JNCIA Class of Service 3

Question 1

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

Accepted Answer

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

Answer

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.

Question 2

In Junos CoS, how are 802.1p bits used?

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

How does Junos CoS interact with MPLS traffic?

Accepted 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

Answer

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.

Question 5

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

Accepted 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

Answer

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.

JNCIA - Juniper Networks Certified Internet Associate Practice Test

JNCIA - Juniper Networks Certified Internet Associate Practice Test

JNCIA Class of Service 3