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Configuring IP Helper to Manage Broadcasts

The ip helper mechanism is an excellent way to forward certain broadcast traffic for manageability and inter-WAN communication. The interface configuration command ip helper-address is utilized to set up an interface to expect and receive broadcasts. By default, Cisco routers do not forward broadcasts to prevent broadcast storms that can easily disrupt a network. This is especially common in large, flat network designs indicative of legacy network environments. One of the most common situations is when a Dynamic Host Configuration Protocol (DHCP) client attempts to get configuration information from a DHCP server on the other side of a router in another broadcast domain. Unless a DHCP service or relay agent is available on the client's network segment, the client does not receive its configuration because broadcast packets are dropped at the router interface, as shown in Figure 3.5.

The ip helper-address command can be used on RouterA and parameterized with either the IP address of the DHCP (BOOTP) server or a direct broadcast address for the segment on which the server resides. The ip helper command works by changing a broadcast message to a unicast message. It is common to have multiple DHCP servers on your network, as in Figure 3.5, for fault tolerance. You can configure multiple entries of the command with different IP addresses as shown in Listing 3.3.

Listing 3.3 The ip helper Settings on RouterA

RouterA(config)# int e0/0
RouterA(config-if)# ip helper-address 172.16.1.200
RouterA(config-if)# ip helper-address 172.16.1.201

DHCP (BOOTP) is not the only service upon which the ip helper command operates. After you enable the ip helper command on RouterA, it automatically forwards on eight UDP ports by default. Table 3.12 displays the services and standard port settings.

Figure 3.5Figure 3.5 The default behavior of Cisco routers is not to forward broadcasts.



Table 3.12 The Default Services and Ports Automatically Enabled by ip helper

IP Service

UDP Port

Time

37

TACACS

49

DNS

53

BOOTP server

67

BOOTP client

68

TFTP

69

NetBIOS name service

137

NetBIOS datagram service

138


If you want to designate a particular broadcast packet to forward on your router globally, you can use the ip forward-protocol command. For example, to forward only the time service, use the following command:

ip forward-protocol udp 37

To shut down an automatically forwarded service, such as Trivial File Transfer Protocol (TFTP), use the following command:

no ip forward-protocol udp 69

CAUTION

Remember that the BOOTP and DHCP services both use port 68 even though it is always called the BOOTP port.

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