Foundry Networks Switch and Router Installation And Configuration Manual page 522

Foundry Switch and Router Installation and Configuration Guide
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.
Figure 15.10 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 sub-net range
because a router with multiple sub-nets configured on an interface cannot distinguish among DHCP discovery
packets received from different sub-nets.
For example, in Figure 15.10 a host from each of the four sub-nets 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 sub-net, 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 Foundry Layer 2 Switch, correct assignments are made because the Layer 2
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 15.11. When the DHCP discovery packet is received at a Foundry 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.
15 - 80
DHCP
Server
Server
207.95.7.6
Server
192.95.5.5
192.95.5.10
192.95.5.35
192.95.5.30
Host 1
192.95.5.x
Sub-net 1
Host 3
202.95.1.x
Sub-net 3
Step 2:
Router assumes the lowest
IP address (192.95.5.1) is the
gateway address.
Router
Step 1:
DHCP IP address requests
for Hosts 1,2,3 and 4 in
Sub-nets 1, 2, 3 and 4
Switch
Host 2
200.95.6.x
Sub-net 2
Hub
Host 4
202.95.5.x
Sub-net 4
IP addresses configured
on the router interface
192.95.5.1
200.95.6.1
202.95.1.1
202.95.5.1
December 2000