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Table of Contents
CR-LDP (Contraint-Based Routing LDP) is an extension of LDP (Label Distribution Protocol) and with these extensions, Contraint-based Routing is supported on LDP. It is used in MPLS networks to establish Label Switched Paths (LSPs) based on specific constraints, such as bandwidth, delay or policy requirements. In MPLS networks, Contraint-Based Routing LDP allows routers to create Traffic-Engineered paths instead of just following the shortest path. In other words, Contraint-based Routing brings Traffic Engineering(TE) mechanism to the LDP.
Basic MPLS Lessons:
CR-LDP LSP (Constraint-Based Path)
Contraint-based Routing is the routing which in which the routing process is done according to some constraints. Both QoS-based routing and policy-based routing belong to Contraint-based Routing. As you know these routing include QoS constraints or policy constraints.
We have learned what is CR-LDP basically. How about its mechanims? How CR-LDP Works? Here, firstly the ingress router defines constraints like minimum bandwidth. Then, Contraint-Based Routing LDP signals a path through the MPLS network that meets these constraints. Routers along the path allocate labels. Lasltly, a Label Switched Path (LSP) is created based on those requirements.
This mechanism provides the optimum path selection. With Contraint-Based Routing LDP, we do not select the shortest path but we can select the path that;
Both CR-LDP and RSVP-TE are used for MPLS Traffic Engineering, but RSVP-TE is the modern and widely used protocol, while Contraint-Based Routing LDP is obsolete. below, you can find the comparison table of CR-LDP vs RSVP-TE.
| Feature | CR-LDP | RSVP-TE |
|---|---|---|
| Protocol Type | Extension of LDP | Extension of RSVP |
| Transport | TCP | IP (Protocol 46) |
| Traffic Engineering | Yes | Yes (Advanced) |
| Resource Reservation | Limited | Strong |
| State Type | Hard State | Soft State (Refresh) |
| Usage | Obsolete | Widely Used |
The additional TLV parameters are new for Contraint-Based Routing LDP:
Value Parameter ----- --------- 821 LSPID 822 ResCls 503 Optical Session Parameters 800 Explicit Route 801-804 ER-Hop TLVS 810 Traffic Parameters 820 Preemption 823 Route Pinning 910 Optical Interface Type 920 Optical Trail Desc 930 Optical Label 940 Lambada Set
For more information about LDP, T-LDP and CR-LDP, check the related following RFCs:
Basic MPLS Lessons:
