QoS on a Access Port

[Anonymous]

Is it possible to setup QoS on egress traffic leaving an access
port? Obviously qos trust cos will not work on a access port to classify
traffic coming into the port, but if I use a DSCP to COS mapping and mark
traffic with an DSCP value that will come into the port, will the traffic get
marked and placed in the expedite queue leaving an access port?

[Anonymous · Accepted solution]

Jeff:

Thank you for asking this question in the support community.  Here is the Configuring Ethernet Switch QoS and CoS in AoS guide that may provide you with a better understanding of Layer 2 Quality of Service (QoS)/Class of Service (CoS).

Understanding Layer 2 QoS

The NetVanta Series of Ethernet switches provides Layer 2 QoS functionality, allowing you to better prioritize and manage packets. If all traffic running on a network has equal priority, each packet has an equal chance of being delivered in a timely fashion. Likewise, if congestion occurs on the network, each packet has an equal chance of being dropped.

Layer 2 QoS is based on 802.1p class of service (CoS) values. Using the CoS value (0-7), the switch maps packets to a designated queue. Up to four transmit queues per output interface are available for classifying incoming packets. This allows delay-sensitive or mission-critical traffic to be delivered in a more predictable fashion. When network congestion occurs, lower priority traffic can be dropped to allow higher priority traffic to be delivered.

Setting the CoS Value for Incoming Packets

Traffic is divided into queues using 802.1p CoS values. CoS operates at Layer 2 of the OSI model, and values range from 0 to 7 (0 being lowest priority). By default, all incoming packets are marked with the default CoS value of 0. Therefore if nothing is changed, all packets are sent to the first transmit queue. The default CoS value can be assigned per interface (using the qos default-cos command), OR the interface can be programmed to trust CoS values of incoming packets if the values have already been set (using the qos trust cos command).

In other words, based on the switch configuration, the CoS value will be determined as follows:

• If the interface is set up to accept (trust) predetermined CoS values, incoming traffic retains its current CoS value.

• If the interface is set up to reclassify CoS values, CoS values of inbound traffic are changed to the value specified using the qos default-cos command. If no value has been specified, they will receive the value 0.

DSCP-to-CoS Mapping

CoS values can also be determined based on the packet’s DSCP value (if DSCP-to-CoS mapping is enabled using the qos dscp-cos command). If DSCP-to-CoS mapping is enabled, CoS values are altered when specified DSCP values are detected on inbound traffic.

I hope that makes sense, but please do not hesitate to reply to this post with any additional information or questions.  I will be happy to help in any way I can.

Levi

[Anonymous]

Jeff,

Are you looking to mark the traffic as it leaves the port to insure the correct priority as it enters the next device?  If so, check out Configuring QoS in AOS

See page 24.

This might help.

Dan

[Anonymous]

Levi/Dan,

I’m trying to find out if QoS is only applied to egress traffic, is it only applied to traffic exiting out dot1q trunk ports or is it also applied to traffic going out access ports as well?

[Anonymous]

Levi/Dan,
I’m also trying to understand if it is possible to classify traffic coming into an access port by using a dscp-cos map, when the traffic that is coming into an access port is already marked with an DSCP value.

Thanks,
Jeff

[Anonymous]

Jeff:

Previously, I provided some information regarding layer 2 QoS/CoS for inbound traffic:

Except from above:

CoS values can also be determined based on the packet’s DSCP value (if DSCP-to-CoS mapping is enabled using the qos dscp-cos command). If DSCP-to-CoS mapping is enabled, CoS values are altered when specified DSCP values are detected on inbound traffic.

Also, danb provided the guide for QoS at layer 3, above. 

When you get a chance, maybe some additional clarification will be beneficial.  Will you provide some additional information about the nature of the application, and what you are trying to achieve?  Are your questions related to only layer 2 QoS/CoS or layer 3 QoS?

Levi

[Anonymous]

Jeff:

I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post and unmark it and select another in its place with the applicable buttons.  If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.

Thanks,

Levi

[Anonymous]

The question was never fully answered

[Anonymous]

Jeff:

Please, reply with what additional/remaining questions you have.  We will be happy to assist you in any way we can.

Levi

[Anonymous]

An excerpt from the Ethernet Switch QoS Configuration Guide on page 2

“Layer 2 QoS is based on 802.1p class of service (CoS) values. Using the CoS value (0-7), the switch maps packets to a designated queue. Up to four transmit queues per output interface are available for classifying incoming packets. This allows delay-sensitive or mission-critical traffic to be delivered in a more predictable fashion. When network congestion occurs, lower priority traffic can be dropped to allow higher priority traffic to be delivered.”

The statement from the above excerpt “Up to four transmit queues per output interface are available for classifying incoming packets” are these transmit queues available on all switch ports no matter how the port is configured, that is no matter if the port is configured as a 802.1q trunk port or as an access port, there is transmit queues on that port for QoS.

[Anonymous]

Jeff:

Class of Service (CoS), commonly referred to as 802.1p, is a 3-bit field called the Priority Code Point (PCP) within an Ethernet frame header when using VLAN tagged frames as defined by IEEE 802.1q. It specifies a priority value of between 0 and 7 inclusive that can be used by QoS disciplines to differentiate traffic.  Therefore, there can be transmit queues for each port regardless of the mode, but CoS values will be tagged on 802.1q trunk ports only (this does not include the native VLAN, which is untagged).

Levi

[Anonymous]

I think what Jeff is asking has to do with packets that are already classified with cos or dscp ingress on lets say port 0/1. L3 Routing sends packets out 0/2 which is a access port.  Will the priority mappings on the packets ingressing port 0/1 be honored on port 0/2 as packets egress?

!

interface eth 0/1

  spanning-tree edgeport

  no shutdown

  switchport voice vlan 2

qos trust cos

!

!

interface eth 0/2

  spanning-tree edgeport

  no shutdown

  switchport access vlan 3

qos trust cos

!