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Label Distribution

After LDP session establishment, labels are distributed from the Tail-End router to the Head-End router for each prefix. This process occurs at the opposite direction of the data plane. At the data plane the traffic flow goes form the Head-End to the Tail-End. But label distribution process is done at the opposite direction.

Label Distribution And Traffic Flow

Label Distribution And Traffic Flow

With these distribution, LIB and the LFIB tables are filled.

FIB, LIB, LFIB


These terms are very important terms in MPLS, We had also talked about this terms before, in the MPLS articles. But to recall, I would like to tell you again below.Lets begin with how this tables are filled.

All the routes to all the destinations are in RIB (Routing Information Base). The best routes to the destinations are choosed and stored in the Routing Table. Then the values in the Routing Table, are copied to the FIB(Forwarding Information Base). Labels for all the destinations are stored in the LIB (Label Information Base). And LFIB(Label Forwarding Information Base) table is filled with the best routes’s label values.

Below you can see the summary of these tables’s job and you can also see how to reach this tables on Alcatel-Lucent service routers.

RIB (Routing Information Base) : All routes to the all destinations

Routing Table : Best routes to the destination

A:># show route route-table

FIB (Forwarding Information Base) : Copy of routing table.

A:># show router fib 1

LIB (Label Information Base) : All labels to all destinations

A:># show router ldp bindings

LFIB(Label Forwarding Information Base) : Labels to the best routes to the destinations

A:># show router ldp bindings active

The default MPLS implementation on ALU routers



– Downstream unsolicited label distribution mode : Labels are distributed to all LDP peers with active LDP sessions ( not like “on Demand”, which send only requested downstream).

– Ordered control mode: LDP router generate and distribute labels for only its own prefixes. If it is egress, the assign a label bindingg to a FEC. (not like independant label distribution control mode, which does not wait for a label from its downstream router and assign a local binding to a FEC).

– Liberal label retention : keep all labels received form all peers, only one label used as active for a given IP prefix (not like conservative label retention, which keeps only labels that will be used).

Here, maybe the above definitions seem complex. But I wrote this with its opposite to recall these MPLS features. Because I think as me, you are also confused on this term usually :)

LDP Label Withdrawal and Release messages

If an LDP router realized a failure, it send a “Withdrawal Message” to notify the peers and remove the binding. When the receiving router gets this message, it send a “Release Message” to the sending node.

The cause of this failures can be , interface failures, MTU changes, missconfiguration or a label clear command like “clear router ldp instance”.

LDP authentication



LDP sessions are established over TCP transport layer. Against the “spoofed” TCP segments MD5 authentication is used.

A:> config router ldp
	peer-parameters
		peer 2.2.2.2
		authentication-key "xyz"
		exit
	exit

MPLS Label Distribution Protocol, LDP – Part 1
MPLS Label Distribution Protocol, LDP – Part 2
MPLS Label Distribution Protocol, LDP – Part 3 (Configuration on ALU)
MPLS Label Distribution Protocol, LDP – Part 4 (ECMP)
MPLS Label Distribution Protocol, LDP – Part 5 (Export and Import Policy, Prefix Aggregation)
MPLS Label Distribution Protocol, LDP – Part 6 (T-LDP)
MPLS Label Distribution Protocol, LDP – Part 7 (CR-LDP)

For more information about LDP, T-LDP and CR-LDP, check the related following RFCs…

RFC 5036: LDP Specification
RFC 3815: Definitions of Managed Objects for the MPLS,LDP
RFC 3478: Graceful Restart Mechanism for Label Distribution Protocol
RFC 5443: LDP IGP Synchronization
RFC 7307: LDP Extensions for Multi-Topology
RFC 7361: LDP Extensions for Optimized MAC Address Withdrawal in a H-VPLS
RFC 3212: Constraint-Based LSP Setup using LDP
RFC 3213: Applicability Statement for CR-LDP 
RFC 3214: LSP Modification Using CR-LDP


…YOU CAN REACH ALL THE “MPLS” ARTICLES AND CONFIGURATIONS BELOW…

MPLS Basics
What is MPLS?
Enabling MPLS on Cisco Router

MPLS Label Distribution Protocol, LDP
MPLS Label Distribution Protocol, LDP – Part 1
MPLS Label Distribution Protocol, LDP – Part 2
MPLS Label Distribution Protocol, LDP – Part 3 (Configuration on ALU)
MPLS Label Distribution Protocol, LDP – Part 4 (ECMP)
MPLS Label Distribution Protocol, LDP – Part 5 (Export and Import Policy, Prefix Aggregation)
MPLS Label Distribution Protocol, LDP – Part 6 (T-LDP)
MPLS Label Distribution Protocol, LDP – Part 7 (CR-LDP)

MPLS, RSVP-TE
MPLS, RSVP-TE – Part 1
MPLS, RSVP-TE – Part 2(Alcatel-Lucent Configuration)

MPLS Protection And Restoration
MPLS Protection and Restoration – Part 1
MPLS Protection and Restoration – Part 2 (End-to-End Protection, Secondary Paths)
MPLS Protection and Restoration – Part 3 (Path Diversity in Secondary Paths)
MPLS Protection and Restoration – Part 4 (Local Protection, Fast Reroute)
MPLS Protection and Restoration – Part 5 (Fast Reroute Protection Types)
MPLS Protection and Restoration – Part 6 (RSVP Objects Used for MPLS Fast Reroute)
MPLS Protection and Restoration – Part 7 (Verification Commands on Alcatel-Lucent Routers)
MPLS Protection and Restoration – Part 8 (Actions After the Failure)
MPLS Protection and Restoration – Part 9 (Convergence Factors)
MPLS Protection and Restoration – Part 10 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 11 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 12 (Juniper Configuration Samples)
MPLS Protection and Restoration – Part 13 (Juniper Configuration Samples)


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About the Author
Gokhan Kosem is a telecommunation and network engineer. His ambition to IP networks and end-to-end system installation made him to prepare this web-site. By sharing his experiences about various networking protocols beside different system installation experiences and Cisco, Juniper, Alcatel-Lucent devices configurations, he is aimed to be helpful for his collegues in all over the world. He is currently lives in Istanbul, Turkey.

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