Two PCs connected with an IP phone

With lock a MAC to a port-VLAN pair feature, you can limit the number of devices connected with an IP phone.

In an above scenario, mixed learn-mode has address limit of three, one static MAC address and two dynamic MAC addresses (PCs). The static MAC address (IP phone) is configured on VLAN 20. The VLAN 20 locks the IP phone and prevents dynamic MAC address learning on same port-VLAN pair. Dynamic MAC addresses are learnt on same port but for VLAN 10.

Sample configuration for two PCs connected with an IP phone:
switch(config)# vlan 10
switch(vlan-10)# untagged 1
switch(vlan-10)# exit
switch(config)# vlan 20
switch(vlan-20)# tagged 1
switch(vlan-20)# voice
switch(vlan-20)# exit
switch(config)# port-security A3 learn-mode mixed
switch(config)# port-security A3 address-limit 3 mac-address 000000-000701 vlan-id 20 action send-alarm
switch(config)# show run interface A3

Running configuration:

interface A3
   tagged vlan 20
   untagged vlan 10
   port-security learn-mode mixed address-limit 3 action send-alarm mac-address
   000000-000701 vlan-id 20

switch(config)# show run

Running configuration:

; JL254A Configuration Editor; 
; Ver
hostname "switch"
module 1 type jl254a
mac-age-time 60
port-security 1 learn-mode mixed
port-security 1 address-limit 3
port-security 1 mac-address 000000-000701 vlan-id 20
port-security 1 action send-alarm
snmp-server community "public" unrestricted
vlan 1
   name "DEFAULT_VLAN"
   no untagged 1
   untagged 2-52
   ip address dhcp-bootp
   ipv6 enable
   ipv6 address dhcp full
vlan 10
   name "VLAN10"
   untagged 1
   no ip address
vlan 20
   name "VLAN20"
   tagged 1
   no ip address