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.

[smehrnia · Accepted solution]

Hello Sarah,

as you already now, trusting CoS value itself has nothing to do with the way qos and traffic management behaves. the important part in how classified packets get serviced depends on the QoS setting that you apply on the switch.

for example, if you put a traffic flow with CoS 5, in an ingress priority queue, this flow doesnt necessarily go to an egress priority queue too, unless you set it so.

but using certain features such as Auto qos for voice, makes the switch manage both ingress and egress queues at the same time

Hope it Helps,

Soroush.

[sarahr202]

Thanks Soroushm.

By default  a switch trusts cos values received on its trunk port.  The cos value in received frame helps switch determine which priority queues  for ingress port, the frame be placed. And If a switch determines that received frame has to forwarded out say f1/2, it will not place the frame in priority queue for egress port (f1/2) based on cos value carried by received frame unless we configure specific features such as auto qos.

Did i get it right ?

[smehrnia]

As for the trunk ports, I believe so Sarah, except for the Native Vlan frames, which dont have Tag Control info in their headers.

it said that in most cases without any special configuration, all user data traffic is sent in a Best Effort basis. so unless u configure ur device to take care of a certain class of traffic, it continues to use the best effort model.

but some configz are like a package! u config one thing, it takes care of ingress and egress; i.e. Auto QoS for voip!

EDIT: I misunderstood!!! you have to trust CoS and DSCP all along the way, then if u trust either CoS or DSCP, then the switch uses a conversion table to convert them in case its needed to be done (default and configurable). on access ports and uplinks you have to explicitly instruct to Trust and untagged traffic gets a 0 value by default.

Thanks Sid !

HTH,

Soroush.

[siddhartham]

Hi Sarah,

"By default  a switch trusts cos values received on its trunk port."

I don't believe the above statement is true, even if its a trunk port you still have to tell the switch to either the trust the COS or DSCP received in the incomming frame.

"Example 2:If the port is trunk port, you can configure either the mls qos trust cos or mls qos trust dscp command. The dscp-cos map table is used to calculate the CoS value if  the port is configured to trust DSCP. Similarly, the cos-dscp map table  is used to calculate the DSCP value if the port is configured to trust  CoS.

As Souroush mentioned since native VLAN traffic will have COS,DSCP values set to zero the switch will reset the COS and DSCP values back to zero even if we specify the trust command.

Example 3: If the port is a dot1q trunk port and the port is configured with the mls qos trust cos command, native VLAN frames will have CoS and DSCP values as 0. Because  native VLAN frames are untagged and the frame is tagged after it enters  the switch, the switch will set the default CoS value to 0 and the  CoS-to-DSCP table sets the DSCP value to 0.

Note: The DSCP value of the packet coming from native VLAN will be reset to 0.

AutoQos is the easiest way to configure LAN QOS.

http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml

https://supportforums.cisco.com/message/3656109#3656109

Siddhartha

[sarahr202]

I should throw my book in thrash.

That is exactly what my book says:

By default, QOS is disabled globally on a switch and all QOS information is allowed to pass from one switch port to another."

thanks for the correction or i will be building my QOS foundation on a wrong concept.

[smehrnia · Accepted solution]

Sarah, if QoS is disabled this is different case i believe we both assumed where u have qos enabled coz we were talking abt priority packets n stuff, in case it is Enabled u have to set trust states, all along the way.

if QoS is disabled, then switch simply doesnt care! it doesnt look for 802.1p header then. think of it like a dumb hub (QoS-wise), would a hub care what header information is attached to a PDU ? No!

HTH,

Soroush.

[sarahr202]

May be i should keep my book.

[siddhartham · Accepted solution]

"May be i should keep my book"

.

As descibed in your book, if QoS is not enabled on your switches it won't care about the QoS markings but they will loose their markings when they cross any layer 3 device.

.

Siddhartha

[sarahr202]

As descibed in your book, if QoS is not enabled on your switches it  won't care about the QoS markings but they will loose their markings  when they cross any layer 3 device.

.

    R1---R2--trunk--sw1-----trunk-sw3-ip phone.

At which node should  we perform mapping cos to dscp? Cos marking will travel over the trunk link. to Sw1.   Can Sw1 map the cos to Dscp and send it  over trunk port to R2 ? Can Sw1 retain COS value and  let R2 map the Cos to Dscp before sending it to R1?

In between can  a router map cos value to dscp value and vice versa ?

thanks and have a good evening.

[siddhartham · Accepted solution]

The switches will calulate both COS and DSCP values, for example if you trust the COS value on a port it will also calculate the respective DSCP value based on the COS-to-DSCP mapping and in the same way if you trust the DSCP value it will calulate the respective COS value based on the DSCP-to COS map.

On the route we can configure using MQC - class maps and policy maps- with class map you can either match the incomming packets based on COS or based on DSCP not on both at the same time, for example

class-map match any voice

cos 5

or

class-map match any voice

dscp 46

Then you can use a policymap and set the value for the outgoing packets

policy-map qos

class-map match-any Platinum-Voice

priority percent 20

set ip dscp ef

At which node should  we perform mapping cos to dscp?

Router and switches

Cos marking  will travel over the trunk link. to Sw1.   Can Sw1 map the cos to Dscp  and send it  over trunk port to R2 ?

Yes

Can Sw1 retain COS value and  let  R2 map the Cos to Dscp before sending it to R1?

Routers can map cos to DSCP and vice versa, so it depends where you want to perform the conversion and it really doesn't matter.

The only requirement is QoS needs to be a end to end configuration.

Siddhartha

[sarahr202]

Siddartham

Please correct me if  ia m wrong.   policy maps for incoming packets are evaluated first before routing.

On the route we can configure using MQC - class maps and policy maps-  with class map you can either match the incomming packets based on COS  or based on DSCP not on both at the same time, for example

class-map match any voice

cos 5

or

class-map match any voice

dscp 46

Let say we identify the   an incoming packet  using above policy map on a router. . Lets call this packet packt1. After we identify the pack1 we use " set" command to change DSCP value say to 45.  Router now perform QOS on pack1 using this new DSCP value by placing it in priority queue for ingress port. Is it correct?

thanks.

[smehrnia · Accepted solution]

Hi Sarah,

i aint Sidd thou!

this is the QoS chart on most of the devices, but note that this could be slightly different in different routers or switches:

Ingress Traffic ----> Classify ---> Policer ----> Marker (Set Cos) ----> Normal / Priority Queue ---> to the egress point

so its a Yes to what you said

HTH,

Soroush.

[siddhartham]

Hi Sarah,

i aint Sidd thou!

I agree with Soroush's explanation in his post.

For routers you can define different calss maps and policy maps for different directions,

for example

Class-map in_G0/0

Policy-map in_G0/0

class-map out_G0/0

policy-map out_G0/0

class-map in_S0/0

policy-map in_S0/0

class-map out_S0/0

policy-map out_S0/0

Siddhartha

[sarahr202]

Hi Soroushm

I already posted this question but still did not understand it.

I hope you can help me sort out this:

Sw1-f1/1-------------------Ip phone

                                     |

                                     |

                                    PC

Int f1/1

switchport acess vlan 22

switchport voice vlan 24.

1) My book says this command tells ip phone to use native vlan 22 ( also called data vlan) and vlan 24 for voip frames.

ip phone will send pc data without any tag and hence the word native vlan

Is this correct?

2) If we use " switchport voice vlan dot1p" the switch tells ip phone to use vlan 0 for voip traffic and use native vlan ( which is vlan 22) for PC.

My question is since we are using vlan 0 for voip, it should be used for voip throughout switched network for example:

Ip phone1-----sw1--------sw2------ipphone2

    |                                            |

  PC                                       PC

If we use " switchport voice vlan dot1p" on sw2,  voip frames will be sent in vlan 0 by iphone2 to sw2.  If ipphone2 has to communicate with iphone1  and they are on same subnet, then should  ipphone1 be on vlan 0 as well?( that means we must also configure  SW1 with " switchport voice vlan dot1p")

  3) If we use " switchport voice vlan untagged" on sw2, my book says both voip and pc data are sent using native vlan. then book also says COS bits still sent.  This boggles my mind. When we use the option " untagged"  the switch should not insert any dot1q tag . So without dot1q tag, how COS bits can still be carried?

=============================================================================

4) sw--------------ipphone

                           |

                         Pc

Let say we configure sw as:

int f1/1

switchport access vlan 2

switchport voice vlan 3

switchport priority extend cos 3

Ip phone will use native vlan2 for Pc which means no dot1q tag and  vlan 3 for voip traffic. The last command tells ipphone to set COS value to 3 for frames received on its port connected to PC.

My question is what is the point as ip phone  will send Pc 'frames using native vlan 2 to sw, without 8021q tag as a result , the cos value 3 can not be sent with PC data frames to SW.

If the above is true why  should one use "switchport priority extend cos 3"  command ?

Sorry for this long winded post. I am almost done with my book, three more chapters to go .I am planing to take switch exam in August.

thanks and happy 4 th of july if you are in states.