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Cisco 7821 SIP Registration Issue

Waleed Khan

Cisco 7821 working as a SIP stuck at "Registering" once it loss the connectivity with CUCM, ( Using CUCM 11.0 ) even after getting connectivity from CUCM, it remains unregister, lets suppose a 7821 is register with CUCM, once we remove CUCM lan cable, 7821 should be unregistered but after plugging the lan cable to CUCM, the 7821 should register also, but 7821 SIP phone remain unregister while other SCCP phone got register automatically, the only solution untill now is removing the POE cable from phone and plugin back so it restart it, then 7821 register otherwise not, anyone have a Idea what is the problem ?


7 Replies 7

Manish Gogna
Cisco Employee
Cisco Employee

Hi Waleed,

I do not see any specific bug for this. Can you check if the 7821 is running the latest firmware and if yes then get the packet captures from the IP phone after recreating the issue.


Its running the latest version, please check Wireshark logs.

Use this filter in Wireshark:

eth.addr == 0C:11:67:81:73:73 ( Mac addr. of 7821)

IP of Phone=


As you can see TCP unseen segment ACK which refers to network issues between the phone and CUCM.

It will be good to test with another separate switch port to see if this resolves... 

It is, but as soon as the CUCM is live, other SCCP phones register, but all 7821 won't.


I have check by upgrading phone firmware to the latest version but still same result.


I have check the issue with colleague, the window size is 0, which means phones is not ready to accept packets results in unregister, but how to tweak it to make it work ?


On VLAN  interface where the phones are connected. You can set the ip MTU value   "ip mtu bytes " to less then 1500

However i believe tcp window can also be set at global level.So better check with network team.

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