Ip Helper

Chapter Description

This guide from Cisteo Press explains the ins và outs of IP management, including how khổng lồ resolve sầu IP.. addressing crises, & how & when lớn use helper addresses.

From the Book


CCNPhường 1: Advanced Routing Companion Guide (Cisteo Networking Academy Program), 2nd Edition


Helper Addresses

This section describes how networks và routers use helper addresses toforward broadcasts to lớn another VPS or router on another network. This sectiondescribes some of the purposes of & scenarquả táo in which lớn use helperaddresses.quý khách hàng vẫn 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 khổng lồ locate TFTP.. servers. Some clients mightneed khổng lồ broadcast lớn locate a TACACS security hệ thống. In a complex hierarchicalnetwork, clients might not reside on the same subnet as key servers. Such remoteclients broadcast khổng lồ locate these servers, but routers, by mặc định, vì chưng notforward client broadcasts beyond their subnet. Some clients are unable lớn make aconnection without services such as DHCPhường. For this reason, the administratormust provide DHCP và DNS servers on all subnets or use the Cisteo IOS softwarehelper address feature. Running services such as DHCPhường or DNS on severalcomputers creates overhead and administrative sầu problems, so the first option isnot very appealing. When possible, administrators use the iphelper-address comm& lớn relay broadcast requests for these key UserDatagram Protocol (UDP) services.

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By using the ip helper-address comm&, a router can be configured toaccept a broadcast request for a UDPhường service & 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 lớn a specific network orsubnetwork.


Figure2-21 Helper Addresses

Configuring IP Helper Addresses

To configure the helper address, identify the router interface that willreceive the broadcasts for UDPhường services. In interface configuration mode, usethe ip helper-address command lớn define the address to lớn 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











BOOTP/DHCPhường. Client




NetBIOS name service


NetBIOS datagram service


Example 2-5 Forwarding UDPhường Services

RTA(config-if)#ip helper-address 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 khổng lồ automatically obtain its IPhường. configuration from the DHCPserver at Because RTA will not forward the Host A DHCPDISCOVERbroadcast, RTA must be configured to help Host A.


Figure2-22 IPhường. Helper Address Example

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

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

RTA(config)#interface e0RTA(config-if)#ip helper-address a directed broadcast khổng lồ the server segment,, 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 lớn the TACACSserver, which does not reside in the hệ thống farm. Configure the RTA e0 khổng lồ makeit work by adding the command ip helper-address

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 Broadcast address is Address determined by cài đặt command MTU is 1500 bytes Helper addresses is not set Directed broadcast forwarding is disabled To allow all the nodes in the VPS farm to receive sầu 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 lớn act as a DHCP server for clients ontwo separate subnets và the IPhường helper address feature to forward DHCPhường requestsfrom a remote subnet.