understanding SRST, Call Manager router config

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Nov 12th, 2007
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We have 5 remote branches that are configured each with their own PRI and a PIP to the main site via MPLS.

Each branch has it's own 800 and local number for that area.

I am told that each branch is configured for SRST.

I do not understand exactly what is happening and what is supposed to happen.

When I do a debug on the remote site router, I can see the inbound call come in to the router, but I am not sure what happens after that.

I see the call manager (located at the main site)is configured in the router also, but what happens when the call manager cannot be reached for example when the MPLS link is down?

I do not see anything in the config that would have the router directing inbound calls to handsets in the remote branches (when we loose MPLS remote site phone do not work).

What is the SRST doing?

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Adam Frankel Mon, 11/12/2007 - 16:55
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The SRST would allow phones to failover using their current configuration, so that all local phones at each site could call each other while retaining their current DN.

John Burnham Sat, 11/17/2007 - 17:50
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On the remote router SRST is set-up with the command "call-manager-fallback". Although I do not see the command "max-ephones" which will define the maximum number of IP phones you have.

Here is a link that will better explain how to set-up SRST on the remote depending on your router and IOS. Make sure that SRST is enabled from the CCM since you are using MGCP.


wilson_1234_2 Mon, 11/19/2007 - 17:39
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Thanks for the reply.

Each remote branch has their own PRI.

Would the theory be that the branches would use Cisco Voip for local calls and their PRI for long distance?

Also, if during a circuit failure to the main site, with SRST, do the remote branch IP phones use the router as their call manager and still have the capability to use the PRI for outbound calls?

On the FXO FXS ports, my understanding is that the FXO port would have the analog phones and modems plugged into them.

Is the FXS port used to plug the PRI cable from the telco DMARC?

John Burnham Tue, 11/20/2007 - 11:02
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How your calls are processed depends on how you have your configuration done within CallManager (CCM). Only when you go into SRST mode will the router determine where the call will go. (Note exception to this rule is when you set-up the VGW as an H.323/SIP gateway.)

FXS ports are used for your phones/modems

FXO is used for ISDN/Telco DMARC


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