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T1 trunk failover when network is lost

Salim Guerid
Level 1
Level 1

Dear all,

I have a configuration with 2 routers each having a T1 (Inbound R1, Outbound R2).

I would like to setup the failover if one router faills. The only test that didn't succeed is when I try to simulate a HW faillure on R1.

It seems that the calla still get routed through R1 by the provider.

I have tried the following on the interface but without success:

interface Serial0/0/0:23

isdn network-failure-cause 38

Any help would be appreciated

4 Replies 4

Andrew Latamore
Level 1
Level 1

Hi Salim,

From the sounds of it you will need to engage your service provider to enable the redundancy you are looking for.  As I understand it you are looking to have inbound calls from the PSTN to your R1 router failover to come in your R2 router if your T1s on the R1 router are down.  Basically in the event of such a failure, it would be up to the PSTN provider to perform the rerouting, as your R1 gateway would have no way to respond to the PSTN provider.  If I am incorrect in my understanding of the issue please let me know.  Otherwise this would be a setup you would need to be done on the provider side.

Hi Andrew,

Thanks for your answer/interest.

In fact the failover to the second T1 is performed well when the T1 cable is unplugged from R1 or R2.

When I removed the cable from the Gig interface calls are still sent to the same router.

Strangely enough, when I shut the Serial 0/0/0:23 calls are still sent to the same router.

It seems that only the layer 1 disconnection triggers the failover to the second T1.

I know there is a specific config to have both cards monitoring each others.

- Salim

Andrew Latamore
Level 1
Level 1

Ah, I understand the issue now.  So if the L1 for the T1 stays active but the upstream connectivity were to go down, the provider would have no knowledge of this and would keep sending the calls to R1.  Those calls will fail because of the upstream network connection issue, so you would want to have a way to have those calls failover to the R2 gateway.

There is a way to do this using the "busyout monitor" set of commands.  These are added to the voice-port for the T1.  Here is an example of the config you can use:

busyout monitor loopback 0

busyout monitor action graceful

The above commands would monitor the loopback 0 interface(though you can also monitor ethernet and serial interfaces).  You can monitor more than one interface for a voiceport as well.  Also, the action here if the monitored interface goes down is to busout the voice-port gracefully(in other words, keep active calls going until the caller disconnects, then busyout the port).

If you monitor the serial interface and the upstream ethernet port with the above command, a failure there should result in the B-channels being shutdown.  This should cause the provider to invoke their failover and send calls to the R2 gateway.  When the monitored interface returns to an active state the gateway should bring the voice-ports back up.

Having said this there are some caveats.  One is that sometimes the provider side will place the T1 into a busyout state if we take the CPE side down.  So when we bring the voice-port back up it fails to come up because the provider side has shut it down.  I have seen this happen with outages as short as a minute.

Another limitation of this is that the monitoring is happening on the state of the monitored port, and not TCP/IP connectivity.  So if the monitored ethernet port is up, but there is a network issue at the next hop, the voice-ports will not be taken out of service and calls will still come to R1.

Here is a link to the IOS Command Reference page on this command, which has further information:
http://www.cisco.com/en/US/docs/ios/voice/command/reference/vr_b1.html#wp1542721

I hope this helps to get the redundancy features you need in your environment!

Hi Andrew,

Sounds good.

I will give it a try.

Thanks for the time you spent writing back.

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