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Catalyst 9300 qos

Folks,

             Looking at 9300 qos document it says that the default behavior of the cat 9300 is to trust DSCP. Does anyone know what is the behavior of the cat 9300 for uplinks(trunks).

 

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Accepted Solutions
Cisco Employee

Re: Catalyst 9300 qos

Hello,

 

It depends on the type of ingress and egress interface:

 

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9400/software/release/16-6/configuration_guide/qos/b_166_qos_9400_cg/b_166_qos_9400_cg_chapter_01.html

 

Look for the table under "Trust Behavior for Wired Port"

 

1) If you come in a layer 3 interface, and leave a layer 3 interfaced. We will trust DSCP. 

2) If you come in a layer 2 interface and leave a layer 2 interface, we don't alter the packet so DSCP is conserved. Since there is no cos because there is no tag, there will still be no cos.

3) If you come in a trunk and leave a trunk, we will trust both DSCP and CoS

4) If you come in a layer 3 interface and leave a trunk, we will trust DSCP. Since the layer 3 interface doesn't have a cos value, we will set it to 0. 

 

Hope that helps!

 

-Bradley Selzer
CCIE# 60833
Cisco Employee

Re: Catalyst 9300 qos

It depends:

 

1) If it is just normal data traffic, we would set cos to 0 since there is no tag on an access port.

2) If it is a phone in a voice vlan, the phone tags traffic even on an access port. We will preserve whatever the tag is. 

 

Essentially, if there is a situation where there is no tag, we will set it to 0. If there is, we will preserve it. However, this is just the default behavior. It can easily be overwritten with a policy-map.

 

Hope that helps!

-Bradley Selzer
CCIE# 60833
3 REPLIES 3
Cisco Employee

Re: Catalyst 9300 qos

Hello,

 

It depends on the type of ingress and egress interface:

 

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9400/software/release/16-6/configuration_guide/qos/b_166_qos_9400_cg/b_166_qos_9400_cg_chapter_01.html

 

Look for the table under "Trust Behavior for Wired Port"

 

1) If you come in a layer 3 interface, and leave a layer 3 interfaced. We will trust DSCP. 

2) If you come in a layer 2 interface and leave a layer 2 interface, we don't alter the packet so DSCP is conserved. Since there is no cos because there is no tag, there will still be no cos.

3) If you come in a trunk and leave a trunk, we will trust both DSCP and CoS

4) If you come in a layer 3 interface and leave a trunk, we will trust DSCP. Since the layer 3 interface doesn't have a cos value, we will set it to 0. 

 

Hope that helps!

 

-Bradley Selzer
CCIE# 60833
Beginner

Re: Catalyst 9300 qos

What is your come in as layer 2 and go out through a trunk?

Cisco Employee

Re: Catalyst 9300 qos

It depends:

 

1) If it is just normal data traffic, we would set cos to 0 since there is no tag on an access port.

2) If it is a phone in a voice vlan, the phone tags traffic even on an access port. We will preserve whatever the tag is. 

 

Essentially, if there is a situation where there is no tag, we will set it to 0. If there is, we will preserve it. However, this is just the default behavior. It can easily be overwritten with a policy-map.

 

Hope that helps!

-Bradley Selzer
CCIE# 60833
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