A client has CUCM 5.1 with 2811 MGCP gateway (IOS 12.4). They have planned to upgrade to CUCM 8.6 in the near future. But they got call quality issue since last week and we narrow down it would be issue related to the gateway.
1) If we replace the 2811 gateway with a new 2911 (IOS 15.1), can the new 2911 work with CUCM 5.1?
2) If IOS 15.1 is not compatible with CUCM 5.1, may I downgrade IOS 15.1 to 12.4 on 2911? Will the licenses with 2911 be lost after downgrade?
Looks like you do not have "network-clock-select" which is needed to properly assign clocking order, this is a very common omision that results in the issue you are describing. I suggest you add it and see if it resolves your issue.
We went through this about 2 years ago. Your best best is going to be to upgrade to CUCM 7.1.5 at the lowest.
If memory serves me correctly 2900s are not officially supported on 12, in conjunction with CUCM, TAC would be best to confirm.
With regards to the licenses they are stored separately from IOS
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1. No, not with MGCP, you will either have to upgrade CUCM or convert to H323
2. No, G2 routers 29XX require 15.0+ IOS
The bigger question here is why do you assume replacing a platform is going to resolve your QoS issues, that is very unlikely, can you tell us how did you come to this conculsion?