Cisco Router 800 Series Software Configuration Guide

							 
7-41
Cisco 800 Series Software Configuration Guide
78-5372-06
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using RFC 1483 Encapsulation
Associating the Policy Map with the ATM PVC and Using TCP MSS Adjust
Fine-Tuning the Size of the PVC ATM Transmit Ring Buffer
Differentiating Between Data and Voice Packets
To give priority to voice packets, the router must differentiate between the 
entering voice and data packets. One way to differentiate the packets is to 
examine their source or destination IP addresses, because data and VoIP devices 
may have different IP addresses. 
Another way to differentiate the packet is use IP Precedence. Usually, data 
packets have precedence 0, while voice packets have IP precedence 5. To learn 
how to configure the IP Precedence for voice packets, refer to the documentation 
for your VoIP device. 
NoteIn 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. 
Configuring an Access List and Voice Class
Assuming that all voice packets have precedence 5 and that all data packets have 
precedence 0, perform these steps to configure an access-list that matches all 
precedence 5 packets, beginning in global configuration mode.
Command Task
Step 1access-list 101 permit ip any any precedenceConfigure an access list to match voice 
packets.
Step 2class-map voiceConfigure a voice class
Step 3match access-group 101Associate the voice class with the access list. 
						

							 
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using RFC 1483 Encapsulation
7-42
Cisco 800 Series Software Configuration Guide
78-5372-06
Configuring a Policy Map and Specifying Voice Queuing
Follow the steps below to configure a policy may and to specify voice queuing, 
beginning in global configuration mode. 
Associating the Policy Map with the ATM PVC and Using TCP 
MSS Adjust
Perform the steps below to associate the policy map with the ATM PVC and to 
use the TCP MSS adjust command to control delay, beginning in global 
configuration mode.
NoteThe default service class for configuring the ATM interface is unspecified bit rate 
(ubr). To attach the policy map to the ATM PVC, you must use a service class of 
vbr (nrt) or vbr (rt).  Command Task
Step 1policy map nameConfigure a policy map.1
1. Total bandwidth for the policy map may not exceed 75 percent of the total PVC bandwidth.
Step 2class voiceSpecify the class for queuing.
Step 3priority bandwidthSpecify the bandwidth for this strict priority 
queue.
Command Task
Step 1interface ATM 0Enter configuration mode for the ATM 
interface.
Step 2dsl equipment-type {co | cpe} Configure the DSL equipment type.
Step 3dsl linerate {number| auto} Specify the ADSL line rate. The range of 
valid numbers is between 72 and 2312.
Step 4ip address ip-address maskSet the IP address and subnet mask for the 
ATM interface. 
						

							 
7-43
Cisco 800 Series Software Configuration Guide
78-5372-06
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using RFC 1483 Encapsulation
Fine-Tuning the Size of the PVC ATM Transmit Ring Buffer
Each PVC has a hardware output first-in first-out (FIFO) queue that temporarily 
stores packets before they are sent out to the transceiver. In order to reduce 
latency for voice packets, you may need to reduce the size of this queue. Reducing 
the queue size reduces the maximum number of data packets that are “ahead” of 
a voice packet in the transmit queue. However, a transmit queue size that is too 
small may affect transmit throughput performance.
Configuration Example
The following example shows a voice QoS configuration in a single-PVC 
environment using AAL5SNAP encapsulation.
access-list 101 permit ip any any precedence critical
class-map voice
match access-group 101
policy-map mypolicy
class voice
Step 5pvc vpi/vciCreate an ATM PVC for each end node with 
which the router communicates.
Step 6encapsulation protocolSpecify the encapsulation type for the PVC. 
Encapsulations can be specified as either 
AAL5SNAP or AAL5MUX PPP.
Step 7service policy out nameAssociate the service policy name.
Step 8vbr-rt pcr scr bsSpecify the service class.
Step 9exitExit configuration mode for the ATM PVC.
Step 10ip tcp adjust-mss mssSpecify the TCP maximum segment size 
(MSS).
Step 11no shutdownEnable the ATM interface.
Step 12exitExit configuration mode for the ATM 
interface. Command Task 
						

							 
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using PPP over ATM and Multilink Encapsulation
7-44
Cisco 800 Series Software Configuration Guide
78-5372-06 priority 480
int atm0
dsl equipment-type CPE
dsl linerate AUTO
ip tcp-mss 1452
pvc 8/35
encapsulation aaal5snap
service-policy out mypolicy
vbr-rt 1000 1000 1
tx-ring-limit 5
!
Configuring a Single-PVC Environment Using PPP 
over ATM and Multilink Encapsulation
This section describes configuring of a single-PVC environment using PPP over 
ATM and multilink encapsulation.
The “Configuring Link Fragmentation and Interleaving with Low Latency 
Queuing” section on page 7-46 describes configuring multilink PPP 
fragmentation and interleaving for a second single-PVC environment. 
In a single-PVC environment using PPP over ATM multilink encapsulation, the 
traffic relies on Cisco IOS to provide priority queuing using LLQ. These tasks are 
involved in configuring a single-PVC environment:
Differentiating Between Data and Voice Packets
Configuring the Policy Map and Specifying Voice Queuing
Associating the Policy Map to the ATM PVC 
Configuring Link Fragmentation and Interleaving with Low Latency 
Queuing
Differentiating Between Data and Voice Packets
To give priority to voice packets, the router must differentiate between the 
entering voice and data packets. One way to differentiate the packets is to 
examine the source or destination IP addresses, because data and VoIP devices 
may have different IP addresses.  
						

							 
7-45
Cisco 800 Series Software Configuration Guide
78-5372-06
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using PPP over ATM and Multilink Encapsulation
Another way to differentiate the packets is use IP Precedence. Usually, data 
packets have precedence 0, while voice packets have IP precedence 5. To learn 
how to configure the IP precedence for voice packets, refer to the documentation 
for your VoIP device. 
NoteIn 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. 
Configuring the Policy Map and Specifying Voice Queuing
Follow the steps below to configure a policy may and to specify voice queuing, 
beginning in global configuration mode. 
Associating the Policy Map to the ATM PVC 
Follow the steps below to associate the policy map to the ATM PVC, beginning 
in global configuration mode.  Command Task
Step 1policy map nameConfigure a policy map.1
Step 2class voiceSpecify the class for queuing.
Step 3priority bandwidthSpecify the bandwidth for this strict priority 
queue.
1. Total bandwidth for the policy map may not exceed 75 percent of the total PVC bandwidth.
Command Task
Step 1interface ATM 0Enter configuration mode for the ATM 
interface.
Step 2dsl equipment-type {co | cpe} Configure the DSL equipment type.
Step 3dsl linerate {number| auto} Specify the ADSL line rate. The range of 
valid numbers is between 72 and 2312. 
						

							 
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using PPP over ATM and Multilink Encapsulation
7-46
Cisco 800 Series Software Configuration Guide
78-5372-06
Configuring Link Fragmentation and Interleaving with Low 
Latency Queuing
Link fragmentation and interleaving (LFI) is available when you are using 
multilink PPP over ATM. 
Two types of traffic can be simultaneously transmitted over the same link: 
Large packets from heavy, delay-insensitive traffic sources
Small packets from delay-sensitive traffic sources
The purpose of LFI is to reduce latency for delay-sensitive traffic. Two things 
happen when LFI is used:
Large packets received from delay-insensitive sources are fragmented. 
Small packets received from delay-sensitive sources are interleaved with the 
large packet fragments. 
Multilink PPP is one example of how LFI is implemented. 
Use the following steps to configure the router for LFI. Begin in global 
configuration mode. 
Step 4ip address ip-address maskSet the IP address and subnet mask for the 
ATM interface.
Step 5pvc vpi/vciCreate an ATM PVC for each end node with 
which the router communicates.
Step 6encapsulation protocolSpecify the encapsulation type for the PVC. 
Encapsulations can be specified as either 
AAL5SNAP or AAL5MUX PPP.
Step 7service policy out nameAssociate the service policy name.
Step 8vbr-rt pcr scr bsSpecify the service class.
Step 9exitExit configuration mode for the ATM PVC. Command Task 
						

							 
7-47
Cisco 800 Series Software Configuration Guide
78-5372-06
Chapter 7      Router Feature Configuration
Configuring a Single-PVC Environment Using PPP over ATM and Multilink Encapsulation
Calculate the fragment size using the following formula:
fragment size = (bandwidth in kbps/8) * fragment-delay i milliseconds (ms)
In this case, the fragment size = (640/8) * 10 = 800. The fragment size is greater 
than the maximum voice packet size of 200, which is that of G.711, 20 ms. Note 
that a low fragment delay corresponds to a fragment size that may be smaller than 
the voice packet size, resulting in reduced voice quality. 
NoteLFI should not be used when you have a link that exceeds 1 Mbps because, at this 
high speed, the latency of sending a big packet is small enough that the benefit of 
LFI is not required. Using LFI may actually increase latency because the extra 
processing time required to fragments packets may become a bottleneck.  Command Task
Step 1bandwidth bandwidth-kptsConfigure the dialer bandwidth, The 
bandwidth configured under the dialer 
interface must be the same as the bandwidth 
allocated to its assigned PVC.
Step 2ppp multilinkEnable ppp multilink.
Step 3ppp multilink interleaveSpecify ppp multilink interleaving.
Step 4ppp multilink fragment-delay millisecondsDefine the fragment delay.
Step 5access-list access-list-number {permit | 
deny} address mask precedence numberCreate an access list.
Step 6class-map match-all voiceCreate a class map.
Step 7match access-group numberLink the class map to the access list.
Step 8policy-map nameCreate a policy map.
Step 9class nameDefine the class.
Step 10priority numberAssign priority bandwidth to the traffic.
Step 11interface dialer numberDefine a dialer rotary group.
Step 12service-policy {input | output} policy-mapCreate a service policy. 
						

							 
Chapter 7      Router Feature Configuration
Configuring a Multiple-PVC Environment
7-48
Cisco 800 Series Software Configuration Guide
78-5372-06
Configuring a Multiple-PVC Environment
In a multiple-PVC environment, the traffic relies on the ATM interface to provide 
priority queuing for voice and fragmentation and interleaving. The following 
sections describe the configurations that you can use.
Voice and Data on Different Subnets
Figure 7-2 shows voice and data packets on different subnets. All voice traffic 
may be on an ATM PVC with a vbr-rt service class, while all data traffic is 
transported on an ATM PVC with a ubr service class.
Figure 7-2 Voice and Data on Different Subnets
Configuring the ATM Interface and Subinterfaces
Follow the steps below to configure the ATM interface and subinterfaces, 
beginning in global configuration mode.
Ethernet 0
P1 P2 P3 P4c82710.0.0.0
11.0.0.0PVC 1/40 VBR (RT), Voice
PVC 8/35 UBR, Data
33494
Command Task
Step 1interface ATM 0.1 point-to-pointSpecify the ATM0.1 subinterface.
Step 2ip address ip-address maskSet the IP address and subnet mask for the 
ATM0.1 subinterface.
Step 3pvc vpi/vciCreate an ATM PVC for each end node with 
which the router communicates. 
						

							 
7-49
Cisco 800 Series Software Configuration Guide
78-5372-06
Chapter 7      Router Feature Configuration
Configuring a Multiple-PVC Environment
Configuration Example 
The following example shows a voice QoS configuration with all data traffic on 
the 30.0.0.1 network and all voice traffic on the 20.0.0.1 network. 
You do not need to enter the commands marked “default.” These commands 
appear automatically in the configuration file that is generated when you use the 
show running-config command.
!
interface ATM0.1 point-to-point
ip address 20.0.0.1 255.0.0.0
no ip directed-broadcast (default)
pvc 1/100
protocol ip 20.0.0.2 broadcast
vbr-rt 424 424 5
encapsulation aal5snap
!
interface ATM0.2 point-to-point
ip address 30.0.0.1 255.0.0.0
no ip directed-broadcast (default)
pvc 1/101
protocol ip 30.0.0.2 broadcast
encapsulation aal5snap
Step 4encapsulation typeSpecify the encapsulation type for the PVC.
Step 5protocol ip address broadcastSet the protocol broadcast for the IP address.
Step 6interface ATM 0.2 point-to-pointSpecify the ATM0.2 subinterface.
Step 7ip address ip-address maskSet the IP address and subnet mask for the 
ATM0.2 subinterface.
Step 8pvc vpi/vciCreate an ATM PVC for each end node with 
which the router communicates.
Step 9encapsulation typeSpecify the encapsulation type for the PVC.
Step 10protocol ip address broadcastSet the protocol broadcast for the IP address.
Step 11exitExit configuration mode for the ATM 
interface. Command Task 
						

							 
Chapter 7      Router Feature Configuration
Configuring a Multiple-PVC Environment
7-50
Cisco 800 Series Software Configuration Guide
78-5372-06
Voice and Data on the Same Subnet Using Virtual Circuit 
Bundling
Figure 7-3 and Ta b l e 7 - 2 show voice and data packets on the same subnet using 
virtual circuit bundling. Virtual circuit bundling allows multiple PVCs on the 
same bundle. Using virtual circuit bundling and assigning precedence 5 to voice 
packets and not data packets ensures that traffic for the two are separated onto two 
PVCs.
Figure 7-3 Voice and Data on the Same Subnet with Virtual Circuit Bundling
P1 P2 P3P4 c82774586
1
2
3
4
Callout Number Description
1Ethernet 0
2Bundle
3PVC Bundle 1/40 BVR (RT), voice
4PVC Bundle 8/35 UBR, data