I85PREP
⌘ K
MPLS study materials
MPLS study resources

MPLS: Traffic Engineering and QoS in MPLS

MPLS · Work through your exam, one topic at a time.

All guide sections

Traffic Engineering and QoS in MPLS

What this domain tests

Traffic Engineering and QoS in MPLS tests constraint-based path selection, RSVP-TE tunnel attributes, and how MPLS carries QoS. You should distinguish TE metrics, admin groups, bandwidth reservations, and EXP/DiffServ handling.

Core rules

  • CSPF computes a path after pruning links that fail constraints such as available bandwidth, admin-group affinity, or TE metric. The result is signaled by RSVP-TE.
  • A TE tunnel is unidirectional. A return path needs a separate tunnel or normal IGP forwarding.
  • RSVP-TE reserves bandwidth at each hop; overbooking is controlled by reservation multipliers and link capacity.
  • MPLS EXP bits can carry QoS marking, but DiffServ tunneling modes matter: uniform mode copies IP DSCP to EXP and back; pipe mode preserves the outer EXP treatment and hides inner marking changes.
  • Queuing, policing, and shaping are applied per hop; MPLS alone does not create QoS without a configured policy.

Worked example

A voice flow needs 20 Mbps from PE1 to PE2 and must avoid a satellite link. The admin group on that link is "high-delay". PE1 configures an affinity that excludes "high-delay" and a bandwidth of 20 Mbps. CSPF removes the link, finds a path with sufficient reserved bandwidth, and RSVP signals it. The voice traffic is steered into the tunnel; each transit router reserves 20 Mbps and applies the configured queue to the EXP value carried by the packet.

Traps

Do not say CSPF reserves bandwidth; RSVP does. Do not confuse TE metric with IGP metric. Do not assume EXP equals DSCP. A tunnel in one direction does not guarantee the reverse path.