Class of Service Flashcards

What is Hierarchical CoS (H-CoS) in Junos?

Answer: A multi-level scheduling architecture that applies CoS at both aggregate (customer/physical port) and per-flow levels simultaneously

H-CoS provides two or more levels of scheduling: physical port shaping at the outer level and per-flow or per-subscriber queue management at the inner level, enabling per-customer SLA enforcement on shared infrastructure.

In Junos CoS, how are 802.1p bits used?

Answer: They are the 3-bit priority field in 802.1Q VLAN tags used to mark CoS for Ethernet frames on trunk/tagged interfaces

The 802.1p (PCP — Priority Code Point) bits are the 3-bit priority field in the 802.1Q VLAN tag. Junos classifiers can map these 8 priority values (0-7) to forwarding classes for CoS on Ethernet networks.

What is a traffic control profile (TCP) in Junos CoS?

Answer: A configuration object that specifies shaping rates and scheduler maps for hierarchical CoS, applied to logical interfaces

In Junos H-CoS, traffic control profiles define aggregate bandwidth constraints (shaping rate, overhead accounting) and reference scheduler maps, providing the building block for multi-level CoS on MX platforms.

How does Junos CoS interact with MPLS traffic?

Answer: MPLS EXP (now TC) bits in the label stack carry CoS markings that Junos classifiers and rewrite rules can read and set at MPLS push/swap/pop points

MPLS label entries contain a 3-bit Traffic Class (TC, formerly EXP) field that carries CoS markings through an MPLS network. Junos can classify incoming MPLS traffic and rewrite TC bits at label operation points.

What is the purpose of the Junos CoS 'shaping-rate' parameter in a scheduler?

Answer: To enforce an upper limit on the transmission rate of a queue, preventing it from consuming more than its allocated bandwidth even when other queues are idle

The 'shaping-rate' in a Junos scheduler sets the maximum bandwidth a queue can use. Unlike 'transmit-rate' (which is a minimum guarantee), shaping-rate creates a hard ceiling regardless of available capacity.

In Junos, how do you verify that CoS is functioning correctly on an interface?

Answer: show class-of-service interface [interface] and show interfaces [interface] extensive to view queue statistics and CoS assignments

'show class-of-service interface [interface]' displays the applied classifiers, scheduler maps, and rewrite rules. 'show interfaces extensive' shows per-queue packet/byte/drop statistics confirming classification is working.