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High CPU utilization on a Catalyst 4500 when connected to an IP phone that uses trunk ports


Core issue

Trunking to an IP phone could be the cause of high CPU utilization. All VLANs for a particular interface are trunked to the phone, increasing the number of STP instances the switch has to manage. This increases the STP CPU utilization. Trunking also causes unnecessary broadcast/mcast/unknown unicast to hit the phone link.

In some cases, the number of STP instances is high because a high number of VLANs are forwarded on all trunk ports. In this case, manually prune the VLANs that are not necessary from the trunk in order to drop the number of STP active ports to well below the recommended value.


In order to fix this, remove the trunk configuration and keep the voice and access VLAN configured along with Quality of Service (QoS). Technically, it is still a trunk, but it is called a Multi-VLAN Access Port (MVAP). Because voice and data traffic can travel through the same voice port, you should specify a different VLAN for each type of traffic. You can configure a switch port to forward voice and data traffic on different VLANs.

In case of IP phones, be sure that you do not configure IP phone ports as trunk ports. This is a common misconfiguration. Configure IP phone ports with a voice VLAN configuration. This configuration creates a pseudo trunk, but does not require you to manually prune the unnecessary VLANs.

Non-Cisco IP phones do not support this voice VLAN or auxiliary VLAN configuration. You must manually prune the ports with non-Cisco IP phones using the command switchport trunk allowed vlan vlan_list.

In order to configure a port to receive voice and data from a Cisco IP phone on different VLANs, refer to Configuring Voice Interfaces.

For more information, refer to the Causes of High CPU Utilization section of CPU Utilization on Catalyst 4500/4000, 2948G, 2980G, and 4912G Switches That Run CatOS Software.