One of the customer is to implement LFA feature for LDP based distribution. First of all LFA is loopfree alternate which helps in faster convergence when there is a disruption in the physical links.

The Juniper Way

A Nice detailed explanation is given below in the KB


How does this Happen  ?

Juniper as most of you know has a forwarding Table Built in and also a Control Plane Table Sepearately. Typically if a link goes down, IGP has to calculate back its way to find an alternate path (if there is no ECMP), which might prove costly for VOIP/VIDEO based application streaming

Implemeting LFA in Juniper Networks Router will simply install alternate path in the forwarding table provisioning for a quickly turn-around in case of a flap.



Lets first check if the protocols are up and vpn site is reachable


Implementing LFA on R1 first, as you can clearly see R1 has only one-link to go forward and there should be no LFA eligible path for R1. Lets verify


Lets implement the same on R2, this time we will also write a load balancing policy to export the Entry into forwarding table, as we know juniper will not install secondary path into forwarding table by default

R2 has another link to reach R4 and there by the End-Destination through R3 in the even R2-R4 link breaks


Lets Examine the LFA possible options


As we can clearly see there is a path-listed in the computatin, let see them via protocols (mpls/forwarding plane). The reference is from Router-R2 to Router-R5 with primary and backup paths


Understanding the results with and without LFA implementation



I hope this helps anyone who is starting with LFA and wanted to understand how it mixes with IGP and installs the best path for rapid convergence.



Rakesh M