VoIP Dial Peer

IOS voice gateways provide many services to connect the packetized, VoIP network to non-packetized, traditional voice services, including analog and digital trunks. IOS gateways perform many tasks, but one of the most important tasks is to convert from packetized voice to nonpacketized voice, and vice versa. In other words, voice traffic entering a router on an analog or digital trunk is not carried inside an IP packet, but the IOS gateway converts the incoming voice to a digital signal (analog trunks only) and adds the appropriate IP, UDP, and RTP headers around the digital voice (both analog and digital trunks). Conversely, when a VoIP packet arrives, and the voice needs to be sent out a trunk, the IOS gateway removes the packet headers, converts the voice to analog (analog trunks only), and sends the traffic out the trunk.

Although this book does not attempt to explain voice configuration and concepts to much depth, some appreciation for IOS gateway configuration is required for some of the functions covered in this book. In particular, Chapter 8, "Call Admission Control and QoS Signaling," which covers Voice call admission control (CAC), requires a little deeper examination of voice. To understand classification and marking using dial peers, however, only a cursory knowledge of voice configuration is required. Consider Figure 3-20, for instance, which shows two analog IOS voice gateways, R1 and R4, along with Examples 3-9 and 3-10, which show the pertinent configuration on R1 and R4.

Figure 3-20 Network with Two Analog Voice Gateways

Figure 3-20 Network with Two Analog Voice Gateways

Example 3-9 R1 Voice Gateway Configuration

hostname R1

!

int fastethernet 0/0

!

251 255

255.255.0

dial-peer voice 3001

voip

destination-pattern

3001

!

192.168

3

254

dial-peer voice 3002

voip

destination-pattern

3002

!

192.168

3

254

dial-peer voice 1001

pots

destination-pattern

1001

!

dial-peer voice 1002

pots

destination-pattern

1002

port 3/1

Example 3-10 R4 Voice Gateway Configuration hostname R4

int fastethernet 0/0

ip address 192.168.3.254 255.255.255.0

dial-peer voice 1001 voip destination-pattern 1001 session target ipv4:192.168.1.251

dial-peer voice 1002 voip destination-pattern 1002 session target ipv4:192.168.1.251

dial-peer voice 3001 pots destination-pattern 3001 port 3/0

dial-peer voice 3002 pots destination-pattern 3002 port 3/1

The highlighted portions of the examples focus on the configuration for the physical voice ports on R1, and the VoIP configuration on R4. Both R1 and R4 use dial-peer commands to define their local analog voice trunks and to define peers to which VoIP calls can be made. In Example 3-9, for instance, the highlighted portion of the configuration shows R1's configuration of the two local analog lines. The two highlighted dial-peer statements use the keyword pots, which stands for plain-old telephone service. The pots keyword implies that the ports associated with this dial peer are traditional analog or digital telephony ports. The physical analog ports are correlated to each dial peer with the port command; in each of these configurations, a two-port FXS card sits inside slot 3 of a 1760-V router. Finally, on R1, the phone number, or dial pattern, associated with each of the analog ports is configured. With just the highlighted configuration in R1, voice calls could be placed between the two extensions (x1001 and x1002).

To place calls to extensions 1001 and 1002 from R4, the dial-peer commands highlighted in Example 3-10 are required. These two dial-peer commands use a voip keyword, which means this dial peer configures information about an entity to which VoIP calls can be placed. The phone number, or dial pattern, is defined with the destination-pattern command again—notice that extensions 1001 and 1002 are again configured. Finally, because these two dial peers configure details about a VoIP call, a local physical port is not referenced. Instead, the sessiontarget ipv4:192.168.1.251 command implies that when these phone numbers are called, to establish a VoIP call, using the IP version 4 IP address shown.

Similarly, R4 defines the local phone numbers and ports for the locally connected phones, and R1 defines VoIP dial peers referring to R4's phones, so that calls can be initiated from R1.

Dial-peer classification and marking, when you know how to configure the basic dial-peer parameters, is easy. POTS dial peers refer to analog or digital trunks, over which no IP packet is in use—so there is nothing to mark. On VoIP dial peers, the dial peer refers to the IP address of another gateway to which a call is placed. So, by placing the ip precedence 5 dial-peer subcommand under each voip dial-peer, the packets generated for calls matching each dial peer will be marked with IP precedence 5. Example 3-11 lists the R4 configuration, with these changes made; the equivalent changes would be made to R1 as well.

Example 3-11 R4 Voice Gateway Configuration hostname R4

interface fastethernet 0/0

ip address 192.168.3.254 255.255.255.0

dial-peer voice 1001 voip destination-pattern 1001 session target ipv4:192.168.1.251 ip precedence 5 no vad

dial-peer voice 1002 voip destination-pattern 1002 session target ipv4:192.168.1.251

Example 3-11 R4 Voice Gateway Configuration (Continued)

|

dial-peer voice 3001

pots

destination-pattern

!

dial-peer voice 3002

pots

destination-pattern

3002

port 3/1

In the example, the highlighted text shows the ip precedence 5 commands under each voip dial-peer. Packets created for VoIP calls for the configured dial patterns of 1001 and 1002 will be marked with IP precedence 5. The identical commands would be added to R1's configuration on the VoIP dial peers to achieve the same effect.

Beginning in IOS Releases 12.2(2)XB and 12.2(2)T the ip precedence command has been replaced with the ip qos dscp command. This allows the dial peer to set the IP precedence or the DSCP value for VoIP payload and signaling traffic. Also keep in mind that the current DQOS exam, at the time this book was published, was based on IOS 12.1(5)T—so this command would not be on the current exam. Check the URLs listed in the Introduction for any possible changes.

The command uses the following syntax: ip qos dscp [number | set-af | set-cs | default | ef][media | signaling]

Table 3-16 outlines the meaning of the parameters of the command.

Table 3-16 IP QoS DSCP Command Options

Table 3-16 outlines the meaning of the parameters of the command.

Table 3-16 IP QoS DSCP Command Options

IP QoS DSCP Options

Function

number

DSCP value. Valid entries are from 0 to 63.

set-af

Sets DSCP to assured forwarding bit pattern. Acceptable values are as follows:

AF11, AF12, AF13, AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, AF43

set-cs

Sets DSCP to class selector code point. Acceptable values are as follows: CS1, CS2, CS3, CS4, CS5, CS6, CS7

default

Sets DSCP to default bit pattern 000000.

ef

Sets DSCP to expedited forwarding bit pattern 101110.

media

Applies the specified DSCP value to the media payload packets.

signaling

Applies the specified DSCP value to the signaling packets.

The ip qos dscp command enables you to have much more granular control of how a VoIP packet is marked than the ip precedence command, while providing a method to preserve backward compatibility. Examples 3-12 and 3-13 show how R1 and R4 can be configured to use the ip qos dscp command to mark voice payload traffic with a DSCP value of EF and voice signaling traffic with a DSCP value of AF31. Figure 3-21 shows the now-familiar network, with the new criteria listed.

Figure 3-21 Mark Voice Payload Traffic

Figure 3-21 Mark Voice Payload Traffic

Example 3-12 R1 IP QoS DSCP Dial-Peer Configuration hostname R1

int fastethernet 0/0

ip address 192.168.1.251 255.255.255.0

Example 3-12 R1 IP QoS DSCP Dial-Peer Configuration (Continued)

dial-peer voice 3001 destination-pattern

voip 3001

ip qos dscp ef media ip qos dscp af31 signaling

session target ipv4

!

192.168.3

254

dial-peer voice 3002 destination-pattern

voip 3002

ip qos dscp ef media ip qos dscp af31 signaling

session target ipv4

!

192.168.3

254

dial-peer voice 1001 destination-pattern port 3/0

!

pots 1001

dial-peer voice 1002 destination-pattern port 3/1

pots 1002

Example 3-13 R4 IP QoS DSCP Dial-Peer Configuration

hostname R4

int fastethernet 0/0

!

254 255

255.255.0

dial-peer voice 1001

voip

destination-pattern

1001

ip qos dscp ef media

ip qos dscp af31 signaling

!

192.168

1.251

dial-peer voice 1002

voip

destination-pattern

1002

ip qos dscp ef media

ip qos dscp af31 signaling

!

192.168

1.251

dial-peer voice 3001

pots

destination-pattern

3001

!

dial-peer voice 3002

pots

destination-pattern

3002

port 3/1

In this example, the highlighted text shows the ip qos dscp commands used to mark voice signaling with DSCP AF31 and voice payload with DSCP EF. For networks that cannot yet support DSCP markings, you can use the set-cs option to mark the voice traffic with IP precedence, providing backward-compatible support.

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