Cisco CallManager uses the device-based AAR calling search space, which is assigned to Cisco IP Phone station devices and gateway devices, when it attempts to route the call to the gateway device that connects to the PSTN or other network.
Actually, AAR uses only the CSS you assign as the AAR CSS on the device.
The decision to use AAR CSS isn't so much a "hunt" or failure to route on the Line or Device CSS. Rather, the call-routing decision is made based on line or device CSS, then most importantly for AAR, Location bandwidth pools are checked for available bandwidth to make the call. If there is sufficient bandwidth, the call is made with "normal" Line or Device CSS choices. If there is insufficient bandwidth available, and if AAR is enabled, Line/Device CSSes are not actually used to place the call. The calling device's AAR CSS is searched for a route to the called device's AAR group prefix + the called device's external number mask. If that route exists, the call is made.
The decision whether you want to configure a dedicated AAR CSS / Partion / Route-pattern combination depends on the desired call flow. Typically, if there is insufficient bandwidth, you will want that call to traverse the PSTN rather than the WAN. So you will ensure that the AAR CSS / Partition has a route-pattern that uses a local gateway to call a remote gateway via PSTN.
However, it may be that your normal dialplan uses the local gateway to make that call anyway ie you have no toll-bypass or the like. If so, there's probably no need for a separate AAR CSS.
Let me know if that helps clarify by rating the post.
One important key to remember when constructing AAR CSS is to include blocking router patterns matching its site's DID ranges to prevent possible infinite loop on PRI. Consider an example of user A DN 1000, who's site is site A, this user now logs in via EM at site B, another user B at site A calls 1000, if there is not enough bandwidth between site A and B AAR will kick in and reroute the call via PSTN. Since the DN of user A was a DID at site A this call will come back from PSTN back to site A, and so on.
So, as a best practice build blocking route patterns for the local DIDs, assign them to a distinctive partition and include this partition in AAR CSS.
Appologies for labouring the point. Are we saying that the line and device CSS is ignored *once* CAC has determined insufficient bandwidth? And that only the AAR CSS is processed by digit analysis?
If my understanding is correct, then you could leave the External Phone Number Mask of the destination IP phone set to the the same pattern as the line DN and have the AAR CSS route the pattern to the gateway, prefixing any AAR Group digits (if any set) or applying any translations at the route pattern/list level.
I have a scenario where I need to route the call via a private TDM VPN if AAR is invoked and therefore, need to maintain the same called number when presenting it to the gateway.
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Topology: IP Phone > Switches > Microsoft NPS setup to forward 802.1x
proxy to > ISE 2.1 patch 3 Authentication: EAP-TLS using Cisco MIC SANs
Phone Models 802.1X support? 802.1x flavor Addtl Comment EAP-MD5 EAP-TLS
Cisco 3905 Y Y N Cisco 6911 Y Y N Cisco ...