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Downstream PC port on 79x5 IP phones - packet loss for PC?

We have a very mixed environment with various switches, end point phones and PCs.

We've observed poor network performance on the desktop PCs where downstream of a 7945 or 7965 phone connected to a 100Mbit switch port with a gigabit capable PC port on the downstream PC port (on the back of the phone) . 

Prior anecdotal experience suggests the following topology leads to congestion/packet loss in the phone's inbuilt switch:

[PC NIC 1Gbit] <-> [Phone PC port 1Gbit] <-> [79x5 phone] <-> [100Mbit Switch port] <-> [LAN etc]

A workaround is to manually configure the PC NIC to 100MBit Full/Duplex.  However, this is not entirely practical to deploy remotely.

Is there any documentation or a bulletin/best practice guide etc that discusses interoperability issues between a gigabit enabled PC on a gigabit enabled Cisco IP phone on a 100Mb enabled LAN?  We have a large number of users potentially affected with more to come as 79x5 phones are rolled out in greater numbers to replace older 79x2 phones.

This issue seems to have also been touched on in this post.

Thanks.

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Super Bronze

Downstream PC port on 79x5 IP phones - packet loss for PC?

Hi Tim

You've not posted your version of CUCM, but check if you have an 'Automatic Port Synchronisation' setting on the phone config page in the 'Product Specific' section.

It's described here : http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7975G_7971g-ge_7970g_7965g_7945g/8_5/english/admin_guide/7970usr.html

When the Cisco Unified CM administrator uses the Remote Port Configuration feature to set the speed and duplex function of an IP phone remotely, loss of packets can occur if one port is slower than the other.

The Automatic Port Synchronization feature synchronizes the ports to the lowest speed among the two ports, which eliminates packet loss. When automatic port synchronization is enabled, it is recommended that both ports be configured for autonegotiate. If one port is enabled for autonegotiate and the other is at a fixed speed, the phone synchronizes to the fixed port speed.

Might be worth a try.

Regards

Aaron

Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
1 REPLY
Super Bronze

Downstream PC port on 79x5 IP phones - packet loss for PC?

Hi Tim

You've not posted your version of CUCM, but check if you have an 'Automatic Port Synchronisation' setting on the phone config page in the 'Product Specific' section.

It's described here : http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7975G_7971g-ge_7970g_7965g_7945g/8_5/english/admin_guide/7970usr.html

When the Cisco Unified CM administrator uses the Remote Port Configuration feature to set the speed and duplex function of an IP phone remotely, loss of packets can occur if one port is slower than the other.

The Automatic Port Synchronization feature synchronizes the ports to the lowest speed among the two ports, which eliminates packet loss. When automatic port synchronization is enabled, it is recommended that both ports be configured for autonegotiate. If one port is enabled for autonegotiate and the other is at a fixed speed, the phone synchronizes to the fixed port speed.

Might be worth a try.

Regards

Aaron

Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
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