Configuration Guide User guide

1040 FastIron Configuration Guide
53-1002494-02
Configuring IP parameters – Layer 2 Switches
DHCP Assist configuration
DHCP Assist allows a Brocade Layer 2 Switch to assist a router that is performing multi-netting on
its interfaces as part of its DHCP relay function.
DHCP Assist ensures that a DHCP server that manages multiple IP subnets can readily recognize
the requester IP subnet, even when that server is not on the client local LAN segment. The Brocade
Layer 2 Switch does so by stamping each request with its IP gateway address in the DHCP discovery
packet.
NOTE
Brocade Layer 3 Switches provide BootP/DHCP assistance by default on an individual port basis.
Refer to “Changing the IP address used for stamping BootP and DHCP requests” on page 1012.
By allowing multiple subnet DHCP requests to be sent on the same wire, you can reduce the
number of router ports required to support secondary addressing as well as reduce the number of
DHCP servers required, by allowing a server to manage multiple subnet address assignments.
FIGURE 120 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.
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.
DHCP
Server
207.95.7.6
192.95.5.35
192.95.5.5
192.95.5.30
192.95.5.10
Router
Layer 2 Switch
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
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 (192.95.5.1) is the
gateway address.
192.95.5.1
200.95.6.1
202.95.1.1
202.95.5.1
IP addresses configured
on the router interface.