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Configuring IP
Step 3:
DHCP Server generates IP
addresses for Hosts 1,2,3 and 4.
All IP address are assigned
Server
DHCP
Server
207.95.7.6
Router
192.95.5.1
200.95.6.1
202.95.1.1
202.95.5.1
192.95.5.35
192.95.5.5
192.95.5.30
192.95.5.10
IP addresses configured
on the router interface
Host 1
200.95.6.x
Host 2
192.95.5.x
Sub-net 1
Sub-net 2
Hub
Powe r
Console
HP Switch 4000
the non-DHCP assist router, causing
incorrect address assignments
to occur.
Step 1:
DHCP IP address requests
for Hosts 1,2,3 and 4 in
Sub-nets 1, 2, 3 and 4
Host 3 Host 4
202.95.1.x
202.95.5.x
Sub-net 3
Sub-net 4
Figure 6.10 DHCP requests in a network without DHCP Assist on the switch
In a network operating without DHCP Assist, hosts can be assigned IP addresses from the wrong sub-net range
because a routing switch with multiple sub-nets configured on an interface cannot distinguish among DHCP
discovery packets received from different sub-nets.
For example, in Figure 6.10 a host from each of the four sub-nets supported on a 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 sub-net, it assumes the lowest IP address or the ‘primary address’ is the
gateway for all ports on the 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 the HP 6208M-SX, correct assignments are made because the switch provides the
stamping service.
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 6.11. When the DHCP discovery packet is received at an HP 6208M-SX 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.
in the 192.95.5.1 range.
DHCP requests for the other sub-nets
were not recognized by
Step 2:
Router assumes the lowest
IP address (192.95.5.1) is the
gateway address.
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