Ip helper

Chapter Description

This guide from Cisco Press explains the ins and outs of IP management, including how to resolve IP addressing crises, and how and when to use helper addresses.

From the Book


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CCNP 1: Advanced Routing Companion Guide (Cisco Networking Academy Program), 2nd Edition

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Helper Addresses

This section describes how networks and routers use helper addresses toforward broadcasts to another server or router on another network. This sectiondescribes some of the purposes of and scenarios in which to use helperaddresses.Bạn đang xem: Ip helper address utilization

Using Helper Addresses

DHCP is not the only critical service that uses broadcasts. Cisco routers andother devices might use broadcasts to locate TFTP servers. Some clients mightneed to broadcast to locate a TACACS security server. In a complex hierarchicalnetwork, clients might not reside on the same subnet as key servers. Such remoteclients broadcast to locate these servers, but routers, by default, do notforward client broadcasts beyond their subnet. Some clients are unable to make aconnection without services such as DHCP. For this reason, the administratormust provide DHCP and DNS servers on all subnets or use the Cisco IOS softwarehelper address feature. Running services such as DHCP or DNS on severalcomputers creates overhead and administrative problems, so the first option isnot very appealing. When possible, administrators use the iphelper-address command to relay broadcast requests for these key UserDatagram Protocol (UDP) services.

Bạn đang xem: Ip helper

By using the ip helper-address command, a router can be configured toaccept a broadcast request for a UDP service and then forward it as a unicast toa specific IP address, as shown in Figure 2-21. Alternatively, the router canforward these requests as directed broadcasts to a specific network orsubnetwork.


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Figure2-21 Helper Addresses

Configuring IP Helper Addresses

To configure the helper address, identify the router interface that willreceive the broadcasts for UDP services. In interface configuration mode, usethe ip helper-address command to define the address to which UDPbroadcasts for services should be forwarded.

By default, the ip helper-address command forwards the eight UDPservices listed in Table 2-10.

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Table 2-10 Default Forward UDP Services

Service

Port

Time

37

TACACS

49

DNS

53

BOOTP/DHCP Server

67

BOOTP/DHCP Client

68

TFTP

69

NetBIOS name service

137

NetBIOS datagram service

138

Example 2-5 Forwarding UDP Services

RTA(config-if)#ip helper-address 192.168.1.254RTA(config-if)#exitRTA(config)#ip forward-protocol udp 517RTA(config)#no ip forward-protocol udp 37RTA(config)#no ip forward-protocol udp 49RTA(config)#no ip forward-protocol udp 137RTA(config)#no ip forward-protocol udp 138

IP Helper Address Example

Consider the complex sample helper address configuration shown in Figure2-22. You want Host A to automatically obtain its IP configuration from the DHCPserver at 172.24.1.9. Because RTA will not forward the Host A DHCPDISCOVERbroadcast, RTA must be configured to help Host A.


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Figure2-22 IP Helper Address Example

To configure RTA e0, the interface that receives the Host A broadcasts, torelay DHCP broadcasts as a unicast to the DHCP server, use the followingcommands:

RTA(config)#interface e0RTA(config-if)#ip helper-address 172.24.1.9With this simple configuration, Host A broadcasts using any of the eightdefault UDP ports that are relayed to the DHCP server's IP address.However, what if Host A also needs to use the services of the NetBIOS server at172.24.1.5? As configured, RTA forwards NetBIOS broadcasts from Host A to theDHCP server. Moreover, if Host A sends a broadcast TFTP packet, RTA alsoforwards this to the DHCP server at 172.24.1.9. What is needed in this exampleis a helper address configuration that relays broadcasts to all servers on thesegment. The following commands configure a directed broadcast to the IP subnetthat is being used as a server farm:

RTA(config)#interface e0RTA(config-if)#ip helper-address 172.24.1.255Configuring a directed broadcast to the server segment, 172.24.1.255, is moreefficient than entering the IP address of every server that could potentiallyrespond to the Host A UDP broadcasts.

Finally, some devices on the Host A segment need to broadcast to the TACACSserver, which does not reside in the server farm. Configure the RTA e0 to makeit work by adding the command ip helper-address 172.16.1.2.

Verify the correct helper configuration with the show ip interfacecommand, as shown in Example 2-6.

Example 2-6 Verifying IP Helper Address Configuration

Example 2-7 Verifying Directed Broadcast Forwarding

RTA#show ip interface e3Ethernet3 is up, line protocol is up Internet address is 172.24.1.1/24 Broadcast address is 255.255.255.255 Address determined by setup command MTU is 1500 bytes Helper addresses is not set Directed broadcast forwarding is disabled To allow all the nodes in the server farm to receive the broadcasts at Layer2, configure e3 to forward directed broadcasts with the following commands:

RTA(config)#interface e3RTA(config-if)#ip directed-broadcastLab 2.10.3 Using DHCP and IP Helper Addresses

In this lab, configure a Cisco router to act as a DHCP server for clients ontwo separate subnets and the IP helper address feature to forward DHCP requestsfrom a remote subnet.