Networks Horizon

Constraint-based Routing LDP (CR-LDP)


Constraint-based Routing LDP (CR-LDP) is a simple, scalable, open, non- proprietary, traffic engineering signaling protocol for MPLS IP networks. It is an extension of the Label Distribution Protocol (LDP), one of the protocols in the Multiprotocol Label Switching architecture. CR-LDP contains extensions for LDP to extend its capabilities such as setup paths beyond what is available for the routing protocol. For instance, a Label Switched Path can be set up based on explicit route constraints, quality of service constraints, and other constraints. Constraint-based routing (CR) is a mechanism used to meet traffic engineering requirements.CR-LDP is almost same as basic LDP, in packet structure, but it contains some extra Type,Length,Value(TLVs) which basically set up the constraint-based LSP.


CR-LDP provides mechanisms for establishing explicitly routed Label Switched Paths (LSPs).  These mechanisms are defined as extensions to LDP.  Because LDP is a peer-to-peer protocol based on the establishment and maintenance of TCP sessions, the following natural benefits exist:

1. CR-LDP messages are reliably delivered by the underlying TCP, and State information associated with explicitly routed LSPs does not require periodic refresh.
2. CR-LDP messages are flow controlled (throttled) through TCP.

CR-LDP allows the specification of a set of parameters to be signaled along with the LSP setup request.  Moreover, the network can be provisioned with a set of edge traffic conditioning functions based on intServ(which could include marking, metering, policing and shaping).  


In particular CR-LDP and RSVP-TE are two signaling protocols that perform similar functions in MPLS networks.  Until now, no consensus on which protocol is technically superior. RSVP-TE has been opted by two major vendors CISCO and JUNIPER. Therefore, it has become a standard now. The IETF MPLS working group has also deprecated CR-LDP and decided to focus purely on RSVP-TE.




Comparision between CR-LDP and RSVP-TE.

• RSVP-TE is a soft state protocol and needing periodic refreshing RSVP-TE uses raw IP but CR-LDP uses TCP for redistributing labels and UDP for discovering neighbors 
• Only RSVP-TE provides failure notification. 
• CR-LDP is promoted by Nortel but RSVP-TE is accepted by CISCO, Juniper.
• CR-LDP is little slower than RSVP-TE as it is based on TCP.
• CR-LDP uses the ATM's traffic engineering terms to map QOS whereas RSVP-TE is paired with IntServ(Cisco Integrated Services) detecton of QOS.