then extracts the gateway address from each request and assigns
an available IP address within the corresponding IP subnet (Figure 137). The IP address is then
forwarded back to the workstation that originated the request.
NOTE
When DHCP Assist is enabled on any port, Layer 2 broadcast packets are forwarded by the CPU.
Unknown unicast and multicast packets are still forwarded in hardware, although selective packets
such as IGMP, are sent to the CPU for analysis. When DHCP Assist is not enabled, Layer 2 broadcast
packets are forwarded in hardware.
DHCP
Server
Hub
207.95.7.6
Router
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
Interface 2
Interface 14
Interface 8
Step 1:
DHCP IP address requests
for Hosts 1, 2, 3 and 4 in
Subnets 1, 2, 3 and 4.
Gateway addresses:
192.95.5.1
200.95.6.1
202.95.1.1
202.95.5.1
Step 2:
Switch stamps each DHCP request
with the gateway address of the
corresponding subnet of the
receiving port.
Step 3:
Router forwards the DHCP request to the
server without touching the gateway
address inserted in the packet by the switch.
Layer 2 Switch