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SRST DHCP

How would I have a backup DHCP server if I lost connection to callmanager and went into SRST. I have set up DHCP on the srst router but the problem I'm having is that I'm getting duplicates because I have a dhcp server that I want the phones to hit when they are not in SRST.

Is there anyway to make the SRST router only give out DHCP when in SRST mode? Any help would be appreciated. Thanks

9 REPLIES
Bronze

Re: SRST DHCP

Hi Jason,

The whole idea behind SRST is that either you get DHCP locally on the SRST router or via CallManager when using the "ip helper-address" command.

In case your WAN is down and you want to get DHCP, you would have to use the local SRST router to service DHCP to the clients. However, even if the phones get their IP ADDRESS, they won't be able to reach their TFTP server (the callmanager) and get their config files, since WAN connectivity is down.

Hope this helps

Tony

Re: SRST DHCP

Tony,

Thanks for the responce. The problem that I'm having is if it is posible to have both a router at a site run DHCP and a Windows server that is at a main location run DHCP. Is the router smart enough to know to only hand out IP addresses if the main DHCP server is unavailable. Thanks

Bronze

Re: SRST DHCP

Hi Jason,

No it is not. Unfortunately, even when using the "ip helper-address" command, it does not disable the local router from serving DHCP requests locally.

Bronze

Re: SRST DHCP

Correct me if I am wrong.

I think the IP Phone will use the last known IP address if they couldnt get to DHCP at main site.

So basically, you dont need DHCP pool at the router. The problem is that will the ip address lease timeout??

Ken

Re: SRST DHCP

SRST is meant as a fallback when there is a outage, that hopefully doesn't last a long time.

Thus, the IP Phones are already up and running and registered to call manager. If there is a outage and no CCM available they will register to router in SRST mode, while they still have the same IP address they had before then re-register to CCM when it becomes available.

Hopefully your DHCP lease time is long enough so they don't expire while the phones are in fallback.

I personally find it easier to use DHCP pools (server) on the router at remote sites. This way no forwarding of broadcast and such. It is in the router config which is hopefully backed up someplace.

If you use DHCP Server on Call Manager, that info is not included in a BARS backup so if your server were to go belly up you would lose all your DHCP server scopes as well and maybe be hurting 2 ways (phones down, and possibly DHCP and PCs/etc may not work if the lease happens to expire).

HTH, Just my 2 cents.

New Member

Re: SRST DHCP

We just had a remote location have a full network outage.The telco had a major fiber cut which caused long distance, local, and cell phone outages for a large portion of the state. We had dial tone on our analog lines going in to a 2811 router. We never went into SRST. The analog line with dial tone did have issues with one direction connects also. Our other 7 locations had no issue. We have tested SRST in the past with success. But my question is how is SRST really suppose to work? Does SRST kick in if something happens to the main location and the remotes loose connection with the CallManager server, then they go into SRST. This is how I understand it...but I am getting confused because of questions being raised here about how it is suppose to preform. Any help would greatly be appreciated. Thanks!!

Re: SRST DHCP

Debby,

SRST should kick in any time your phones lose connection to callmanager. Not sure if your link was flapping rather than having a total outage.

Bronze

Re: SRST DHCP

SRST is initiated after the phone loses 3 keepalives with the CallManager (either because of WAN outage or because of CallManager failure). In this case, the phone will next try and contact the secondary CallManager (if configured). If the phones can't contact the secondary CallManager, then it will try and register to the SRST reference that is configured underneath the dial-peer.

Here is a link that describes the process:

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_guide_chapter09186a00806674d2.html#wp1328133

Also, here is link to enable SRST on the gateway.

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_guide_chapter09186a00806674c3.html#wp1351962

Bronze

Re: SRST DHCP

If you would like, I can review your SRST config. Just make sure to remove all passwords and blank out the ip address fields if you post the config. Thanks.

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