Configuration Guide User guide

FastIron Configuration Guide 1041
53-1002494-02
Configuring IP parameters – Layer 2 Switches
For example, in Figure 120, 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 Brocade 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 121. When the DHCP discovery packet is received at a
Brocade 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 121 DHCP requests in a network with DHCP Assist operating on a FastIron Switch
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:
FastIron 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