Chapter Description

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

From the Book


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


Helper Addresses

This section describes how networks and routers use helper addresses toforward broadcasts khổng lồ another hệ thống or router on another network. This sectiondescribes some of the purposes of and scenargame ios in which lớn use helperaddresses.

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Using Helper Addresses

DHCP is not the only critical service that uses broadcasts. Cisteo routers andother devices might use broadcasts khổng lồ locate TFTP servers. Some clients mightneed to lớn broadcast lớn locate a TACACS security server. In a complex hierarchicalnetwork, clients might not reside on the same subnet as key servers. Such remoteclients broadcast lớn locate these servers, but routers, by mặc định, vì chưng 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 Cisteo IOS softwarehelper address feature. Running services such as DHCP or DNS on severalcomputers creates overhead and administrative sầu 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.

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

Figure2-21 Helper Addresses

Configuring IPhường 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 khổng lồ define the address lớn which UDPbroadcasts for services should be forwarded.

By mặc định, the ip helper-address comm& forwards the eight UDPservices listed in Table 2-10.

Table 2-10 Default Forward UDP.. Services









BOOTP/DHCPhường Server






NetBIOS name service


NetBIOS datagram service


What if Company XYZ needs to forward requests for a service noton this list? The Cisteo IOS software provides the global configuration commandip forward-protocol lớn allow an administrator to forward any UDP port inaddition to the default eight. To forward UDPhường on port 517, use the globalconfiguration comm& ip forward-protocol udp 517. This commvà is usednot only to add a UDPhường port to lớn the default eight, but also to lớn subtract anunwanted service from the mặc định group. When forwarding DHCP., TFTP, & DNSwithout forwarding Time, TACACS, và NetBIOS, the Cisteo IOS software requiresthat the router be configured according khổng lồ the syntax shown in Example2-5.

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Example 2-5 Forwarding UDP.. 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 to lớn automatically obtain its IP configuration from the DHCPVPS at Because RTA will not forward the Host A DHCPDISCOVERbroadcast, RTA must be configured to help Host A.

Figure2-22 IP Helper Address Example

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

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

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

Finally, some devices on the Host A segment need to lớn broadcast to lớn the TACACShệ thống, which does not reside in the hệ thống farm. Configure the RTA e0 khổng lồ makeit work by adding the comm& ip helper-address

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

Example 2-6 Verifying IP Helper Address Configuration

RTA#show ip interface e0Ethernet0 is up, line protocol is up Internet address is Broadcast address is Address determined by thiết đặt command MTU is 1500 bytes Helper addresses are Directed broadcast forwarding is disabled Notice that the RTA interface e3 in Example 2-7, which connects lớn the serverfarm, is not configured with helper addresses. However, the output shows thatfor this interface, directed broadcast forwarding is disabled. This means thatthe router does not convert the logical broadcast inkhổng lồ a physicalbroadcast with a Layer 2 address of FF-FF-FF-FF-FF-FF.

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 lớn 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.. và IP.. Helper Addresses

In this lab, configure a Cisco router lớn act as a DHCP hệ thống for clients ontwo separate subnets & the IPhường. helper address feature lớn forward DHCPhường requestsfrom a remote subnet.

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