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Bronze

QOS scenarios

Hi everybody

Please consider the following case:

PC1-----Acess link--f1/1--SW1-- Acess link---------F 0/1-R1-acess link-------Sw2-----trunk--------SW3-f1/4--------PC2

All switches are multilayer switches ( 3560)

Pc1 and PC2 are running Cisco Soft phones.

If  we configure SW3 with:

int f1/4

mls qos trust dscp.

1)Does the above command require SW3's  f1/4 be configured as layer 3 port not layer 2?

( My reasoning is since Sw3 has to get to Ip header to process DSCP values, therefore Sw3'f1/4 should be configured  for layer 3 operation.)

2) What if sw3 were not 3560 but layer 2 switch such as old 2900 series. can layer 2 switch be configured with:

mls qos trust dscp.?   Will layer 2 switch be able to interpret dscp values and perform QOS ?

( My understanding is layer 2 switch should not be able to read dscp values )

I  will be posting few more questions on the above scenario.

Thanks and have a nice week.

  • LAN Switching and Routing
Everyone's tags (2)
4 ACCEPTED SOLUTIONS

Accepted Solutions

QOS scenarios

Hello,

You are slightly confused.

If your port is an access port or a layer 3 port, then you should trust the DSCP markings. The reason you cannot trust cos on an access or layer 3 is, COS bits are present only in 802.1q frame. Whereas, on the trunk links you can trust the COS or DSCP. So, if you trust dscp on trunk link, then dscp-cos mapping table would be used to resolve the cos value. if you trust cos, then cos-dscp table would be used.

Hope this helps.

Regards

Vivek

Silver

QOS scenarios

The below examples describe different scenarioes in Port based QoS configurations

Example 1:

If the port is an access port or Layer 3 port, you need to configure themls qos trust dscp command. You cannot use the mls qos trust cos command because the frame from the access port or Layer 3 port does not  contain dot1q or ISL tag. CoS bits are present in the dot1q or ISL  frame only.

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.

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.

Siddhartha

Siddhartha
Silver

QOS scenarios

My question is : why does SW3 have to set both COS and DSCP values ? Is  it because SW3 does not know if SW2 is configured  to trust cos or dscp ?

You are right.

Have a look at the below picture and the link, according to that switch creates a QoS label based on DSCP values. I am not familiar with QoS label concept

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_35_se/configuration/guide/swqos.html

https://supportforums.cisco.com/docs/DOC-8093#_Toc191205666

My book says Phone has an internal switch. Does this switch behave the same way when it comes to performing  QOS based on COS? 

I am not sure whether the internal switch in the phone can perform QoS or not.

Siddhartha

Siddhartha
Silver

QOS scenarios

Usually both cisco and avaya phones have an option to mark the packets with both COS and DSCP markings

Siddhartha

Siddhartha
7 REPLIES

QOS scenarios

Hello,

You are slightly confused.

If your port is an access port or a layer 3 port, then you should trust the DSCP markings. The reason you cannot trust cos on an access or layer 3 is, COS bits are present only in 802.1q frame. Whereas, on the trunk links you can trust the COS or DSCP. So, if you trust dscp on trunk link, then dscp-cos mapping table would be used to resolve the cos value. if you trust cos, then cos-dscp table would be used.

Hope this helps.

Regards

Vivek

Silver

QOS scenarios

The below examples describe different scenarioes in Port based QoS configurations

Example 1:

If the port is an access port or Layer 3 port, you need to configure themls qos trust dscp command. You cannot use the mls qos trust cos command because the frame from the access port or Layer 3 port does not  contain dot1q or ISL tag. CoS bits are present in the dot1q or ISL  frame only.

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.

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.

Siddhartha

Siddhartha
Bronze

Re: QOS scenarios

Thanks Siddhartha.

PC1-----Acess link--f1/1--SW1-- Acess link--F 0/1-R1-acess link---Sw2--trunk--SW3-f1/4---PC2

If  we configure SW3 with:

int f1/4

mls qos trust dscp.

When Sw3 receives a  ip packet with DSCP value say 24.  Sw3  use mapping DCSP-COS  and then sends the packet out of trunk port with COS value.  The frame traversing the trunk between sw3 and sw2, carries COS  in dot1q tag/isl and DSCP value in ip header. 

My question is : why does SW3 have to set both COS and DSCP values ? Is it because SW3 does not know if SW2 is configured  to trust cos or dscp ?

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

Some cisco ip phone has one data port and internal port for voip stream.

External  SW----------f1/1 ip phone---Attached PC

My book says Phone has an internal switch. Does this switch behave the same way when it comes to performing  QOS based on COS? 

For example  if attached Pc generates a   frame and ip phone generates a  a frame carrying voip data, the internal switch must priorities Voip data over pc data by performing QOS before sending them out port( f1/1) connected to an external switch.  Similarly when ip phone receives frames  over its port f1/1 connected to external switch, the internal switch should perform QOS based on COS, putting voip frame in priority queues  for ingress port( f1/1)

Is it correct?

thanks

Silver

QOS scenarios

My question is : why does SW3 have to set both COS and DSCP values ? Is  it because SW3 does not know if SW2 is configured  to trust cos or dscp ?

You are right.

Have a look at the below picture and the link, according to that switch creates a QoS label based on DSCP values. I am not familiar with QoS label concept

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_35_se/configuration/guide/swqos.html

https://supportforums.cisco.com/docs/DOC-8093#_Toc191205666

My book says Phone has an internal switch. Does this switch behave the same way when it comes to performing  QOS based on COS? 

I am not sure whether the internal switch in the phone can perform QoS or not.

Siddhartha

Siddhartha
Bronze

Re: QOS scenarios

Thanks Siddhartham

A cisco ip phone( hardware one) set cos to 5. Does it also set DScp value as well?

thanks and have a nice weekend.

Silver

QOS scenarios

Usually both cisco and avaya phones have an option to mark the packets with both COS and DSCP markings

Siddhartha

Siddhartha
Super Bronze

QOS scenarios

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The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

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Posting

sarahr202 wrote:

int f1/4

mls qos trust dscp.

1)Does the above command require SW3's  f1/4 be configured as layer 3 port not layer 2?

Usually not, as switches that "understand" DSCP can examine packet on L2 ports.

sarahr202 wrote:

2) What if sw3 were not 3560 but layer 2 switch such as old 2900 series. can layer 2 switch be configured with:

mls qos trust dscp.?   Will layer 2 switch be able to interpret dscp values and perform QOS ?

( My understanding is layer 2 switch should not be able to read dscp values )

"Pure" L2 switches can not examine L3 info, such as DSCP.

"Enhanced" or "intelligent" L2 switches often have the capability to examine some L3 packet info, such as DSCP even when they otherwise only provide L2 switching.

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