The Big show and Big D for Troubleshooting ISDN

Lab 15b: Configuring Voice over IP ”Part II

Lab Walkthrough

Example 6-4 shows the configurations that you will use on each of these routers to accomplish your Voice over IP lab. The configuration of the voice ports, dial peers, and QOS mechanisms are discussed. For the purpose of this lab, the Cisco voice-capable routers have host names of voip1 and voip2.

Example 6-4 Configuring Voice over IP on voip1

 Current configuration: ! version 12.1 service timestamps debug uptime service timestamps log uptime no service password-encryption !  hostname voip1  ! enable password cisco ! ! ! ! ! ! ces 1/0 ! ip subnet-zero no ip domain-lookup ! lane client flush ! ! ! ! ! controller T1 1/0 ! ! ! interface FastEthernet0/0  no ip address  shutdown  duplex auto  speed auto ! interface FastEthernet0/1  no ip address  shutdown  duplex auto  speed auto ! interface ATM1/0  no ip address  shutdown  no atm scrambling cell-payload  no atm ilmi-keepalive !  interface Serial2/0   no ip address   encapsulation frame-relay   no ip mroute-cache   no fair-queue   !   interface Serial2/0.1 point-to-point   ip address 150.150.10.1 255.255.255.0   no ip mroute-cache   frame-relay class voice   frame-relay interface-dlci 100   frame-relay ip rtp header-compression  ! interface Serial2/1  no ip address  shutdown ! interface Serial2/2  no ip address  shutdown ! interface Serial2/3  no ip address  shutdown ! router rip  network 150.150.0.0 ! ip classless no ip http server ! !  map-class frame-relay voice   frame-relay voice bandwidth 78000   frame-relay fragment 1500   frame-relay ip rtp priority 16384 16383 312   no frame-relay adaptive-shaping   frame-relay cir 768000   frame-relay bc 1000   frame-relay mincir 120000   frame-relay fair-queue  ! voice-port 3/0/0 ! voice-port 3/0/1 ! voice-port 3/1/0 ! voice-port 3/1/1 !  dial-peer voice 11 pots   destination-pattern 1111   port 3/0/0   !   dial-peer voice 2222 voip   destination-pattern 2222   session target ipv4:150.150.10.2   ip precedence 5  ! ! line con 0  password cisco  transport input none line aux 0  password cisco line vty 0 4  password cisco  login ! end 

The configurations for the VoIP lab and VoFR lab are similar, with a few exceptions. These exceptions include the VoIP dial peer, the IP precedence associated with the VoIP dial peer, the Frame Relay IP RTP header compression on the Serial subinterface, and the Frame Relay IP RTP priority defined in the map class.

Let's start with the dial peers. The POTS dial peer is the same as the one defined in the VoFR lab. However, you can see that the session target defined on the VoIP dial peer now points to an IP address rather than an interface and DLCI number. The IP address of the session target can be any interface on the router that you are directing your call to. Best practice would dictate that this address should be the IP address of a loopback interface because it is the most stable. That is, it should always be logically up as long as the router is powered up. In the example, we pointed the session target at the far end IP address of the Serial subinterface.

IP RTP header compression reduces the 40-byte IP+UDP+RTP header to 2 to 4 bytes, thereby reducing the bandwidth required per voice call on point-to-point links. The header is compressed at one end of the link and decompressed at the other end. Another standard name for this technique is CRTP, which stands for compressed RTP. To configure IP RTP header compression, you need to configure the ip rtp header-compression command under the serial interface, or the frame-relay ip rtp header-compression command under the Frame Relay subinterface.

Frame Relay IP RTP priority is used to reserve a strict priority queue on a Frame Relay PVC for a set of RTP packet flows belonging to a range of User Datagram Protocol (UDP) destination ports. The theory behind IP RTP priority is that VoIP traffic is identified by the fact that it uses a well-known UDP port range (16384-32767). Although the actual ports used are dynamically negotiated between end devices or gateways, all Cisco VoIP products utilize the same port range. When the router recognizes the VoIP traffic, it places this traffic into a strict priority queue. The amount of traffic that is placed into the priority queue is controlled at a user -configurable rate by the system. This priority queue always is serviced before any other traffic, which means that jitter and delay of VoIP traffic is minimized.

IP precedence gives voice packets a higher priority than other IP data traffic. The ip precedence command is used by the Cisco 3600 router to differentiate voice traffic from data traffic. So, you need to ensure that the data IP packets do not have the same IP precedence as that of the voice packets. In IP precedence, the numbers 1 through 5 identify classes for IP flows; the numbers 6 through 7 are used for network and backbone routing and updates. It is recommended that IP precedence 5 be used for voice packets.

Example 6-5 shows the configuration for the voip2 router.

Example 6-5 Configuring Voice over IP on voip2

 Current configuration: ! version 12.1 service timestamps debug uptime service timestamps log uptime no service password-encryption !  hostname voip2  ! enable password cisco ! ! ! ! ! ! ces 1/0 ! ip subnet-zero no ip domain-lookup ! lane client flush ! ! ! ! ! controller T1 1/0 ! ! ! interface FastEthernet0/0  no ip address  shutdown  duplex auto  speed auto ! interface FastEthernet0/1  no ip address  shutdown  duplex auto  speed auto ! interface ATM1/0  no ip address  shutdown  no atm scrambling cell-payload  no atm ilmi-keepalive !  interface Serial2/0   no ip address   encapsulation frame-relay   no ip mroute-cache   no fair-queue   !   interface Serial2/0.1 point-to-point   ip address 150.150.10.2 255.255.255.0   no ip mroute-cache   frame-relay class voice   frame-relay interface-dlci 200   frame-relay ip rtp header-compression  ! interface Serial2/1  no ip address  shutdown ! interface Serial2/2  no ip address  shutdown ! interface Serial2/3  no ip address  shutdown ! router rip  network 150.150.0.0 ! ip classless no ip http server ! !  map-class frame-relay voice   frame-relay voice bandwidth 78000   frame-relay fragment 1500   frame-relay ip rtp priority 16384 16383 312   no frame-relay adaptive-shaping   frame-relay cir 768000   frame-relay bc 1000   frame-relay mincir 120000   frame-relay fair-queue  ! voice-port 3/0/0 ! voice-port 3/0/1 ! voice-port 3/1/0 ! voice-port 3/1/1 !  dial-peer voice 22 pots   destination-pattern 2222   port 3/1/0   !   dial-peer voice 1111 voip   destination-pattern 1111   session target ipv4:150.150.10.1   ip precedence 5  ! ! line con 0  password cisco  transport input none line aux 0  password cisco line vty 0 4  password cisco  login ! end 

Now that both router configurations are complete, pick up the phone set on voip1 and dial the digits 2222. If your configurations are correct and your routers are physically connected as described earlier, you should be able to complete a successful call. Hang up the phone set on voip1 and try placing a call from voip2 by dialing 1111.