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Will the switch change the DSCP value in Layer 3 forwarding by default? Thanks!


Switch configured vrrp for hosts, and hosts send traffic with DSCP. By default, when switch forwarding the traffic to network, how the DSCP with tag, the original one or change to 0? Thanks!

8 replies

Userlevel 6
Hey Evan

The switch should not change the DSCP by default when forwarding at L3. We can change the DSCP by changing the DSCP value of a queue and re-write it.

One thing to be aware of is that by default .ip has precedence over DSCP in the QoS of a packet. So if a packet is set as .1p value of 0 and a DSCP of 46 and we don't change the precedence of how the switch looks at QoS we will look at the 0 value before we look at the DSCP value.

Hope that helps

P
Paul Russo wrote:

Hey Evan

The switch should not change the DSCP by default when forwarding at L3. We can change the DSCP by changing the DSCP value of a queue and re-write it.

One thing to be aware of is that by default .ip has precedence over DSCP in the QoS of a packet. So if a packet is set as .1p value of 0 and a DSCP of 46 and we don't change the precedence of how the switch looks at QoS we will look at the 0 value before we look at the DSCP value.

Hope that helps

P

How can we change the precedence of how switch look at 0 value before look at DSCP
Userlevel 6
Paul Russo wrote:

Hey Evan

The switch should not change the DSCP by default when forwarding at L3. We can change the DSCP by changing the DSCP value of a queue and re-write it.

One thing to be aware of is that by default .ip has precedence over DSCP in the QoS of a packet. So if a packet is set as .1p value of 0 and a DSCP of 46 and we don't change the precedence of how the switch looks at QoS we will look at the 0 value before we look at the DSCP value.

Hope that helps

P

Hello the only way to change precedence to have the switch look at DSCP over .10 is to disable .1p or use an acl to look at DSCP and assign it to a queue.
Hey Palu

Thanks a lot for the reply. It helps much.

We need to keep the DSCP to the network for end-to-end QoS deployment. If switch will not change the DSCP, then we need not configure any QoS on Switch. 🙂

Thanks

BR/Evan
Userlevel 6
Hey Evan

Yes the DSCP will stay in tacked but if you want the switch to honor the code-point then we need to set up the queue and map the DSCP into that queue.

I hope that makes sense. I just didn't want you to think that the switch would automatically keep that traffic at a higher throughput.

Let us know if we need to provide more information.

Thanks
P
Userlevel 6
Hello Evan,

To piggyback off what Paul said you will need to disable dot1p examination because this takes precendence over diffserv (DSCP) and is enabled by default. The command to do this is "disable dot1p examination ports all". As far as the diffserv configuration you can follow the article below:

https://gtacknowledge.extremenetworks.com/articles/How_To/Configuring-QOS-profiles-and-verify-QOS-is...

Let us know if you have any questions.
Userlevel 7
Starting with 16.1 and above, you shouldn't be able to stack QoS config on a port.
Hey Paul, Hello Patrick,

Thanks much for your reply, for the time being, it is OK.

Thanks

BR/Evan

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