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New Member

Limiting IP Phone SRST registration

My customer has a site with SRST, but more than 24 phones. I am looking for the best way to limit registration to selected phones. The IOS recommendation seems to be mutilple voiice vlans, and strict match on the source address fallback command. One problem I see is that they would need to configure certain swiitch ports for the SRST-capable vlan, and vice versa. This coule cause problems as phones ar moved, etc.

I was wondering if anyone had tried an access--list to permit only certain IP addresses to access the source address on port 2000? I realize this has potential problems with possible IP address changes, but that might be the lesser evil. Any other suggestions are welcome. Thanks

Cisco Employee

Re: Limiting IP Phone SRST registration

As long as you are running 3.1(3a) or later you have the option on the phone configuration screen to tell the phone whether to do SRST or not. This is the best way.

Thank you,


New Member

Re: Limiting IP Phone SRST registration

Thanks, but for a while yet, the customer will be at 3.1.2c. Does anyone have experience with the various options to limit SRST?

Cisco Employee

Re: Limiting IP Phone SRST registration

Those are the only options: use strict-match and multiple vlans; point the other phones to a different def gateway altogether (if possible); if really old CM version, then only use the SRS phone load that you want to fail over; or access-list, for example:

access-list 101 permit tcp host host eq 2000

access-list 101 deny tcp any host eq 2000

access-list 101 permit ip any any


where is the SRS router (i.e. the default gateway) and is one IP phone that you want to allow. You'd need to put a range of ip addresses or add a line for each individual one.

New Member

Re: Limiting IP Phone SRST registration

Thanks, that;s what I thought, but it's good to get confirmation

Re: Limiting IP Phone SRST registration

Or you could upgrade to 3.2 where you can tick on a per phone basis, those phones you want to be SRST extensions.


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