WIXLIB

Entering Configuration ModeEntering Configuration ModeYou configure Cisco IOS software for voice over IP by typing commands on the command line. Thismethod of entering commands is called configuration mode.Note Voice over IP commands require the IP Plus, Desktop Plus, or Enterprise Plus image,Cisco IOS Release 11.3(1)T or later.To enter configuration mode, follow this procedure:Step 1Connect a console to the router. Configure the console for 9600 baud, 8 data bits, 1 stopbit, and no parity. Power up the router.Step 2If the current configuration is no longer valid (for instance, because you added aninterface), after about one minute you see the following prompt:Would you like to enter the initial configuration dialog? [yes]:Answer no. You now enter the normal operating mode of the router.Note If the current configuration is valid, you enter the normal operating modeautomatically.Step 3After a few seconds, you see the user EXEC prompt (Router ). Type enable and thepassword to enter enable mode:Router enablePassword:Configuration changes can be made only in enable mode. The prompt changes to theprivileged EXEC (enable) prompt (Router#):Router#Step 4Enter the config terminal command to enter configuration mode:Router# config terminalRouter(config)#The router enters global configuration mode, indicated by the Router(config)# prompt.Step 5If you have not configured the router before, or want to change the configuration,configure global parameters, passwords, network management, and routing protocols. Inthis example, IP routing, AppleTalk routing, and IPX routing are all enabled:Router(config)# ip routingRouter(config)# appletalk routingRouter(config)# ipx routingFor complete information about global configuration commands, and about configuringLAN and WAN interfaces on your router, refer to the Cisco IOS configuration guides andcommand references.10 Voice over IP Quick Start Guide

Entering Configuration ModeThe rest of this guide explains how to configure Cisco IOS software for voice over IP traffic.1293Timesaver Voice configuration uses a number of new Cisco IOS commands. For completeinformation about these commands, see the Voice over IP Software Configuration Guide. You canalso enter a question mark after a command or partial command at the Router(config)# promptto get help with syntax and arguments.6Remember that when you finish configuring the software, you must save the new configuration toNVRAM. This procedure is described in the section “Saving the Configuration” later in this guide.You may also want to save periodically during the configuration process.12936Timesaver At any point, you can see the current operating configuration, including changes youjust made, by entering the show running-config command.Voice over IP Quick Start Guide11

Configuring the IP Network for Real-Time Voice TrafficConfiguring the IP Network for Real-Time Voice TrafficVoice traffic is much more sensitive to timing variations than data traffic. For good voiceperformance, you might need to configure your data network so voice packets are not lost or delayed.This section describes three important methods of improving quality of service, or QoS (the level ofnetwork performance needed for voice over IP connections): RSVPMultilink PPP interleavingRTP header compressionCisco IOS software provides many other tools for ensuring QoS, such as custom queuing, priorityqueuing, weighted fair queuing, and IP precedence. For further information and more detailedexamples of QoS configuration, see the Voice over IP Software Configuration Guide.Note QoS measures the level of network performance. It does not directly measure the quality ofthe voice signal.Configuring voice over IP on a Frame Relay link involves special considerations. These arediscussed in the section “Configuring Frame Relay for Voice over IP.”Do You Need QoS?On a relatively low-bandwidth connection, such as a PPP or HDLC serial link, you should considerusing methods to ensure QoS. If you have a high-bandwidth network, such as Ethernet or FastEthernet, and voice and data traffic together occupy only a small fraction of the bandwidth available,you may not need to provide QoS. (See Figure 11.) If you are not sure, you may want to try skippingthe following sections and continuing with “Configuring FXS Interfaces” later in this guide. Returnto this section if you need to improve the reliability of voice connections.Figure 11Bandwidth v. Quality of ServiceHigh bandwidthMay not need QoS12 Voice over IP Quick Start Guide10689Low bandwidthNeed QoS

Do You Need RSVP?Do You Need RSVP?Resource Reservation Protocol (RSVP) enables routers to reserve enough bandwidth for reliableperformance. RSVP works well on PPP, HDLC, and similar serial line interfaces. It does not workwell on multiaccess LANs.You should configure RSVP if you have a serial interface and any of the following: Links slower than 2 MbpsLinks with high utilizationNeed for the best possible voice qualityNote If you configure multilink PPP interleaving, you can use the ip rtp reserve command insteadof configuring RSVP. See the next section on “Do You Need Multilink PPP Interleaving?”Configuring RSVPBy default, RSVP is disabled for compatibility with routers that do not implement it. To enableRSVP on an IP network, enter the ip rsvp bandwidth command from interface configuration mode.The following example shows how to configure RSVP on serial interface 0/0:Router enablePassword:Router# configure terminalRouter(config)# interface serial 0/0Router(config-if)# ip rsvp bandwidthThe default maximum bandwidth is 75 percent of the bandwidth available on each interface.RSVP must be enabled at each LAN or WAN interface that voice packets will travel across. Youmust also configure each remote dial peer to request an RSVP session, using the req-qos command.See the section “Calling Between Routers” later in this guide.Do You Need Multilink PPP Interleaving?On a dialer, ISDN PRI or BRI interface, or a virtual template, you can configure multilink PPPinterleaving, which helps to make sure that voice packets are transmitted without delay.You should configure multilink PPP interleaving if you have one of these interfaces, and either ofthe following: Point-to-point connections using PPP encapsulationLinks slower than 2 MbpsNote Do not use multilink PPP on links faster than 2 Mbps.Voice over IP Quick Start Guide13

Configuring the IP Network for Real-Time Voice TrafficConfiguring Multilink PPP InterleavingTo configure multilink PPP and interleaving on a dialer, ISDN PRI, or ISDN BRI interface, or avirtual template, you must first configure multilink PPP and interleaving on the interface or templateby entering the following commands in interface configuration mode:Router(config-if)# encapsulation pppRouter(config-if)# ppp multilinkRouter(config-if)# ppp multilink interleaveOptionally, configure a maximum fragment delay:Router(config-if)# ppp multilink fragment-delay millisecondsYou can also reserve a special queue for real-time packet flows to specified destination UDP ports,allowing real-time traffic to have higher priority than other flows. This command is needed only ifyou have not configured RSVP:Router(config-if)# ip rtp reserve lowest-UDP-port range-of-portsFor virtual templates only, apply the virtual template to the multilink bundle:Router(config-if)# multilink virtual-template 1Do You Need RTP Header Compression?RTP header compression on a PPP, HDLC, or similar serial interface compresses the packet headerto reduce network overhead.You should configure RTP header compression on a serial interface if you have either of thefollowing: Links slower than 2 MbpsNeed to save bandwidthNote Do not use RTP header compression on links faster than 2 Mbps.Configuring RTP Header CompressionEnable RTP header compression at both ends of the serial link by entering the following commandin interface configuration mode:Router(config-if)# ip rtp header-compression14 Voice over IP Quick Start Guide

Configuring Frame Relay for Voice over IPConfiguring Frame Relay for Voice over IPConfiguring voice over IP on a Frame Relay link involves certain special considerations to ensureacceptable voice quality. For Frame Relay links with slow output rates (64 kbps or less), where dataand voice are being transmitted over the same PVC, you should configure the following: Lower MTU size—Voice packets are generally small. If you decrease the MTU size to 300 bytes,large data packets can be broken up into smaller data packets that are more easily interleaved withvoice packets.The following example configures an MTU size of 300 bytes over serial interface 0/0:Router# interface serial 0/0Router(config-if)# mtu 300 RSVP—Configure RSVP on the subinterfaces (which correspond to PVCs) to reserve bandwidthfor voice channels. See the section on “Do You Need RSVP?” earlier in this guide.The following example configures RSVP over serial subinterface 0/0.1:Router(config-if)# interface serial 0/0.1 point-to-pointRouter(config-if)# ip address 192.168.19.0 255.0.0.0Router(config-if)# ip rsvp bandwidth RTP header compression—Configure RTP header compression on the subinterfaces to minimizethe size of the voice packet. See the previous section on “Do You Need RTP HeaderCompression?”The following command configures RTP header compression over the currently selectedsubinterface:Router(config-if)# frame-relay ip rtp header-compression Traffic shaping—Use traffic shaping to control the outbound traffic rate; otherwise, voice packetsmight be discarded. In Cisco IOS Release 11.3, Frame Relay traffic shaping is not compatiblewith RSVP. Use generic traffic shaping instead, setting the CIR equal to the port speed. Doingthis prevents the router from exceeding the CIR rate, which could lead to frames being discarded.The following command configures generic traffic shaping with a CIR of 32000 bps:Router(config-if)# traffic-shape rate 32000For further information and more detailed examples of Frame Relay configuration, see the Voiceover IP Software Configuration Guide.Voice over IP Quick Start Guide15

Configuring FXS InterfacesConfiguring FXS InterfacesThis section explains how to configure FXS VICs. Ports on this VIC connect directly to a standardtelephone, fax machine, or similar device.Figure 12 shows a basic voice network. A small business uses a Cisco 3600 series router (namedWest) to provide telephone and fax connections among employees in its office. Figure 12 shows twoof these telephones, each connected to an FXS VIC port in the West router.Figure 12Basic Voice Network (West Router)408 555-3737FXS VIC2/0/0IP cloudWestFXS VIC2/0/110007408 555-4141Note You can name your router by using the hostname command in global configuration mode.Table 1 lists telephone numbers and voice ports for the West router. (For information about portnumbering, see the section on “Voice Port Numbering” earlier in this guide.)Table 1West Router Telephone Numbers and Voice PortsTelephone NumberVoice Port408 555-37372/0/0408 555-41412/0/1Note Additional telephones and fax machines could be connected in this example, up to a total of12 if the router is a Cisco 3640 configured with three two-slot voice network modules. The modulein the remaining slot would provide interfaces for IP connectivity to the LAN or WAN and for datatraffic. To accommodate more than 12 voice devices, you would need to add more routers, or to usean E&M VIC and a local PBX, rather than connecting every telephone to its own FXS VIC.16 Voice over IP Quick Start Guide

Local Dial PeersLocal Dial PeersTo route a received voice call to the right destination, the router needs to know which telephonenumber belongs to each voice port. For instance, if a call comes in for 408 555-3737, the router needsto know that this telephone is connected to voice port 2/0/0 (as shown in Figure 12). In other words,the router needs to know the information in Table 1.To hold this information, Cisco IOS software uses objects called dial peers. A telephone number, avoice port, and other call parameters are tied together by associating them all with the same dial peer.Configuring dial peers is similar to configuring static IP routes—you are telling the router what pathto follow to route the call.Dial peers are identified by numbers, but to avoid confusing these numbers with telephone numbers,they are usually referred to as tags. Dial peer tags are integers that can range from 1 to 231 -1(2147483647), which should be enough for most purposes. Dial peers on the same router must haveunique tags, but you can reuse the tags on other routers.Table 2 assigns each phone number-voice port pair on the West router a dial peer tag. Within theallowed range, you can choose any dial peer tag or any system that is convenient or makes sense toyou. This type of dial peer is called a POTS dial peer or a local dial peer. The term “POTS” (plainold telephone service) means that the dial peer associates a physical voice port with a local telephonedevice. (Another type of dial peer is explained in the section “Calling Between Routers.”)Table 2West Router Local Dial PeersTelephone NumberVoice PortDial Peer Tag408 555-37372/0/0401408 555-41412/0/1402You should construct a table similar to Table 2 for your own routers, assigning your own telephonenumbers and dial peer tags.Note The telephone numbers used in this guide are only examples, and are generally invalid forpublic use in the United States. When you configure your network, be sure to substitute your owntelephone numbers.To configure the router with the information in the table, enter the following commands in globalconfiguration mode:West(config)# eer)#West(config)# eer)#West(config-dial-peer)#West(config)#voice 401 potsdest-pat 14085553737port 2/0/0voice 402 potsdest-pat 14085554141port 2/0/1exitThese commands are summarized in Figure 13.Voice over IP Quick Start Guide17

Configuring FXS InterfacesFigure 13West Router Configured for Local Dial Peersdial-peer voice 401 potsdest-pat 14085553737port 2/0/0IP cloudFXS VIC2/0/0WestFXS VIC2/0/110008dial-peer voice 402 potsdest-pat 14085554141port 2/0/1The dial-peer command always takes the argument voice. The number following it is the dial peertag, and pots is the type of dial peer.Cisco IOS software refers to a telephone number as a destination pattern, because it is thedestination for an incoming or outgoing call. Enter these numbers with the destination-pattern(abbreviated dest-pat) command. A destination pattern always begins with a plus sign ( ). It canalso include asterisks (*) and pound signs (#) from the telephone keypad, and commas (,) andperiods (.), which have special meaning. Parentheses ( () ), hyphens (-), slashes (/), and spaces ( ),which are often used to make telephone numbers easier for humans to read, are not allowed.Notice that the commands in the examples append the prefix 1 (used in the United States to indicatea long-distance number) and an area code to the destination pattern. You may need to include similarcodes for your country if the voice over IP equipment needs to establish a connection to the PSTN.In other situations, you might be able to simplify configuration by omitting this information.Note The Cisco IOS software does not check the validity of the telephone number. It accepts anystring of permitted characters as a valid number.The business that owns the West router also has a branch office in the East. Figure 14 shows the Eastnetwork, and Table 3 lists phone numbers, voice ports, and dial peer tags for this office.Figure 14Basic Voice Network (East Router)919 958-8282IP cloudFXS VIC3/1/0East919 958-959518 Voice over IP Quick Start Guide10009FXS VIC3/1/1

Local Dial PeersTable 3East Router Local Dial PeersTelephone NumberDestination PatternVoice PortDial Peer Tag919 958-8282 191995882823/1/0901919 958-9595 191995895953/1/1902The following commands configure local ports on the East Router:East(config)# eer)#East(config)# eer)#East(config-dial-peer)#East(config)#voice 901 potsdest-pat 19199588282port 3/1/0voice 902 potsdest-pat 19199589595port 3/1/1exitThese commands are summarized in Figure 15.Figure 15East Router Configured for Local Dial Peersdial-peer voice 901 potsdest-pat 19199588282port 3/1/0IP cloudFXS VIC3/1/0FXS VIC3/1/1dial-peer voice 902 potsdest-pat 19199589595port 3/1/110010EastChecking the ConfigurationIf you configured your router following these examples, you should now be able to place callsbetween telephones connected to the same router. You can also use the show dial-peer voicecommand to verify that the data you configured is correct.12936Timesaver If the voice port is offline, use the no shutdown command from interfaceconfiguration mode to enable it.Note Although placing calls directly between ports on the same router helps to verify yourconfiguration, it is not recommended for general telecommunications use.Voice over IP Quick Start Guide19

Configuring FXS InterfacesWild Cards and Number ExpansionOffice PBXs are usually configured so a user can dial a local call (within the same PBX) by dialingthe extension only—for instance, the four-digit extension 3737 or the five-digit extension 53737instead of the full telephone number, 1 408 555-3737.You can provide the same shortcut on a voice over IP network by using the number-expansion(num-exp) command. This command tells the router to expand a particular sequence of dialednumbers into a complete telephone number (destination pattern). For instance, to expand 3737 into408 555-3737, enter the following command:West(config)# num-exp 3737 14085553737To expand 4141 into 1 408 555-4141, enter the following command:West(config)# num-exp 4141 14085554141More generally, you can use the period (.) as a wild-card character representing a s

2 Voice over IP Quick Start Guide Configuring the IP Network for Real-Time Voice Traffic, page 12—Describes how to configure the IP network to accommodate voice traffic. Configuring FXS Interfaces, page 16—Describes how to configure FXS voice interface cards for connecting a router to telephones, fax machines, and similar devices.