Cisco SPA 504G backup IP address

[Raimondas Mazeika]

Hi, 

Apologies if this is a repeated question, but I was not able to locate any similar info... 

I know Cisco SPA504G is coming to its end of life, but because of its simplicity and low price is still very popular among business we support. Recently I noticed an emerging trend with these phones and their "backup IP address" that I would like to get some answers. I will try to explain the issue in brief.

When a SPA504G is connected to a network, it displays "Initializing Network..." and after some time an IP address is assigned to them by a DHCP server, then it registers with relevant IP-PBX and happy life. When it come to DHCP renew, this works with no issue most of the times. However, some sites have some phone that get stuck to "Initializing Network..." and do not progress anywhere. This indicates that the phones do not get IP address issued although plenty of IP addresses available in the pool. I am not sure if this status is a cause or a consequence of something else, but we also noticed that during IP lease renew, some phones reverts itself back to their previous IP address (this is what we call "backup" IP) and this could potentially start causing IP conflicts in the network. Inability to obtain an IP address is most likely related to DHCP server and this is being investigated on a separate basis. 

At this stage, I want to find out: 

• What is the timer between the first IP discovery packet is sent and reverting itself to the previously issue IP address and is it possible to change it? 
• Is there any official reference document available in relation to this action? 

To add, the phones are on 7.5.3 version. If we upgrade to 7.5.6, the phones do not revert to the "backup" IP address anymore; this solves the issue to certain extent. Unfortunately, not all phones can be upgraded to 7.5.6 due to various setups. 

[Jaime Valencia]

SPA endpoints are covered in the SMB community, might want to move this over there.

[Raimondas Mazeika]

Thanks, Jaime. I found similar issues reported in your suggested section. 

[Dan Lukes]

 some sites have some phone that get stuck to "Initializing Network..." and do not progress anywhere

This needs to be analyzed in details. Fortunately, it's rather easy. I stopped DHCP server, started tcpdump, power cycled the phone (7.6.1 firmware version) - and voila, here are the results.

Phone fire DHCP requests at T+0, T+4, T+12, T+28, T+60, T+64, T+72, T+88. The retransmission strategy follow RFC (although not exactly) and is not configurable.

If no address received and there's saved unexpired address it become used on T+120 onward.

IMHO, if the DHCP server fail to respond eight requests during 90s window then it is broken badly. You should start with this issue if you wish for reliable VoIP network.

[Raimondas Mazeika]

Thanks, Dan. Good shout. I was not aware of 7.6.1, so it will be another factor to consider. 

[Dan Lukes]

May be I missed something, but the matter seems not to be related to phone at all, thus exact firmware version is unimportant. IMHO the core cause is the unreliable DHCP server not responding the requests for so long time ...