SBC (CUBE) and Inbound Firewall Rules

[mmelbourne]

I have a situation where a SIP Trunk provider is stating they don't need any inbound ports open towards the SBC from their platform; all the communication is done via outbound connections from the SBC.

I assume the SBC will make a SIP connection to the Provider and hold this open, but my simple question here, is how does this work in establishing two RTP streams per call, when the SDP messages inform the other side which port it is expecting media to arrive on? The implication is that the SBC must originate two UDP connections per call?

[Maren Mahoney]

It's possible your provider is expecting that your SBC will establish a tunnel not unlike the traversal tunnel between Expressways C and E. Just a guess though. Here is a diagram and explanation of that process from a Cisco Live talk on Expressways. Once the tunnel is established, traffic inbound toward the Expressway-C from the Expressway-E is treated as "established" traffic that is in response to the outbound keepalives.

You'd have to check with your provider to see if this is what is happening.

From Enabling External Collaboration with Expressway (Cisco Live 2020 - BRKUCC-2801) 

From "Enabling External Collaboration with Expressway" Cisco Live 2020

Maren