[Asterisk-Users] 802.1p , precedence and TOS

[SCollins]

Questions:
1) How can I set the 802.1p precedence field from Snom and asterisk?

2) Which is the differences between the IP Precedence field and TOS
  field?

IP Precednce and ToS are synonymous. There is a 3-bit ToS field in the IP  
header that allows for 8 levels of classification of IP traffic - these  
are known as the IP precedence bits.

802.1p or CoS (Class of Service) is a Layer 2 Prioitization protocol.  
802.1p uses a 3-bit prioitization field, in an 802.1q TAG,to define 8  
priority levels.  To obtain the correct Layer 2 CoS on your 3300 I believe  
you need to set the prioritization on the IP phone, using 802.1q and then  
setting the prioritization, and then configure the clasifiers and profiles  
on the 3Com 3300 so that it will remark the packets on ingress.  The CoS  
traffic classification will need to be configured on ALL Switches in your  
network for the 802.1p prioritzation of the Voice traffic.

Most layer 3 Switches support both ToS and CoS and will re-mark the 802.1p  
Prioritization into the ToS field in the IP header, so that if the traffic  
is being routed, between VLANs for example, the traffic will maintain it's  
priority.


Hope this helps.

Sean


On Tue, 19 Apr 2005 15:28:43 +0200, Eugenio De Vena <edevena at gsy.it> wrote:

> Hello everyone, I have some doubt on the QoS matter and I hope that  
> someone
> could
> bring me some light.
> I see my 3Com 3300 switch supports 802.1p priorization ,
> I see through Microsoft network monitor that the packet coming from my  
> SIP
> phones
> have in IP header  a field which is marked "Precedence" and when the  
> packet
> is sent from
> phone to asterisk the value is "CRITICAL/EXP" , when the packet is sent  
> back
> from asterisk
> to phone the value is "Normal". To correct this I set the value "Tos" in
> sip.conf to 160 and now
> the packet sent back from asterisk to phone is marked "CRITICAL/EXP" too.
> Fine.
> Under the field "Precedence" there is a value called "Type of Service" (
> TOS ) and the values is
> "Normal" both ways.
>
> Googling around I see that 802.1p is a Layer2 and not Layer3  
> specification
> so it must work at MAC level,
> so I think that these field which are contained into the IP header are  
> not
> considered  by my switch and are just ignored.
> The Ip precedence field I think would be considered when routed by a QoS
> router ( which is not my case ).
>
> Questions:
> 1) How can I set the 802.1p precedence field from Snom and asterisk?
> 2) Which is the differences between the IP Precedence field and TOS  
> field?
>
> Thanks Everyone
> Eugenio
>
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