Configuring QoS to give SIP priority

[ngthen]

I have a Cisco PIX 515e for employee Internet access on a 20Mbps up/down circuit.  Recently tennants/clients in my building purchased phone service through an external SIP provider and have the Small Business SPA303 phones.  At times I have noticed that calls are being dropped and I would like to give these phones priority.  These phones are connected to 2950 series switches which connect back via fiber to a 3550-12G.  From here, it goes to the PIX.

I had a few questions...

1) Is there anything special that needs to be done to configure the SPA303s for QoS?

2) If the SPA303s tag the packets or even a desktop/workstation for that matter, then would I need to do anything on the 2950s or 3550?  If not, I am assuming that the tagging would remain in place until the packet reaches the PIX.  Then here I could give priority.  I do not need to do anything internally with priority, just outbound to the Internet.

[Mohamed Sobair]

Hi,

1) Is there anything special that needs to be done to configure the SPA303s for QoS?

You need to make sure that the SPA303s is setting CoS value for the IP-Phones, if its , you will hav to trust those valuse on the Switch, if not, then you can set CoS value for the IP-Phones on the 2950s.

2)  If the SPA303s tag the packets or even a desktop/workstation for that  matter, then would I need to do anything on the 2950s or 3550?  If not, I  am assuming that the tagging would remain in place until the packet  reaches the PIX.  Then here I could give priority.  I do not need to do  anything internally with priority, just outbound to the Internet

NO, you NEED to trust those values on the Switch ports, the Switch doesnt forward any CoS value by default unless you trust it, and then you need to map it to DSCP and trunst those DSCP marking on the UPlinks as well as ensures the (Priority-Queue Out) enabled on those links for expedite forwarding.

On the 3560, you can apply your QoS policy outbound towards the Pix.

HTH

Mohamed

[ngthen]

Do you have a simple example of how I can trust it on my 2950s, 3550 and pass it to the PIX?

[Mohamed Sobair]

On the 2950s (Access Ports where the IP-Phones are connected):

mls QoS

interface x/y

Mls qos trust cos

mls qos Cos 5

priority-queue out

mls qos map cos-dscp 0 0 0 0 46 0 0

interface x/z (Uplink to 3560)

Mls Qos trust dscp

priority-queue out

On the 3560:

MLS QoS

Interface x/z (uplink to 2950)

mls qos truct dscp

priority-queue out

Class-map match-any voice

matc ip dscp ef

policy-map voice

class voice

priority xxx (Bandwidth in Kbps)

Interface a/b (the Layer-3 intefrcae connected to the pix)

service-policy output voice

HTH

Mohamed

[ngthen]

I see what you are showing, but I wanted to clearify to make sure I understand it.  Correct me if I am wrong...I put my comments/questions in red.  I am using a 3550 not 3560...I am assuming this would still apply.

mls QoS (turns on QoS)

interface x/y

Mls qos trust cos (what is the difference between qos trust cos and qos Cos 5)

mls qos Cos 5

priority-queue out (packets outbound are prioritized)

mls qos map cos-dscp 0 0 0 0 46 0 0 (I am assuming this just maps the dscp to cos...why all the 0s)

interface x/z (Uplink to 3560)

Mls Qos trust dscp (why trust dscp and not cos?)

priority-queue out

On the 3560:

MLS QoS

Interface x/z (uplink to 2950) (does this just prioritize stuff going back to the 2950?)

mls qos truct dscp

priority-queue out

Class-map match-any voice

matc ip dscp ef

policy-map voice

class voice

priority xxx (Bandwidth in Kbps) (why do I need a bandwidth here?  wouldn't I need this on the pix?)

Interface a/b (the Layer-3 intefrcae connected to the pix)

service-policy output voice (will the dscp info be in the packets when they arrive at the pix so I can prioritize them?)

[Mohamed Sobair]

Answers inline:

mls QoS (turns on QoS): YES

interface x/y

Mls qos trust cos (what is the difference between qos trust cos and qos Cos 5). If the IP-phone sets cos value, then no need for (mls qos cos 5) hence the command trust cos, if the IP-phone dosent set Cos value, then the command (MLS QoS  CoS 5) inherit the CoS to be 5 for the traffic recieved from the IP-phone.

mls qos Cos 5

priority-queue out (packets outbound are prioritized), this command enables the expedite queue (priority queue) for all voice traffic

mls qos map cos-dscp 0 0 0 0 46 0 0 (I am assuming this just maps the dscp to cos...why all the 0s): this command maps all Cos Value to the a Specified DSCP paramenters.Our Example here focuses on Voice Payload, thats why the CoS 5 is mapped to a dscp value of 46 and the rest is ignored.

interface x/z (Uplink to 3560)

Mls Qos trust dscp (why trust dscp and not cos?) : Because you already mapped a CoS value to a DSCP, you dont need to trust CoS here again, but DSCP.

priority-queue out

On the 3560:

MLS QoS

Interface x/z (uplink to 2950) (does this just prioritize stuff going back to the 2950?): This trust the incoming DSCP Value which was set by the  2950.

mls qos truct dscp

priority-queue out

Class-map match-any voice

matc ip dscp ef

policy-map voice

class voice

priority xxx (Bandwidth in Kbps) (why do I need a bandwidth here?  wouldn't I need this on the pix?):  This will ensure QoS LLQ is applied for Voice traffic before reaches the Pix.

Interface a/b (the Layer-3 intefrcae connected to the pix)

service-policy output voice (will the dscp info be in the packets when they arrive at the pix so I can prioritize them?): Yes, the DSCP infor will still be in the IP header.

HTH

Mohamed

[ngthen]

Thanks, this helps.  One last question I think... if the phones send a CoS value do I still need mls qos Cos 5?

[Mohamed Sobair · Accepted solution]

If the Phone Sets CoS 5, then You DONT need this command anyn more, you just need (mls QoS trust CoS) on the interface.

Pls rate this post with the star box below,

Regards,

Mohamed