T1 · T1-R02
Offer and answer
Learn it in 5 minutes
- SDP is the offer or the answer, not a second signaling protocol you ignore.
- Early offer puts the offer in the INVITE.
- The answer lists what was accepted, not a new unrelated codec list.
- A re-INVITE can change hold, codec, or fax.
- RFC 3264 is the rule for which lines may be answered.
Visual
What is going on
Early offer is common on CUCM SIP trunks. Delayed offer puts the offer in the 200 and the answer in the ACK. Mixing them on a CUBE without a plan produces a call that signals and then has no media, or a codec neither side wanted. Read m= lines and the direction attributes before you blame the WAN.
Confirm before relying. Confirm the early-offer checkbox name in the CUCM version you run.
Worked example
An INVITE carries G.711 and G.729. The 200 answers with G.711 only. The call is G.711. If the 200 has no audio m-line you accepted, you do not have a voice call yet.
Quiz
1. The answer may add a codec the offer did not list.
False. The answer narrows the offer.
Ecosystem
Sister pieces sit on the ecosystem page: the glossary, the mind map, the integration map, and the sibling labs.
- Early offer — SDP in the INVITE.
- Delayed offer — SDP offered in the reliable response or the 200.
Config. Check the SIP trunk's early offer setting before you compare two traces.
Troubleshooting. One-way audio sometimes starts as an unanswered direction attribute, sendonly against sendonly.
Bridge. Webex Calling local gateway guides expect early offer and SRTP. Confirm the current guide.
Mind map, sister technologies, and future hooks live on the ecosystem page.
Sources
Product screens move. Confirm the guide for your release before you change a live system.
Sources and go deeper
Opened 2026-10. A row marked confirm has no link, because that page was not verified.
- RFC. RFC 3264, offer/answer last checked 2026-10
- RFC. RFC 3261, SIP last checked 2026-10
- RFC. RFC 4566, SDP last checked 2026-10
- RFC. RFC 3550, RTP last checked 2026-10
- Cisco. Cisco Unified Communications Manager product support last checked 2026-10
- Cisco. System Configuration Guide for CUCM, Release 15 last checked 2026-10
- Protocol and ports. CUCM 15 TCP and UDP port usage last checked 2026-10
- Protocol and ports. IANA SIP parameters last checked 2026-10
- Protocol and ports. IANA RTP parameters last checked 2026-10
- Community. Cisco Community, Collaboration knowledge base last checked 2026-10
- Wikipedia. Session Initiation Protocol (Wikipedia) last checked 2026-10
- Wikipedia. Real-time Transport Protocol (Wikipedia) last checked 2026-10
- Cisco. Collaboration 12 SRND book. confirm. The historical Collaboration 12 SRND path opened as a product listing, not the design book. Use the Preferred Architecture index, which was opened. last checked 2026-10
- Wikipedia. Session Description Protocol (Wikipedia) last checked 2026-10