One of the most frequently asked question, when connecting Cisco Unified Communications Manager (CUCM) to Public Switch Telephony Network or to any other VoIP system with a SIP Trunk, is how to configure SIP Early Offer media negotiations for this SIP trunk. By default, CUCM SIP trunks send the INVITE message without an initial offer (Delayed Offer). CUCM has the following three configurable options to enable a SIP trunk to send the offer in the INVITE message (Early Offer). Let's see what all these options are.
So the SIP Early Offer for the CUCM SIP Trunk can be enabled with:
1. Using the MTP Required option on the SIP trunk settings.
Enabling this option on the SIP trunk assigns an MTP for every outbound call. The SIP Trunk will always include the MTP's IP Address, UDP port number and codec in the SDP body of the initial SIP INVITE instead of those of the endpoint. Early offer using the Media Termination Point Required option can be useful in cases where voice media for inbound and outbound calls must be anchored to a single IP address (that of the MTP).
But this way of configuration has some disadvantages. This option does not support codec pass-through mode, which imposes a single codec (G.711 or G.729) limitation over the SIP trunk, thus limiting media to voice calls only. With this option enabled, calls over the trunk use MTPs that are assigned to the trunk rather than using calling device MTPs, which forces the media to follow the same signaling path.
Enabling the Media Termination Point Required option on the SIP trunk increases MTP usage because an MTP is assigned for every inbound and outbound call rather than on an as-needed basis. Also with his configuration it is recommended to use IOS MTPs always, because sometimes the CUCM based MTPs freeze at a high load unfortunately.
Due to all these problems, Enabling early offer using the Media Termination Point Required option has been superseded by the Early Offer Support for Voice and Video Calls Mandatory (Insert MTP If Needed) and Early Offer Support for Voice and Video Calls Best Effort (No MTP Inserted) options in SIP Profile settings since CUCM ver. 8.5.
2. SIP Profile Early Offer dropdown list option - Mandatory (insert MTP if needed)
Enabling this CUCM configuration option on the SIP profile that is associated with the SIP trunk inserts an MTP only if the calling device cannot provide CUCM with the media characteristics that are required to create the outbound Early Offer (for example, where an inbound call to CUCM is received on a delayed-offer SIP trunk or a slow-start H.323 trunk and on calls from older SCCP-based phones such as Cisco IP Phones 7940 or 7960 that are registered to CUCM).
For calls from trunks and devices, that can provide their media parameters at the beginning of the call, this information can be sent in the SDP body of the initial SIP INVITE on the outbound Early Offer Trunk. No MTP is used for the Early Offer in this case.
But when Unified CM receives an inbound call on an H.323 slow-start or SIP delayed-offer trunk, the media capabilities of the calling device are not available when the call is initiated. If trunks and devices that cannot send their media information, when they initialize the call, the CUCM will use device's MTP resources first (according to the device's MRGL) or the outbound trunk's MTPs (trunk's MRGL) to create a SIP Early Offer.
If no MTP resources are available, the call will proceed as a delayed-offer call.
3. SIP Profile Early Offer dropdown list option - Best Effort (No MTP Inserted)
If this CUCM SIP profile configuration option is enabled, the SIP trunk will never use MTPs to create an Early Offer, but will send either an Early Offer or a Delayed Offer, depending on the capabilities of the calling device.
Sometimes Best Effort (No MTP Inserted) configuration option is called Best Effort Early Offer. It can be enabled on the SIP profile that is associated with the SIP trunk, and it is the recommended configuration for all CUCM and CUCM Session Management Edition trunks.
So if the calling device can provide its media parameters at the beginning of the call, the trunk uses Early Offer. It not - then the trunk operates with Delayed Offer.
Using Best Effort Early Offer for all SIP trunks in your enterprise simplifies Cisco Collaboration Systems network design and deployments, and it eliminates the need to use MTPs to generate an offer.
SUMMARY
SIP Trunk Delayed Offer - this is the default option
SIP Trunk Early Offer using MTP Required - MTP is inserted for every call. Do not use this option!
SIP Trunk Early Offer with SIP Profile Mandatory (insert MTP if needed) - MTPs are inserted for calls from older devices. Use this option for Early Offer if you have CUCM version 8.5/8.6/9.x/10.0
SIP Trunk Early Offer with SIP Profile Best Effort (No MTP Inserted) - this is the best and recommended option if you have CUCM 10.5 or newer.
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