Switch Config for voip QoS

Unanswered Question
Aug 18th, 2008

I've read of 2 ways to configure switch ports for cisco voip phones with attached PCs. Use switchport voice vlan for the phone and either use switchport mode access or mode trunk for data from the PC. Which is better, access or trunk? Why? Is QoS / CoS the same for both options? I'm using 2950s and 3750s. I'd like know how the switch handles vlan and QoS for traffic it receives from the voip phone and PC in both trunk and access mode cases.


I have this problem too.
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Marwan ALshawi Mon, 08/18/2008 - 16:57

first off all

the two ways u have read are configurable and both works fine but with trunk it will be less secure becuase any one can use the trunk and do what is called vlan hoping!!

if u wonder why trunking for voice.. the answer is cos marking is based on layer two

Many LAN switches today can mark and react to a Layer 2 3-bit field called the Class of Service (CoS) located inside an Ethernet header. The CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link (ISL) trunking is used. (The IEEE 802.1P standard actually defines the usage of the CoS bits inside the 802.1Q header.) You can use the field to set 8 different binary values, which can be used by the classification features of other QoS tools

The term CoS really refers to two different fieldsa field inside either the 802.1Q trunking header, or a field inside the ISL header. The IEEE 802.1Q standard uses the 3 most-significant bits of the 2-byte Tag Control field, called the user-priority bits. The Cisco proprietary ISL specification uses the 3 least-significant bits from the 1-byte User field, which is found inside the ISL header's user field byte. In general conversation, and in the QoS courses from Cisco, the term CoS applies to either of these two fields

so when u make the port access port and creat voice vlan

the port will be called muli-access port

although it access port but technicaly will be as 802.1Q trunk so it will reconise the voice vlan and when u trust the phone or cos on the port will take the cos value from the phone thta it send it by default as cos 5 for voice and cos 3 for call signaling and the PC connected to the phone the phon by default will mark all Pc traffic as cos 0 unless u manuly change it with extend command to whatever value u want.

in ur case this concepts aplly to all Cisco switchs

so configure it as access port with voice vlan

good luck

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