SIP Trunking questions

[prasanna.k1]

Please clarify the following questions/assumptions:
1. For PRI trunk, the VoIP gateway will be having physical ports which will be the end points which in turn will be having the channels.  On the other hand, SIP trunk is the logical trunk that is configured in the Call Manager. When configuring a SIP trunk, multiple gateway IP can be specified along with port number (by default 5060). With this we understood that a SIP trunk is a logical trunk that is configured in Call Manager and when there is a SIP call on the call manager’s cluster, the call will be routed to one of the configured gateway’s SIP port (5060) and there it will be routed to the service provider. 

2. If the above statement is correct, can we configure the same gateway IP address to multiple SIP trunks? 

3. Can a VoIP gateway have more than one SIP port?

4. My assumption is, when we install CUBE software update on any of the supported Cisco Integrated Services Routers, the device will act as a Session Border Controller. and SBCs are no special hardware other than VoIP Gateway. Please clarify.

5. How we can know if a SIP trunk is up/down?

 

[Vivek Batra · Accepted solution]

1. For PRI trunk, the VoIP gateway will be having physical ports which will be the end points which in turn will be having the channels.  On the other hand, SIP trunk is the logical trunk that is configured in the Call Manager. When configuring a SIP trunk, multiple gateway IP can be specified along with port number (by default 5060). With this we understood that a SIP trunk is a logical trunk that is configured in Call Manager and when there is a SIP call on the call manager’s cluster, the call will be routed to one of the configured gateway’s SIP port (5060) and there it will be routed to the service provider. 

You are right. SIP trunk doesn't use physical port unlike ISDN and is a peer-to-peer application layer protocol. When you configure SIP trunk in a call manager, you have a choice to route the call to intended destination on the basis of IP addresses configured.

2. If the above statement is correct, can we configure the same gateway IP address to multiple SIP trunks? 

Practically you never need to configure the same gateway IP on multiple trunks.

My assumption is, when we install CUBE software update on any of the supported Cisco Integrated Services Routers, the device will act as a Session Border Controller. and SBCs are no special hardware other than VoIP Gateway. Please clarify.

Yes, you can use CUBE into your existing ISR. CUBE in short handles IP-IP call legs which can be served by existing ISR with desired configuration.

How we can know if a SIP trunk is up/down?

CUCM supports 'OPTION' method to check periodically whether gateway is up or down.

[prasanna.k1]

Hi Vivek,

Thank you for your response.

The IP addresses we give on the CUCM SIP Trunk configuration is the gateway IP address right (VoIP Gateway's which is probably in the same network as CUCM)?

Is it not the IP address given by the service provider (Some SIP proxy server IP on the SP side??)?

The  SIP OPTIONS PING command you stated will check the trunk status only and not the gateway's as gateway we will directly check by PING command. Could you help by providing some sample response of SIP OPTIONS PING? (The link http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/8x/uc8x/trunks.html#wp1123287 don't have command level descriptions).

Other than configuring SIP trunks in the gateway, do we need to do anything on the gateway side w.r.t SIP trunks?

Thanks,
Prasanna

[Vivek Batra]

The IP addresses we give on the CUCM SIP Trunk configuration is the gateway IP address right (VoIP Gateway's which is probably in the same network as CUCM)? 

Yes, IP address which you configured on SIP trunk is most of the times the IP address of gateway but it's not compulsory gateway to be in the same network. In centralized call processing, you cluster may be in San Jose but call manager may be controlling gateway located in Mumbai using SIP trunk.

Is it not the IP address given by the service provider (Some SIP proxy server IP on the SP side??)?

Technically you can connect call manager directly to internet telephony service provider but it is not recommended and never used. That's is the reason you have CUBE in between call manager and internet telephony service provider. CUBE gives enhanced security features and SIP packet customization required for inter-operating with various service provider.

The  SIP OPTIONS PING command you stated will check the trunk status only and not the gateway's as gateway we will directly check by PING command. Could you help by providing some sample response of SIP OPTIONS PING?

In my opinion, ping is not a correct word to be used here (as it will get contradict with ICMP). CUCM uses pure SIP defined method viz 'OPTION' message to check whether gateway is reachable or not. Following is the example of SIP OPTION message taken from RFC 3261, alternatively you would like to have look at RFC 3261 to check more about this.

Also note that when you are checking SIP trunk status using SIP OPTION, you are eventually checking gateway status (reachable or not).

OPTIONS sip:carol@chicago.com SIP/2.0
      Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bKhjhs8ass877
      Max-Forwards: 70
      To: <sip:carol@chicago.com>
      From: Alice <sip:alice@atlanta.com>;tag=1928301774
      Call-ID: a84b4c76e66710
      CSeq: 63104 OPTIONS
      Contact: <sip:alice@pc33.atlanta.com>
      Accept: application/sdp
      Content-Length: 0

Other than configuring SIP trunks in the gateway, do we need to do anything on the gateway side w.r.t SIP trunks?

There is a bunch of configuration to be done on gateway side. All depends on your installation scenario and services you are using.

[prasanna.k1]

Thank you Vivek for your help in clarifying the questions.

Regards,
Prasanna

[Vivek Batra]

You are welcome!