Brocade Communications Systems Layer 3 Routing Configuration ICX 6650 - Manual (Page 110)

92 Brocade ICX 6650 Layer 3 Routing Configuration Guide
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Configuring IP parameters – Layer 2 Switches
FIGURE 11 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
subnet range because a router with multiple subnets configured on an interface cannot distinguish
among DHCP discovery packets received from different subnets.
For example, in Figure 11 , 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 12 . 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.
Step 3:
DHCP Server generates IP
addresses for Hosts 1,2,3 and 4.
All IP address are assigned
in the 192.168.5.1 range.
DHCP requests for the other sub-nets
were not recognized by
the non-DHCP assist router causing
incorrect address assignments.
DHCP
Server
10.95.7.6
192.168.5.35
192.168.5.5
192.168.5.30
192.168.5.10
Router
Layer 2 Switch
Host 1 Host 2
Host 3 Host 4
192.168.5.x
Subnet 1
10.95.6.x
Subnet 2
10.95.1.x
Subnet 3
10.95.5.x
Subnet 4
Hub
Step 1:
DHCP IP address requests
for Hosts 1, 2, 3 and 4 in
Sub-nets 1, 2, 3 and 4.
Step 2:
Router assumes the lowest
IP address (192.168.5.1) is the
gateway address.
192.168.5.1
10.95.6.1
10.95.1.1
10.95.5.1
IP addresses configured
on the router interface.