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Ordered LSP Control vs Independent

MPLS has two ways of binding labels to FEC’s; either  through ordered LSP control, or independent LSP control.  There is a distinct difference in the way labels are bound between the two modes.

 

Ordered LSP Control

This mode uses a simple rule.  Only bind a label to a FEC if it is the egress LSR, or the router received a label binding for a FEC from the next hop router.  The diagram below illustrates this process.

Although all the other routers may have a entry in the RIB for 10.1.1.1/32, the only router that is allowed to bind a label to it is the Egress LSR.  Only when this router distributes this binding through LDP or TDP towards LSR3 is he allowed to then build his own label binding for this particular FEC (and that is if the Egress-LSR is the actual next hop – i.e. the best path for this FEC).  LSR3 will then distribute it’s own label binding for this FEC towards LSR2.  LSR2 will then bind a label for this FEC so long as LSR3 is the next hop for it.  The process continues until it reaches the ingress LSR.

Independent LSP Control

In this mode, an MPLS router will create a label binding for each FEC and distribute it to its neighbors so long as he has a entry in the RIB for the destination.  So label bindings are made without any dependencies on another router advertising a label for a particular FEC.  Each router makes it own independent decision to create a label for each FEC.  The diagram below illustrates this.

By default IOS uses Independent LSP Control

5 Comments

Krishna PrakashJune 7th, 2013 at 7:41 am

Clear Explaination

Anurag JJanuary 18th, 2014 at 6:16 am

Too good explanation. Really got something which i was missing while studying from RFC 3031.

TobiasDecember 15th, 2014 at 8:28 am

Great and clear explanation yet again! Keep up the good work!

Pradeepkumar BaratekkeApril 10th, 2015 at 10:12 am

Very good explanation with relevant diagrams!!

Sreekanth BadabagniAugust 31st, 2017 at 1:52 am

Thanks for this info, got a good start.

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