Consider this complex
sample helper address configuration (see Figure ). Assume you
want Host A to automatically obtain its IP configuration from the
DHCP server at 172.24.1.9. Because RTA will not forward Host A's
DHCPDISCOVER broadcast, you must configure RTA to help Host A.
To configure RTA's E0 (the interface that receives Host A's
broadcasts) to relay DHCP broadcasts as a unicast to the DHCP
server, use the following commands:
RTA(config)#interface e0
RTA(config-if)#ip helper-address 172.24.1.9
With this simple configuration, Host A broadcasts using any of the
eight default UDP ports will be relayed to the DHCP server's IP
address. However, what if Host A also needs to use the services of the
NetBIOS server at 172.24.1.5? As configured, RTA will forward
NetBIOS broadcasts from Host A to the DHCP server. Moreover, if Host
A sends a broadcast TFTP packet, RTA also will forward this to the
DHCP server at 172.24.1.9. What is needed in this example is a
helper address configuration that relays broadcasts to all servers
on the segment. The following commands configure a directed
broadcast to the IP subnet that is being used as a server farm:
RTA(config)#interface e0
RTA(config-if)#ip
helper-address 172.24.1.255
Configuring a directed broadcast to the server segment
(172.24.1.255) is more efficient than entering the IP address of
every server that could potentially respond to Host A's UDP
broadcasts.
Finally, some devices on Host A's segment need to broadcast to the
TACACS server, which does not reside in the server farm. You can
configure RTA's E0 to make it work by adding the command
ip helper-address 172.16.1.2.
You can verify the correct helper configuration with the show
ip interface
command, as shown in Figure .
Notice in Figure that RTA's interface E3 (which connects to the
server farm) is not configured with helper addresses. However, the output
in Figure also shows that, for this interface, directed
broadcast forwarding is disabled. This means that the router will
not convert the logical broadcast 172.24.1.255 into a physical
broadcast (with a Layer 2 address of FF-FF-FF-FF-FF-FF). To allow
all the nodes in the server farm to receive the broadcasts at Layer
2, you will have to configure E3 to forward directed broadcasts with
the following command:
RTA(config)#interface e3
RTA(config-if)#ip
directed-broadcast
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Interactive
Lab Activity
(Flash,
387 kB) |
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In
this lab activity you will configure SanJose2 to
act as a DHCP server. Then you will configure
SanJose1 to forward UDP broadcasts for DHCP
requests.
Finally,
you will test your configuration using a DHCP
client. |
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