Wide Metrics and Traffic Engineering Extensions
What the exam tests
This domain tests the TE TLVs and sub-TLVs, the difference between IGP metric and TE metric, and the prerequisites for MPLS TE CSPF.
Core rules
- TLV 22 advertises extended IS reachability; TLV 135 advertises extended IP reachability.
- TLV 22 sub-TLVs carry TE metric, maximum link bandwidth, maximum reservable bandwidth, unreserved bandwidth, administrative group and SRLG.
- TE requires wide metrics; narrow metrics cannot carry the required attributes.
- MPLS TE uses CSPF, which prunes links that fail bandwidth or administrative-group constraints, then computes the shortest path using the TE metric.
- The TE metric can differ from the IGP metric, so IP forwarding and TE LSP paths may diverge.
- Nokia SR OS enables TE under the IS-IS instance and allows a per-interface TE metric.
Worked example
Two parallel links both have IGP metric 10. Link 1 has TE metric 100 and 1 Gbps reservable; link 2 has TE metric 200 and 10 Gbps reservable. IP traffic ECMPs across both because IGP costs are equal. A CSPF LSP requesting 500 Mbps prefers link 1 because its TE metric is lower, provided the bandwidth constraint is met. A request for 5 Gbps is forced onto link 2.
Traps
Enabling TE without wide metrics; assuming TE metric equals IGP metric; forgetting that CSPF checks bandwidth and admin groups; not advertising unreserved bandwidth; expecting TE paths to affect normal IP routing.