RTP Header Compression

In this lab, you will configure routers R1 and R2, which are shown in Figure 5-33, for RTP header compression (cRTP) and compare bandwidth usage before and after enabling cRTP.

Figure 5-33. Lab Topology

Task 1: Change the Load Interval on Router R2's Serial Interface

When you issue the show interfaces command for a router's interface, many of the statistics shown are, by default, five-minute average values. In this lab you will view the input rate on one of router R2's serial interfaces, and to avoid having to wait the default load interval of 5 minutes, in this task you will set the load interval to its minimum value of 30 seconds.

Complete these steps:

Step 1.	Enter serial interface configuration mode on router R2, for your serial interface that connects router R2 to router R1. For example, in the sample topology shown in Figure 5-33, you would enter interface configuration mode for R2's Serial 0/0 interface.
Step 2.	Configure a load interval of 30 seconds with the load-interval 30 command.
Step 3.	Exit to privileged mode with the end command.

Task 2: Take a Baseline Measurement

In this task, you will determine the amount of bandwidth being consumed by a VoIP phone call, without cRTP.

Complete these steps:

Step 1.	Place a call from extension 2222 to extension 1111.
Step 2.	After answering the call, wait at least 30 seconds in order for the load interval to elapse.
Step 3.	On router R2, use the show interfaces command for your interface that connects router R2 to router R1 (for example, interface Serial 0/0 in the topology shown).
Step 4.	Note the value of the 30 second input rate value.

Task 3: Enable cRTP

In this task, you will enable cRTP on routers R2 and R1.

Complete these steps:

Step 1.	Enter interface configuration mode for R2's serial interface that connects to router R1.
Step 2.	Enable cRTP for this interface, such that this interface will begin sending compressed RTP headers after it receives compressed RTP headers, using the ip rtp header-compression passive command.
Step 3.	Enter interface configuration mode for R1's serial interface that connects to router R2.
Step 4.	Enable cRTP for this interface, such that this interface sends compressed RTP headers, even though it might not yet have received a compressed RTP header, using the ip rtp header-compression command.

Task 4: Verify cRTP

In this task, you will measure the 30 second input rate of router R2's serial interface that you configured for cRTP in Task 3.

Complete these steps:

Step 1.	Wait at least 30 seconds from the time you enabled cRTP on router R1 to allow sufficient time for the load interval to elapse.
Step 2.	On router R2, issue the show interfaces command for the interface on which you enabled cRTP.
Step 3.	Note the 30 second input rate value. This value should be significantly lower than the rate measured in Task 2. The bandwidth savings you are witnessing is a result of enabling RTP header compression.

Suggested Solution

Although your physical hardware might differ, Examples 5-1, 5-2, and 5-3 offer one solution to the preceding exercise. Note from these examples that the 30 second input rate prior to implementing cRTP was 26 kbps, and that rate dropped to 10 kbps after enabling cRTP, resulting in significant bandwidth savings.

Example 5-1. Router R2's Configuration and Bandwidth Utilization Prior to cRTP

R2#configure terminal
R2(config)#interface serial 0/0
R2(config-if)#load-interval30
R2(config-if)#end
R2#show interfaces serial 0/0
Serial0/0 is up, line protocol is up
  Hardware is PowerQUICC Serial
  Internet address is 10.1.1.1/24
  MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
     reliability 255/255, txload 51/255, rxload 51/255
  Encapsulation PPP, LCP Open
  Open: CDPCP, IPCP, loopback not set
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:26:55
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: weighted fair
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
    Conversations 0/2/32 (active/max active/max total)
     Reserved Conversations 2/2 (allocated/max allocated)
     Available Bandwidth 0 kilobits/sec
  30second input rate 26000 bits/sec, 51 packets/sec                           
  30second output rate 26000 bits/sec, 51 packets/sec
    8206 packets input, 524947 bytes, 0 no buffer
    Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
    8146 packets output, 524470 bytes, 0 underruns
    0 output errors, 0 collisions, 2 interface resets
    0 output buffer failures, 0 output buffers swapped out
    21 carrier transitions
    DCD=up DSR=up DTR=up RTS=up CTS=up
R2#configure terminal
R2(config)#interface serial 0/0
R2(config-if)#iprtp header-compression passive
R2(config-if)#end

Example 5-2. Router R1's cRTP Configuration

 R1#configure terminal
R1(config)#interface serial 0
 R1(config-if)#ip rtp header-compression
 R1(config-if)#end

Example 5-3. Router R2 Bandwidth Utilization after Enabling cRTP

R2#show interfaces serial 0/0
Serial0/0is up, line protocol is up
  Hardware is PowerQUICC Serial
  Internet address is 10.1.1.1/24
  MTU 1500 bytes, BW 128 Kbit, DLY 20000 usec,
     reliability 255/255, txload 23/255, rxload 19/255
  Encapsulation PPP, LCP Open
  Open: CDPCP, IPCP, loopback not set
  Last input 00:00:00, output 00:00:00, output hang never
  Last clearing of "show interface" counters 00:30:39
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total outputdrops: 0
  Queueing strategy: weighted fair
  Output queue: 0/1000/64/0 (size/max total/threshold/drops)
     Conversations 0/3/32 (active/max active/max total)
     Reserved Conversations 2/2 (allocated/max allocated)
     Available Bandwidth 0 kilobits/sec
  30 second input rate 10000 bits/sec, 52 packets/sec                          
  30 second output rate 12000 bits/sec, 52 packets/sec
     19775 packets input, 1018699 bytes, 0 no buffer
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
     19681 packets output, 1024385 bytes, 0 underruns
     0 output errors, 0 collisions, 2 interface resets
     0 output buffer failures, 0 output buffers swapped out
     21 carrier transitions
     DCD=up DSR=up DTR=up RTS=up CTS=up

RTP Header Compression