Segment Routing TI-LFA

TI-LFA - Topology Indpendent - Loop Free Alternate is fast reroute for Segment Routing. This video describes the P-Space and Q-Space and how they're used to create a post convergence path immediately.

[@tolgayucel1442]

It's surprising for me to learn your channel. Excellent video. Appreciate it

[@BarbaraAnne]

So nice of you!

[@algabsi6323]

Thank you, Barbara. Great Video!

[@BarbaraAnne]

Thank you!

[@josemanuelpinalesfiguereo6859]

Excellent and very detailed explanation. I was struggling getting the P and Q knows. Now I have a better understanding. Also, a great point was how to the P4 router calculates what label to push, whether the prefix sid or a adj sid label.

[@BarbaraAnne]

Thank you!

[@ashrafzakaria1267]

Very Good Explanation, Thanks Barbara

[@BarbaraAnne]

Thanks Ashraf!

[@networkstuff5152]

Great video, congratulations for your very clear explanation

[@BarbaraAnne]

Thanks! Glad you liked it.

[@eng.bensonkalokin.6463]

Exactly the explanation I needed! Great 👍

[@BarbaraAnne]

Glad it was helpful!

[@matthewjohnson9184]

Hi Barbara, Can you explain why P5 is considered in the Q-space as it is not on the post computed shortest path.

[@BarbaraAnne]

Hi Matthew - In this example, P5 is in the Q-space because it's shortest path from P5 to P9 never traverses the potential broken link between P4 and P5.
For the post computed shortest path, it is looking for a node that is in the P space and the Q space. P5 doesn't fit into that category and so it's not on the post computed path.

[@babamohamed452]

Hi @Barbara,
Thanks for your explanation. I have one question please, why PE2, PE3 are not selected in the Q space knowing that they can reach Dest without passing through the protected link?

[@BarbaraAnne]

Hi Mohamed. Their link metrics are 5000 so they are not on a shortest path. The green links have a metric of 1 and the orange have a metric of 5000.

[@babamohamed452]

@@BarbaraAnne Thanks dear for your reply. But I still think that PE2 can be in Q space :)
Please check my below understanding of the topology:
- PE3 can reach P9 through (P5--P6--P9) --> PE3 is not in Q space as it passes through P5 (I was wrong in my 1st comment ;)
- PE2 can reach P9 through (P6--P9) --> PE2 can be in Q sapce as it is not passing through the protected link P4-P5.
Please correct me if I'm wrong
Thanks

[@BarbaraAnne]

@@babamohamed452 TI-LFA is trying to find the shortest path so that when the IGP reconverges, it doesn't have to change the Service Path. PE2->P6->P9 has a path cost of 5000+1=5001 whereas P2->P3->P6->P9 has a cost of 1+1+1=3 so that's the route the converged IGP will take. It's possible to take that route but it is not the BEST route because of the link metrics. TI-LFA is trying to find the best route.