For all Unified CM SIP trunks, you must check the MTP required check box on the trunk configuration page to enable SIP Early Offer in IPv4 (IPv6 does not support Early Offer). When MTP required is checked, a media termination point (MTP) is used in the media path for all inbound and outbound calls. This statically assigned MTP affects all calls in the following ways:

• Because the MTP is placed in the media path for all calls, rather than having one call leg from the calling phone to the called phone, the insertion of the MTP creates two legs: one from the calling phone to the MTP, and the other from the MTP to the called phone. For signaling purposes, this can be considered to be two calls. The calling phone and MTP negotiate media capabilities (such as codec, IP addresses, and UDP port numbers to be used), as do the MTP and the called phone at the far end of the SIP trunk.

• The statically assigned MTP (MTP required checked) must be configured to use one codec type (G711 or G729). Assigning a single voice codec to this statically assigned MTP disables the use of the pass-through codec. This, in turn, prevents the negotiation of the pass-through codec that is required for video calls or encrypted calls. (T.38 fax calls are supported with statically assigned MTPs.) Therefore, if support for video or encryption is required over the SIP trunk, SIP Delayed Offer (no statically assigned MTP) must be used.

  Note	The pass-through codec should be configured on all dynamically inserted MTPs. To enable the use of the pass-through codec, configure the MTP with both a standard codec and the pass-through codec.

If SIP Early Offer is required for dual-stack SIP Unified CM trunks, then you must configure the Cisco IOS MTP to use both an IPv6 and IPv4 address. For details, see Media Resources and Music on Hold Overview.

Delayed Offer trunks do not have a statically assigned MTP and therefore MTP resources are not used for every call. For Delayed Offer calls, Unified CM attempts to set up the call using a single call leg between the calling and called device, and in doing so must consider the IP addressing mode configuration of both the Unified CM trunk and the IP phone registered with Unified CM. In certain calls where there are IP addressing mode mismatches between the Unified CM trunk and the registered phone, Unified CM dynamically inserts an MTP to resolve this mismatch. The pass-through codec is supported by this dynamically inserted MTP, and video calls and encrypted calls can be established with this MTP in the call path. The pass-through codec should be configured on all dynamically inserted MTPs. To enable the use of the pass-through codec, configure the MTP with both a standard codec and the pass-through codec.

The following factors affect which IP addressing version is used for signaling on Unified CM SIP trunks:

• Call direction

• IP addressing mode of the trunk

• Configured destination addresses of the trunk

• Trunk's IP addressing mode preference for signaling

• Cluster-wide IP addressing mode preference for signaling

Note	This is not supported in CSR 12.0

SIP Early Offer calls involve two call legs: one from the phone to trunk MTP, and the other from the trunk MTP to the SIP voice gateway. The Cisco IOS MTP is configured to support both IPv4 and IPv6 addresses. ANAT has not been enabled on the SIP trunk in the following figure, so as with a standard SIP trunk, only a single IP addressing version is exchanged.

Call Leg from Phone to Trunk MTP: Standard Unified CM In-Cluster Negotiation

The MTP is dual-stacked and can match the media addressing type of the phone if it is set to IPv4-only or IPv6-only. If the phone is also dual-stacked, the cluster-wide IP Addressing Mode Preference for Media (IPv4 or IPv6) determines which IP addressing version is used for media.

Call Leg from MTP Trunk to Voice Gateway: ANAT Not Enabled, and One Media Address Is Sent in SDP (IPv4 or IPv6)

For outbound Early Offer calls where ANAT is not enabled, the IP Addressing Mode of the SIP trunk determines what is sent in the SDP body of the SIP Offer, as follows:

• IP Addressing Mode = IPv4 only—The IPv4 address of the MTP is sent in the SDP body.

• IP Addressing Mode = IPv6 only—The IPv6 address of the MTP is sent in the SDP body.

• IP Addressing Mode = IPv4 and IPv6—The cluster-wide IP Addressing Mode Preference for Media (IPv4 or IPv6) is used to determine which MTP address is sent in the SDP body.

Note	IPv6 is not supported.

SIP Early Offer calls involve two call legs: one from the phone to the trunk MTP, and the other from the trunk MTP to the SIP voice gateway. The Cisco IOS MTP is configured to support both IPv4 and IPv6 addresses. ANAT has not been enabled on the SIP trunk in Figure 7-6, so as with a standard SIP trunk, only a single IP addressing version is exchanged in the SIP Offer and Answer.

Call Leg from Trunk MTP to Phone: Standard Unified CM In-Cluster Negotiation

The MTP is dual-stacked and can match the media addressing type of the phone if it is set to IPv4 only or IPv6 only. If the phone is also dual-stacked, the cluster-wide IP Addressing Mode Preference for Media (IPv4 or IPv6) determines which IP addressing version is used for media.

Call Leg from Voice Gateway to Trunk MTP: ANAT Not Enabled, and One Media Address Is Received in SDP

For inbound Early Offer calls where ANAT is not enabled, the IP Addressing Mode of the SIP trunk determines whether the address received in the SDP body of the SIP Offer is accepted or rejected, as follows:

• IP Addressing Mode = IPv4 only:

  • If an IPv4 address is received in the SDP body, proceed with the call.

  • If an IPv6 address is received in the SDP body, reject the call.

• IP Addressing Mode = IPv6 only:

  • If an IPv6 address is received in the SDP body, proceed with the call.

  • If an IPv4 address is received in the SDP body, reject the call.

  Note	For these trunk calls, Unified CM does not insert an MTP to resolve a media addressing version mismatch between the two voice devices.

• IP Addressing Mode = IPv4 and IPv6:

  • If an IPv4 address is received in the SDP body, proceed with the call.

  • If an IPv6 address is received in the SDP body, proceed with the call.

ANAT is used in the SIP Offer and Answer exchange between dual-stack SIP trunks. ANAT allows devices to send both IPv4 and IPv6 addresses in the SDP body of the SIP Offer, and to return in the SDP body of the SIP Answer, a preferred IP address (IPv4 or IPv6) with which to establish a media connection.

The use of ANAT on a dual-stack SIP trunk is indicated in the header of the SIP Invite. The field Require: sdp-anat is used by Unified CM SIP trunks using Early Offer, and the field Supported: sdp-anat is used by Unified CM SIP trunks using Delayed Offer. The Require: sdp-anat value indicates to the far end of the SIP trunk connection that an ANAT response must be supported. The Supported: sdp-anat value indicates to the far end of the SIP trunk connection that an ANAT response should be supported.

We support ANAT on dual-stack SIP trunks only; that is, on SIP trunks configured with an addressing mode of IPv4 and IPv6.

The receipt of Require: sdp-anat or Supported: sdp-anat does not affect how Unified CM responds to inbound Invites on trunks configured for SIP Early Offer, but it does effect how MTPs are assigned dynamically for inbound calls to Unified CM SIP trunks using Delayed Offer.

Unified CM supports ANAT over dual-stack (IPv4 and IPv6) SIP trunks. If ANAT is enabled, it should be configured on both ends of the SIP trunk. If Require: sdp-anat is sent in the SIP Invite and the receiving SIP trunk does not support ANAT, all calls are rejected. To enable ANAT, check the Enable ANAT check box on the SIP Profile associated with the SIP trunk. ANAT can be used with both Early Offer and Delayed Offer calls.

ANAT should be enabled only on SIP trunks with an IP Addressing Mode setting of IPv4 and IPv6. Enabling ANAT on a single-stack SIP trunk (IPv4 only or IPv6 only) does not really make sense because only one IP address can be offered. Therefore, we do not support ANAT on single-stack (IPv6 only or IPv4 only) SIP trunks.

Note	This is not supported in CSR 12.0

The following figure shows a simplified version the SIP Early Offer and SIP Answer using ANAT on dual-stack SIP trunks.

SIP Early Offer calls involve two call legs: one from the phone to the trunk MTP, and the other from the trunk MTP to the SIP voice gateway. The Cisco IOS MTP is configured to support both IPv4 and IPv6 addresses. ANAT has been enabled on this SIP trunk, so both IPv4 and IPv6 addresses are exchanged in the SIP Offer and Answer.

Call Leg from Phone to Trunk MTP: Standard Unified CM In-Cluster Negotiation

The MTP is dual-stacked and can match the media addressing type of the phone if it is set to IPv4-only or IPv6-only. If the phone is also dual-stacked, the cluster-wide IP Addressing Mode Preference for Media (IPv4 or IPv6) determines which IP addressing version is used for media.

Call Leg from MTP Trunk to Voice Gateway: ANAT Enabled, and Two Media Addresses Sent in SDP (IPv4 and IPv6)

Unified CM selects the media address preference indicated in the SDP body of the ANAT SIP Offer by using the cluster-wide setting for IP Addressing Mode Preference for Media. The IP Addressing Mode of the trunk must set to IPv4 and IPv6. The trunk's IP Addressing Mode could be set to IPv4-only or IPv6-only, but this would defeat the purpose of ANAT because only one address would be sent. (The trunk's IP Addressing Mode overrides the ANAT setting.)

The called device (voice gateway) selects which addressing version to use for the voice call. The caller's preference does not have to be honored.

Outbound SIP Early Offer

The SIP header of the Invite with the outbound SIP Early Offer contains the Require: sdp-anat field, indicating that ANAT must be supported by the far-end SIP device. For outbound SIP Offers on Unified CM SIP trunks configured for Early Offer, for all calls the SDP body of the SIP Offer includes the IPv4 address and UDP port number and the IPv6 address and UDP port number of the trunk's statically assigned MTP. The preferred addressing version for Unified CM is also indicated in the SDP body, and the field a=group:ANAT 2 1 indicates that the second address (the IPv6 address) is preferred by Unified CM. For Early Offer calls, this preference is selected based on the cluster-wide IP Addressing Mode Preference for Media.

Inbound SIP Answer

When the far-end SIP trunk receives an Invite with Require: sdp-anat, it must support ANAT and should return an ANAT-based response in its SIP Answer. If ANAT is not supported by the far-end SIP trunk, it should reject the call. In the previous figure, a=group:ANAT 2 indicates the gateway's choice of its IPv6 address and port number for the voice call. Notice that the gateway's IPv6 address and IPv4 address are both included in the Answer; however, only the IPv6 UDP port number is returned, and the IPv4 UDP port number is set to zero.

Note	This is not supported in CSR 12.0

The following figure shows a simplified version the SIP Early Offer and SIP Answer using ANAT on dual-stack SIP trunks.

SIP Early Offer calls involve two call legs: one from the phone to the trunk MTP, and the other from the trunk MTP to the SIP voice gateway. The Cisco IOS MTP is configured to support both IPv4 and IPv6 addresses. ANAT has been enabled on this SIP trunk, so both IPv4 and IPv6 addresses will be exchanged in the SIP Offer and Answer.

Call Leg from Trunk MTP to Phone: Standard Unified CM In-Cluster Negotiation

The MTP is dual-stacked and can match the media addressing version of the phone if it is set to IPv4-only or IPv6-only. If the phone is also dual stacked, the cluster-wide IP Addressing Mode Preference for Media (IPv4 or IPv6) is used to select which IP addressing version is used for media.

Call Leg from Voice Gateway to Trunk MTP: ANAT Enabled, and IPv4 and IPv6 Media Addresses Received in SDP

Unified CM does not honor the indicated address preference in the SDP body of the received SIP Offer. For dual-stack Unified CM SIP trunks (IP Addressing Mode = IPv4 and IPv6), Unified CM selects the addressing version for the voice call based on the setting of the cluster-wide IP Addressing Mode Preference for Media.

Inbound SIP Early Offer

The SIP header of the Invite with the outbound SIP Early Offer contains the Require: sdp-anat field, indicating that ANAT must be supported by Unified CM. The SDP body of the SIP Offer includes the IPv4 address and UDP port number and the IPv6 address and UDP port number of the calling device. The preferred addressing version of the calling device is also indicated in the SDP body, and the field a=group:ANAT 2 1 indicates that the second address (the IPv6 address) is preferred.

Outbound SIP Answer

When the Unified CM SIP Early Offer trunk receives an Invite with Require: sdp-anat, it must support ANAT and should return an ANAT-based response in its SIP Answer. If ANAT is not supported by the Unified CM SIP trunk, it will reject the call. For Unified CM trunks configured for Early Offer, Unified CM returns the IPv4 and IPv6 addresses of the trunk MTP in its SIP Answers. In the previous figure, a=group:ANAT 2 indicates Unified CM’s choice for the IPv6 address and port number of the MTP for the voice call. Notice that the MTP's IPv6 address and IPv4 address are both included in the Answer; however, only the IPv6 UDP port number is returned, and the IPv4 UDP port number is set to zero.

Note	Unified CM selects the addressing version for the voice call based on the setting of the cluster-wide IP Addressing Mode Preference for Media. The incoming preference is not honored by Unified CM.

As shown in the following figure, SIP Delayed Offer calls typically involve a single call leg from the phone to the SIP voice gateway. ANAT has not been enabled on this SIP trunk, so as with a standard SIP trunk, only a single IP addressing version is exchanged in the SIP Offer and Answer.

For outbound Delayed Offer calls, the IP Addressing Mode settings of both the trunk and the phone influence the call setup in the following ways:

• The IP Addressing Mode setting of the trunk determines whether the received SIP Offer is accepted or rejected.

• The IP Addressing Mode setting of the phone determines which address (phone or MTP) is returned in the SIP Answer from Unified CM.

In this scenario, Unified CM can dynamically insert an MTP, if needed, into the call to convert the IP addressing version of the voice media stream between the calling and called devices. As mentioned previously, dynamically inserted MTPs support the pass-through codec, allowing video calls and encrypted calls to be established.

IP Addressing Mode of the Trunk

• IP Addressing Mode = IPv4 only:

  • If an IPv4 address is received in the SDP body, proceed with the call.

  • If an IPv6 address is received in the SDP body, reject the call.

• IP Addressing Mode = IPv6 only:

  • If an IPv6 address is received in the SDP body, proceed with the call.

  • If an IPv4 address is received in the SDP body, reject the call.

  Note	For trunk call signaling, Unified CM does not insert an MTP to resolve a media addressing version mismatch.

• IP Addressing Mode = IPv4 and IPv6 (Recommended configuration):

  • If an IPv4 address is received in the SDP body, proceed with the call.

  • If an IPv6 address is received in the SDP body, proceed with the call.

For SIP trunks using Delayed Offer and not using ANAT, the recommended trunk IP Addressing Mode setting is IPv4 and IPv6 because both IPv6 calls and IPv4 calls are accepted by the trunk.

IP Addressing Mode of the Phone

• IP Addressing Mode = IPv4 only:

  • If an IPv4 address is received in the SDP body, proceed with the call and return the IPv4 address of the phone in the SDP body of the SIP answer.

  • If an IPv6 address is received in the SDP body, dynamically insert an MTP into the media path to convert IP addressing versions, then proceed with the call. Return the IPv6 address of the MTP in the SDP body of the SIP answer.

• IP Addressing Mode = IPv6 only:

  • If an IPv6 address is received in the SDP body, proceed with the call and return the IPv6 address of the phone in the SDP body of the SIP answer.

  • If an IPv4 address is received in the SDP body, dynamically insert an MTP into the media path to convert IP addressing versions, then proceed with the call. Return the IPv4 address of the MTP in the SDP body of the SIP answer.

• IP Addressing Mode = IPv4 and IPv6:

  • If an IPv4 address is received in the SDP body, proceed with the call and return the IPv4 address of the phone in the SDP body of the SIP answer.

  • If an IPv6 address is received in the SDP body, proceed with the call and return the IPv6 address of the phone in the SDP body of the SIP answer.

When an MTP Is Required, Will the MTP of the Phone or the Trunk Be Used?

The cluster-wide IP Addressing Mode Preference for Media determines whether the MTP of the phone or of the trunk is used to convert the voice media stream between IPv4 and IPv6. This preference is used to select an MTP so that the longest Real-Time Transport Protocol (RTP) call leg in the cluster matches the cluster-wide preference.

Deployment Considerations for Delayed Offer Calls over Trunks without ANAT

If a call from an IPv4-only phone receives a SIP Offer that contains an IPv6 address, or if a call from an IPv6-only phone receives a SIP Offer that contains an IPv4 address, Unified CM dynamically inserts an MTP to convert between IPv4 and IPv6. In deployments with large numbers of IPv4-only phones, any SIP trunk call to or from an IPv6-only device requires an MTP for conversion between IPv4 and IPv6. Therefore, we recommend that you provide MTP resources for IPv4-only and IPv6-only devices in the Unified CM cluster.

As shown in the following figure, SIP Delayed Offer calls typically involve a single call leg from the phone to the SIP voice gateway. ANAT has not been enabled on this SIP trunk, so as with a standard SIP trunk, only a single IP addressing version is exchanged in the SIP Offer and Answer.

For inbound Delayed Offer calls, the combined settings of the IP Addressing Mode of both the trunk and the phone determine which IP addressing version and which device's IP address is sent in the SDP body of the SIP Offer.

For inbound Delayed Offer calls, if a mismatch exists between the IP addressing modes of the phone and the trunk, Unified CM can dynamically insert an MTP into the call path to convert the IP addressing version of the voice media stream from the IP phone, so that it matches that configured on the trunk. In this case, the address of the MTP is sent in the SDP body of Unified CM's SIP Offer.

For SIP trunks using Delayed Offer and not using ANAT, the recommended IP Addressing Mode setting for the trunk is IPv4 and IPv6. With this setting, Unified CM does not need to insert MTPs for inbound SIP Delayed Offer calls.

When an MTP Is Required, Will the MTP of the Phone or the Trunk Be Used?

The cluster-wide IP Addressing Mode Preference for Media determines whether the MTP of the phone or of the trunk is used to convert the voice media stream between IPv4 and IPv6. See the following table for details. This preference is used to select an MTP so that the longest Real-Time Transport Protocol (RTP) call leg in the cluster matches the cluster-wide preference.

As mentioned previously, dynamically inserted MTPs do support the pass-through codec, allowing video calls and encrypted calls to be established.

In the following call scenarios, the SIP trunks use ANAT to exchange IPv4 and IPv6 addresses for the media connection between the called and calling endpoints:

• Outbound Delayed Offer calls with ANAT.

• Inbound Delayed Offer calls with ANAT, where Supported: sdp-anat is received.

• Inbound Delayed Offer calls with ANAT, where Require: sdp-anat is received.

Alternative Network Address Types (ANAT)

ANAT is used in the SIP Offer and Answer exchange between dual-stack SIP trunks. ANAT allows SIP devices to send both IPv4 and IPv6 addresses in the SDP body of the SIP Offer, and to return in the SDP body of the SIP Answer, the preferred IP address (IPv4 or IPv6) with which to establish a media connection.

The use of ANAT on a SIP trunk is indicated in the header of the SIP Invite. The field Require: sdp-anat is used by Unified CM SIP trunks using Early Offer, and the field Supported: sdp-anat is used by Unified CM SIP trunks using Delayed Offer. The Require: sdp-anat value indicates to the far end of the SIP trunk connection that an ANAT response must be supported. The Supported: sdp-anat value indicates to the far end of the SIP trunk connection that an ANAT response should be supported.

For inbound calls to Unified CM SIP trunks using Delayed Offer, the receipt of these require or supported sdp-anat values by Unified CM has the following effects on how MTPs are assigned dynamically:

• If Unified CM receives an Invite with Require: sdp-anat, it returns two IP addresses in the SDP body of its ANAT SIP Offer (and therefore inserts an MTP for calls to IPv4-only and IPv6-only devices).

• If Unified CM receives an Invite with Supported: sdp-anat, it returns the IP addresses supported by the called device in the SDP body of its SIP Offer. In the case of an IP addressing version mismatch between the calling and called device for calls between Unified CM clusters, the calling Unified CM cluster inserts an MTP for conversions between IPv4 and IPv6.

• MTPs are not needed for calls to ANAT-enabled dual-stack Unified CM SIP trunks where Supported: sdp-anat is received; whereas when Require: sdp-anat is received by Unified CM, MTPs are needed for single-stack (IPv4-only or IPv6-only) endpoints.

Unified CM supports ANAT over dual-stack (IPv4 and IPv6) SIP trunks. If ANAT is enabled, it should be configured on both ends of the SIP trunk. (If Require: sdp-anat is sent in the SIP Invite and the receiving SIP trunk does not support ANAT, all calls are rejected.) To enable ANAT, check the Enable ANAT check box on the SIP Profile associated with the SIP trunk. ANAT can be used with both Early Offer and Delayed Offer calls.

ANAT should be enabled only on SIP trunks with an IP Addressing Mode setting of IPv4 and IPv6. Enabling ANAT on a single-stack SIP trunk (IPv4 only or IPv6 only) does not really make sense because only one IP address can be offered.

The following figure shows a simplified version of the SIP Delayed Offer and SIP Answer using ANAT on dual-stack SIP trunks.

SIP Delayed Offer calls typically involve a single call leg from the phone to SIP voice gateway. For outbound SIP Delayed Offer calls, Unified CM sends Supported: sdp-anat in its SIP Invite.

Outbound SIP Invite

The SIP header of the outbound Delayed Offer SIP Invite contains Supported: sdp-anat, indicating to the far-end device that ANAT is supported by this Unified CM trunk and should be supported by the far-end trunk. If ANAT is not supported by the far-end trunk, the call can still proceed, and only a single IP address is returned in the (non-ANAT) SIP Offer. In this case, Unified CM selects a media address (and inserts MTPs if required), as described in Media Selection for Outbound Delayed Offer Calls Over Unified CM SIP Trunks Without ANAT.

Inbound SIP Delayed Offer

The SDP body of the inbound SIP Offer includes the IPv4 address and UDP port number as well as the IPv6 address and UDP port number of the voice gateway. The preferred addressing version of the gateway is also indicated in the SDP body, and a=group:ANAT 2 1 indicates that the second address (the IPv6 address) is preferred by the gateway. For Cisco IOS gateways, the ANAT IP addressing version preference is configured at the voice service voip level using the protocol mode dual-stack preference CLI command.

Outbound SIP Answer

With ANAT supported but not required, the SIP Answer from Unified CM does not have to contain both an IPv4 and an IPv6 address. If the calling device supports IPv4 Only or IPv6 Only, then only a single IP address is sent in the SDP body of the outbound SIP Answer. For the call shown in the previous figure, both the calling phone and trunk support both IPv4 and IPv6, in which case both addresses of the phone are sent in the SIP Answer. The a=group:ANAT 2 indicates Unified CM's choice of the phone's IPv6 address and port number for the voice call. In this example, the phone's IPv6 address and IPv4 address are both included in the SIP Answer; however, only the IPv6 UDP port number is returned, and the IPv4 UDP port number is set to zero.

Note	Unified CM does not have to honor the IP addressing version preference received in the SIP Offer. The media addressing version preference sent by Unified CM in the SDP Answer is set by the cluster-wide IP Addressing Mode Preference for Media.

When only one valid IP address and UDP port number is available to be returned by Unified CM in the SIP Answer, a second invalid address (typically the IPv4 or IPv6 address received in the SIP Offer) is returned in the SIP Answer, with its UDP port number set to 0.

Based on the far-end trunk configuration, inbound SIP Invites to Unified CM from a trunk using Delayed Offer and ANAT could contain either Require: sdp-anat or Supported: sdp-anat in the SIP header. Dual-stack Unified CM SIP trunks respond as follows to inbound calls with each of these settings:

• With Require: sdp-anat, Unified CM always sends a valid IPv4 address and a valid IPv6 address in the SIP Offer.

• With Supported: sdp-anat, Unified CM sends the IP addresses supported by the called device.

For inbound calls to ANAT-enabled dual-stack Unified CM SIP trunks where Supported: sdp-anat is received in the SIP Invite, Unified CM does not have to use MTPs; whereas when Require: sdp-anat is received by Unified CM, MTPs must be used for single-stack (IPv4-only or IPv6-only) endpoints.

Note	Unified CM trunks always send Supported: sdp-anat in Delayed Offer SIP Invites. The default setting for Cisco IOS gateways is to send Require: sdp-anat Early Offer calls.

The following figure shows a simplified version of the SIP Offer and SIP Answer using ANAT, where the calling trunk sends Supported: sdp-anat in its SIP Invite. As illustrated, SIP Delayed Offer calls typically involve a single call leg from the phone to the SIP voice gateway.

Inbound SIP Invite

 protocol mode dual-stack preference

Outbound SIP Offer

With ANAT supported but not required, Unified CM's outbound SIP Offer does not have to contain both an IPv4 address and an IPv6 address. If the called device supports IPv4 Only or IPv6 Only, then only a single IP address is sent in the SDP body of the SIP Offer. For the call shown in the previous figure, both the called phone and the trunk support both IPv4 and IPv6, in which case the SDP body of the SIP Delayed Offer includes the IPv4 address and UDP port number as well as the IPv6 address and UDP port number of the called IP phone. The preferred addressing version of Unified CM is also indicated in the SDP body, and a=group:ANAT 2 1 indicates that the second address (the IPv6 address) is preferred by Unified CM. For outbound Delayed Offer calls, this preference is selected based on the cluster-wide IP Addressing Mode Preference for Media.

Inbound SIP Answer

If Unified CM sends a single address in its SIP Offer, the calling trunk should respond as if it is a Delayed Offer call without ANAT enabled. For the call shown in the previous figure, both the called phone and the trunk support both IPv4 and IPv6. In the received SIP Answer, a=group:ANAT 2 indicates the gateway's choice of its IPv6 address and port number for the voice call. Both the gateway's IPv6 address and its IPv4 address are included in the SIP Answer; however, only the IPv6 UDP port number is returned, and the IPv4 UDP port number is set to zero.

Note	The called device does not have to honor the IP addressing version preference of the calling device.

If only one IP address is available to be sent in the SIP Offer, then only this single IP address is sent, and accordingly only one address (of the same IP addressing version) is expected in the SIP Answer.

The following figure shows a simplified version of the SIP Offer and SIP Answer using ANAT, where the calling device sends Require: sdp-anat in its SIP Invite. As illustrated, SIP Delayed Offer calls typically involve a single call leg from the phone to the SIP voice gateway.

Inbound SIP Invite

protocol mode dual-stack preference

Outbound SIP Offer

With ANAT required, Unified CM's SIP Offer must contain both an IPv4 and an IPv6 address. If the called device supports an addressing mode of IPv4 Only or IPv6 Only, then Unified CM dynamically inserts an MTP and sends its IPv4 address and its IPv6 address in the SDP body of the SIP Offer. For the call shown in the previous figure both the called phone and trunk support both IPv4 and IPv6, in which case the SDP body of the SIP Delayed Offer includes the IPv4 address and UDP port number as well as the IPv6 address and UDP port number of the called IP phone. Unified CM's preferred addressing version is also indicated in the SDP body, and a=group:ANAT 2 1 indicates that the second address (the IPv6 address) is preferred by Unified CM. For Outbound Delayed Offer calls, the cluster-wide IP Addressing Mode Preference for Media determines this preference.

Inbound SIP Answer

When ANAT is required by the calling trunk, it sends an ANAT-based response in its SIP answer. In the received SIP Answer, a=group:ANAT 2 indicates the gateway's choice of its IPv6 address and port number for the voice call. Both the gateway's IPv6 address and its IPv4 address are included in the SIP Answer; however, only the IPv6 UDP port number is returned, and the IPv4 UDP port number is set to zero.

Note	The called device does not have to honor the IP addressing version preference of the calling device.

For inbound Delayed Offer calls with Require: sdp-anat in the received Invite, the IP Addressing Mode of the trunk is set to IPv4 and IPv6. If a mismatch exists between the phone's and the trunk's IP Addressing Modes, Unified CM dynamically inserts an MTP into the call path and sends the MTP's IPv4 and IPv6 addresses in the SDP body of SIP Offer from Unified CM.

When an MTP Is Required, Will the MTP of the Phone or the Trunk Be Used?

The cluster-wide IP Addressing Mode Preference for Media determines whether the MTP of the phone or of the trunk is used to convert the voice media stream between IPv4 and IPv6. This preference is used to select an MTP so that the longest Real-Time Transport Protocol (RTP) call leg in the cluster matches the cluster-wide preference.

Dynamically inserted MTPs do support the pass-through codec, allowing video calls and encrypted calls to be established.