SIP SRST: Is all RTP between SRST registered phones supposed to merge on the SRST?

[Nadav]

Hi everyone,

My setup is composed of an ISR G2 SIP SRST, and a few SIP phones with up to date firmware.

I'm checking the SRST functionality when the phones can't reach the CUCM cluster.

It seems as though when phones registered to the SRST are in a call with one another, all RTP traffic travels through the SRST much like an audio media gateway. When they aren't registered via SRST, but rather the same CUCM cluster, they speak to each other via RTP directly.

Is this correct behavior from the SIP SRST?

Thanks!

[R0g22]

That's right. SIP to SIP line-side on IOS does not support flow around. Only flow-through is supported. SIP to SIP trunk call flows ONLY can support flow-around. SCCP - SCCP calls on SRST are flow-around.

[Nadav]

Hi,

1) According to:

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cusrst/command/reference/srstcr/srsa_n_z.html

It is possible to configure the media mode to flow around (under "voice service voip"). Does this only apply to calls over the SIP trunk between CUCM clusters and SRSTs? 

2) Are you aware of any Cisco documentation that states that SRST SIP-to-SIP calls (not over the SIP trunk) only support flow through? I wasn't able to find such a statement.

3) As far as I can tell, no video RTP is supported with endpoints registered under SRST. Even jabber supports only phone-mode under SRST.

Is this accurate? If so, then I just need to make sure that there is no aggregate audio RTP bottleneck to and from my SRST for voice traffic. If there are endpoints which support video under SRST, I'd need to make sure there is no aggregate video RTP bottleneck.

Within:

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/voice/cube/configuration/cube-book/media-path.html

It states that "Video codecs are not supported for Media Flow-Through" yet it could be referring to only CME rather than SRST as well. The same document also states that Flow-Around is supported by video endpoints.

Thanks for the assistance.

[R0g22 · Accepted solution]

1. Flow around is for CUBE only. You are not supposed to configure that for either CME
or SRST.

2. I am not sure if there is any docs or not. I don’t think there is a doc that states that flow around is supported and/or should be used in SRST.

3. Video should work in ESRST only starting version 10.0 but AFAIK it is for select endpoint series including Jabber. Check the SRST admin guide.

http://www.cisco.com/c/en/us/products/collateral/unified-communications/unified-survivable-remote-site-telephony/prod_qas0900aecd8028d113.html

4. You are confusing b/w SRST
and CUBE. The config guide is for CUBE and not SRST. Not everything mentioned in the guide applies to SRST deployments.

[Nadav]

Alright, thank you :)