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Cisco Bug: CSCus99883 - RTP behavior of 7975 phone system during times of network congestion

Last Modified

Feb 19, 2018

Products (36)

  • Cisco Unified IP Phone 7900 Series
  • Cisco Unified IP Phone 7971G-GE
  • Cisco Unified IP Phone Expansion Module 7914
  • Cisco Unified IP Phone 9951
  • Cisco Unified IP Phone 7962G
  • Cisco Unified IP Phone 7945G
  • Cisco Unified Wireless IP Phone 7920
  • Cisco Unified IP Phone 7941G
  • Cisco Unified IP Phone 6941
  • Cisco Unified IP Phone 6901
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Known Affected Releases


Description (partial)

Unicast L2 traffic is sent to the mac of the IP Phones from the spirent traffic generator.  The traffic load range goes from 100% to 8% of the 1Gbps link, as is described in the following tests below.  The traffic is sent to both switches separately, either one or the other is under test not both at the same time.

The sequences of the test are the following :
1. we enable traffic to reach the phone port with spirent. While the traffic is enabled the phone can not register with CM and does not receive RTP audio packets
2. as soon as we stop traffic the phone registers with CM and audio is back

Test1 (capture name Test2Jim.cap)
L2 unicast traffic is sent to the IPPhone, after around 40 secs synch with CM is lost, the phones show 'registering' mode and timeout logs are received in phone doctor. As soon as the traffic is stopped the IP Phone registers again with the CM
In the .cap we can see there is ospf traffic is sent during the traffic load, nevertheless no synch traffic from the phone is seen.

Test 2 (capture Test3JimenezCALL.cap)
In this second test a call is set up between both IPPhones. With no traffic load, the audio works in both directions. As soon as the traffic is enabled the audio does not work on the loaded IP phone (end user can not hear anything), even if during the traffic load the RTP packets are still flowing in both directions as we see in the capture

*Both tests have been done with a traffic load of 8% of 1Gbps link
**No drops are seen in the port where the IP Phone is connected

We have also experienced in separated tests that with 8 Mbps of L2 unicast traffic arriving in the switchport where the phone is connected, the synchronization with the CUCM is NOT lost (at least in a 10 minutes test), but with 9 Mbps and higher load the sync is lost.

From this tests we get the following conclusions:

No traffic is dropped on the switch
RTP packets are reaching the 'loaded' IPPhone, but user can not hear anything.

Administrative tasks such as an internal port scan or security scan.
Attacks that occur on the network, such as a denial-of-service attack
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