COS values and outgoing interface queues.

[sarahr202]

Hi everybody.

The switch uses its queues, which are available on a per-port basis, to  buffer incoming and outgoing frames. The switch can use the CoS values  to place the frames in the appropriate queues. Voice frames should be  placed in the priority queue for minimal delay.

Let say i a switch trust cos of incoming frames and places in the priority queue for minimal delay. The switch does not alter cos values. Will this switch also place these frames with cos values in the appropriate queues for outgoing interface ?

thanks and have a great weekend.

[sarahr202]

Thanks Reza for your kind remarks.

I used GNS3 but there are certain topics in switching exam such as qos , voip,  I can not use gns3 for them( I don't have certain software ).

Have a nice evening.

[smehrnia · Accepted solution]

With regards to the receiving end point, since 802.1p uses vlan 0 as a spacial case to carry a CoS value for voip frames, and cos is only used by intermediate devices for QoS purposes and having in mind that upon receiving the Voice packet, Switch assumes the Access vlan which is set on the interface in this case.

then i'd say, the Switch will simply strip off the vlan ID from the frame and and forward it to ip phone's IP address out the interface as it'll do on a normal access switchport.

this was my assumption based on the facts.

HTH,

Soroush.

[sarahr202]

Thanks Soroushm

With regards to the receiving end point, since 802.1p uses vlan 0 as a  spacial case to carry a CoS value for voip frames, and cos is only used  by intermediate devices for QoS purposes and having in mind that upon  receiving the Voice packet, Switch assumes the Access vlan which is set  on the interface in this case.

then  i'd say, the Switch will simply strip off the vlan ID from the frame  and and forward it to ip phone's IP address out the interface as it'll  do on a normal access switchport.

Please correct me if iam wrong.

If switch removes the vlan id and sends it over to ip phone where it is received by internal switch inside the ip phone.How will this internal switch perform qos if there is no vlan id, ? Since this internal switch receives frames for ipphone itself and attached pc,therefore it must prioritize voip frame by placing them in priority queue for ingress port( the connected to external switch). In absence of vlan id or dot1q tag, which means no cos values, internal switch will not be able to prioritizes voip traffic.

thanks

[smehrnia · Accepted solution]

when voice packet is received at the ip phone with its Mac Address attached, the phone will just pick it up n otherwise will send it out the other interface, is a small 1 to 2 / 2 to 1 switch module. i dont think the built-in switch module in an ip phone is that sophisticated to perform queuing and other QoS stuff, this needs some hardware muscle... n it doesn't really need it.

its just faster to read the address and switch.

whatever qos and marking we implement at the beginning (ip phone) is just for Access, Distribution and Core network handling.

EDIT:

a 2nd view: reasong behind using CoS & DSCP marking is that the intermediate device has a lot of packets to handle and the only way it could know abt the urgency of the frame is reading these markings and follow instructions. as for the ip phone it could be hardcoded in itself that packets destined for the phone itself has the highest priority over all others!

Soroush.