Configuring IP

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

to occur.

DHCP

Server

207.95.7.6

v

192.95.5.5

192.95.5.10

192.95.5.35

192.95.5.30

Step 2:

Router assumes the lowest IP address (192.95.5.1) is the gateway address.

IP addresses configured on the router interface

Router 192.95.5.1 200.95.6.1 202.95.1.1 202.95.5.1

 

Step 1:

 

DHCP IP address requests

 

for Hosts 1,2,3 and 4 in

HP Switch 4000

Sub-nets 1, 2, 3 and 4

 

 

Host 1

Host 2

192.95.5.x

200.95.6.x

Sub-net 1

Sub-net 2

 

Hub

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.

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