How to configure QoS trust boundary on Cisco Switches

Lessons Discussion
108

Hello Rene/Laz,

  1. If no QoS configuration change is made to a brand new Catalyst 3850 switch (with default QoS configuration), how does it (Catalyst 3850) treat QOS marking on its inbound Access Ports (end device is marking the packets)? Does the switch erase the marking and set it to all 0s (best effort)?

  2. Does a Cisco IP camera or Cisco IP phone mark traffic with COS (L2) value or TOS (L3) value or both?

  3. Can a router’s LAN L3 interface (inbound) be configured to mark traffic with COS (L2) value?

  4. Can a QoS Shaper be configured on a router’s WAN L3 interface (outbound) based on the COS marking (L2)?

  5. What is the use of using COS marking? To my understanding, COS value can be used only to manipulate traffic in terms of QoS on the trunk links; is this correct? Can you please provide a configuration example for a Catalyst 3850 switch?

  6. According to Rene’s one of the posts, only L2 trunk ports are capable of marking traffic with COS value. If that is the case, in the scenario below where end devices are marking traffic with DSCP value, how some traffic can be prioritized over others through the trunk link from Cat-3850-SW2 to Cat-3850-SW1?

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Thanks in advance.