Cisco Unified Communications Manager starts a lightweight HTTP server as part of the Cisco TFTP service. When a phone requests a firmware upgrade, it switches from the normal TFTP transfer to HTTP for faster throughput. The CUCM-embedded HTTP server listens, by default, on TCP port 6970. Because HTTP is a TCP-based protocol, the transport is always TCP, never UDP. Therefore, the default image-upgrade service for Cisco IP phones uses TCP port 6970.

A. 8080/TCP – Alternative HTTP port; CUCM TFTP does not listen here for firmware downloads.
B. 6970/UDP – HTTP is not UDP-based; CUCM does not open UDP 6970.
C. 443/TCP – HTTPS port; CUCM TFTP HTTP service is unsecured on 6970 by default.

1. Cisco Unified Communications Manager Features and Services Guide, Release 12.5(1)SU, “Cisco TFTP Service – Ports Used”, Table 2: “HTTP (TCP) 6970 – image/log file download for IP phones.”

2. Cisco Collaboration SRND 12.x, Chapter “Endpoint Firmware Distribution”, p. 158: “Phones retrieve firmware over HTTP on TCP port 6970 from the CUCM TFTP service.”

3. Cisco IP Phone 88xx Series Administration Guide, Release 12.0, Appendix B “Network Ports Used”, entry: “TCP 6970 – HTTP firmware and log file transfer (TFTP service).”