SIP keepalive in CUCM

[W S H FERNANDO]

Hi 

I have CUCM with SIP phones, also they have SRST as 4th TFTP. In CUCM default keepalive time is 120 sec. so Phone will send KPA with every 120 sec, if it failed is it going to send KPA message to 2nd TFTP ? if that also failed then to 3rd immediately ? then to SRST right ?

Please explain me the process, this will be a great help 

Regds

$

[Sreekanth Narayanan]

Hi Sasith,

I don't think there is a 4th TFTP server that can be configured for the phone. I think you mean, the 4th call agent, after Pri, Sec and Ter call agents.

Here's a doc that talks about what you want to know.

https://supportforums.cisco.com/document/74666/cucm-ip-phone-sccp-keepalive-and-failover-architecture

Thanks

Sreekanth

[W S H FERNANDO]

Hi Sreekanth 

yes 4th Agent as SRST gateway. 

I found this doc, but I'm using SIP Phones, not SCCP ...

Regds

$

[Sreekanth Narayanan · Accepted solution]

Hi Sasith,

Unless you change the default values, the phone should send a keepalive every 120 seconds to its primary call agent.

SIP Station KeepAlive Interval

Once the phone no longer receives the 200 OK for its register from the primary, it starts behaving like the SCCP phone does. I am not 100% sure about this though. Will have to check and get back to you. The best thing is to try this in the lab while taking packet captures.

[Sreekanth Narayanan]

Hi Sasith,

I checked internally and found out that SIP phones do not have the same well-defined sequence for failover as SCCP, so my earlier statement was wrong.

They keep sending a REGISTER message with expires=0 to their backup CUCM servers.

The moment the sip phones realize that the TCP connection with the primary breaks, they immediately register with the secondary. If the phone realizes that it cannot establish the connection with the secondary, it tries the tertiary. If the same happens with the tertiary, it tries the SRST router.

All this time, the phone keeps trying a TCP SYN with its primary CUCM server.

Once the primary CUCM responds to the TCP SYN, the phone will send a Register with expires=0 (keepalive). If the CUCM responds with 200 ok, then the phone sends a REFER with token registration to register again with the primary.

"X-cisco-remotecc:token-registration"