Often times this is one of the most misunderstood aspect of a CUBE configuration. This stems from our knowledge of how bind commands work with h323 gateway. Typically when deploying h323 gateway with CUCM we need to configured h323 bind commands on the interface that we will use for h323 traffic.
Once this is defined, this interface IP address is seen on CUCM for the gateway. Hence all h225 signaling and h245 media negotiations will be directed towards this interface.This is good for H323 and doesn't create any issues for us. However this doesn't work the same way on a SIP to SIP CUBE solution. Question is why?
On a traditional H323 to TDM PSTN connection, we only have one leg of VoIP traffic, so all h323 signaling is directed towards that leg. The other leg is a TDM either digital PRI, or Analogue FXO ports. In this setup where we apply signaling and media traffic doesn't need much consideration as it can only be done on one leg of the call.
On a H323 to SIP connection, even though we have two VoIP legs, the leg that usually connects to the PSTN is the SIP leg. In this case we usually apply sip signalling and media to the outside leg that connects to the ITSP..This works okay too without any issues….
However since most people are migrating to a full homogenous SIP solution this becomes very important to know where to apply your sip bind commands. This is because in most deployment, customers typically have two different interfaces. One interface connects to the LAN, and the other Interface connects to the ITSP. In this scenario applying the bind command wrongly on a SIP to-SIP CUBE, has the potential to remove the listener from the external interface and in some cases create a one way audio problems
Lets look at an example to understand this last point
Here is the scenario:
CUCM-----SIP---CUBE---SIP---ITSP
NB: CUCM IP is 192.168.12.190, ITSP 10.205.20.50
CUBE Interface config
interface GigabitEthernet0/0
Decription LAN interface
ip address 192.168.10.5 255.255.255.0
!
interface GigabitEthernet0/2
Decription Connection to ITSP
ip address 172.29.25.246 255.255.255.252
duplex auto
speed auto
voice service voip
sip
bind all source-interface gig0/0
In this example, all sip signaling and media traffic will be sent to gig0/0, the LAN interface on the CUBE. What is wrong with that you may ask…Well a lot!
Let’s look at a sample trace for an outbound call originating from CUCM towards the CUBE..
Received:
INVITE sip:0447014824@192.168.10.5:5060 SIP/2.0
Via: SIP/2.0/TCP 192.168.12.190:5060;branch=z9hG4bK78adb3ecc5092
From: "5399" <sip:5399@192.168.12.190>;tag=1729466~70e9433b-1d79-44ae-9a16-09a52be377c5-30642592
To: <sip:0447014824@192.168.10.5>
Sent:
INVITE sip:0447014824@10.205.20.50:5060 SIP/2.0
Via: SIP/2.0/UDP 192.168.10.5:5060;branch=z9hG4bK2C9207D
Remote-Party-ID: "5399" <sip:8065399@192.168.10.5>;party=calling;screen=yes;privacy=off
From: "5399" <sip:8065399@192.168.10.5>;tag=242A3EF4-1315
In this scenario what we see is this…All sip traffic originates and terminates on the LAN interface. Hence when CUBE sends an INVITE to the ITSP it uses the Local LAN interface also. This presents a problem because the ITSP cant talk to CUBE on the Local LAN Interface. In one case I looked at the INVITE didn't even make it to the ITSP because we never got any response from them. This is most likely because the LOCAL interface doesn't have any route to the ITSP network.
Some folks rather than binding on the local interface will bind on the interface facing the ITSP.
The major problem with this is:
CUCM will not accept any sip traffic coming from an IP address that is not configured on its sip trunk.
Lets look at a sample trace..CUCM SIP trunk configured with ip address 192.168.10.5
Received:
INVITE sip:0344556677@172.25.29.246:5060 SIP/2.0
Via: SIP/2.0/TCP 10.10:5060;branch=z9hG4bK78adb3ecc5092
From: <sip: 0447014824@10.205.20.50:5060>;tag=1729466~70e9433b-1d79-44ae-9a16-09a52be377c5-30642592
To: <sip: 0344556677@172.25.29.246>
Sent:
INVITE sip:5399@192.168.12.190:5060 SIP/2.0
Via: SIP/2.0/UDP 172.25.29.246:5060;branch=z9hG4bK2C9207D
From: <sip: : 0447014824@172.25.29.246>;tag=242A3EF4-1315
In this case, we can see that CUBE has sent out on INVITE to Cisco call Manager using the external interface.
The result of this is call failure. This is because CUCM is going to reject the call with “501 service unavailable” due to the fact that the IP address from which sip traffic originates and to which CUBE has indicated it want to receive a response back (through the via header) is not the ip configured on the SIP trunk on CUCM.
How do we then address SIP binds on deployments like this:
There are two ways to address this. The first is
- No SIP BIND COMMANDS AT ALL
Don't use sip binds command at all. Yes you heard me, this is because you shouldn't need a SIP bind on SIP to SIP because the IOS on CUBE will source from the interfaces closest to the destination of the SIP packet.
In this scenario without a sip bind, CUBE will receive traffic on the Local interface and will send out INVITE to the ITSP on the ITSP interface and all will be well.
However there are scenarios where this won’t work either.I recently had one of those scenarios.
CUBE was selecting the MPLS interface on the router as the destination closest to CUCM, but because this interface was not the one configured on the sip trunk, CUCM rejected the call.
We then decided to configure this interface on the sip trunk..Everything worked except there was no audio. It turned out that this subnet was not reachable from the IP Phones Subnet. So question is what do we do..If we configure bind on one interface we had issues. When we didn’t, CUBE selected the wrong interface to originate and terminate sip traffic..This brings us to our second method
- USE BIND COMMANDS on DIAL-PEER
Example..........
dial-peer voice 15 voip
description outbound dial-peer to CUCM
voice-class sip bind control source-interface GigabitEthernet0/0
voice-class sip bind media source-interface GigabitEthernet0/0
dial-peer voice 1 voip
description inbound from CUCM
voice-class sip bind control source-interface GigabitEthernet0/0
voice-class sip bind media source-interface GigabitEthernet0/0
dial-peer voice 2 voip
description inbound from ITSP
voice-class sip bind control source-interface GigabitEthernet0/2
voice-class sip bind media source-interface GigabitEthernet0/2
dial-peer voice 20 voip
description ***outbound dialpeer to ITSP***
voice-class sip bind control source-interface GigabitEthernet0/2
voice-class sip bind media source-interface GigabitEthernet0/2
In the config above, when traffic originates or terminate from or to CUCM-CUBE, the local interface will be used for both SIP signalling and Media traffic. When calls originate or terminate from ITSP to CUBE the external interface to ITSP will be used.
Sample trace
Received:
INVITE sip:0447014824@192.168.10.5:5060 SIP/2.0
Via: SIP/2.0/TCP 192.168.12.190:5060;branch=z9hG4bK78adb3ecc5092
From: "5399" <sip:5399@192.168.12.190>;tag=1729466~70e9433b-1d79-44ae-9a16-09a52be377c5-30642592
To: <sip:0447014824@192.168.10.5>
Sent:
INVITE sip:0447014824@10.205.20.50:5060 SIP/2.0
Via: SIP/2.0/UDP 172.25.29.246:5060;branch=z9hG4bK2C9207D
Remote-Party-ID: "5399" <sip:8065399@172.25.29.246>;party=calling;screen=yes;privacy=off
From: "5399" <sip:8065399@172.25.29.246>;tag=242A3EF4-1315
What we see here is how things should be..
CUCM sends invite to CUBE on its local interface and CUBE sends INVITE to ITSP on the external interface.
