MPLS/RSVP configuration & troubleshooting #3

This is the 3rd post of my MPLS/RSVP series. In the first and second, I set up an MPLS cloud with some sort of redundancy. In this post, I will enable traffic engineering support on OSPF in order to use CSPF and fast reroute feature. To explain fast reroute I need the topology again;


In a standard MPLS setup without fast reroute, if you have an LSP from J35 to J40 (Path: J34->J30->J29) and link between J30 and J29 breaks, it will take time for PATH error message to be received by J35 ingress router. However, if you enable fast reroute every router along the path will have alternate PATH available in case its link breaks and detours very quickly and will keep forwarding the traffic till the new LSP is established by the ingress router.
Please note that this is a temporary workaround to keep the traffic flowing without any disruption. Now it is time to get into the CLI to see how this works;

We must enable traffic engineering on OSPF and CSPF on MPLS. Otherwise fastreroute doesn’t work. This is what I have seen at least. In addition to this, my OSPF setup is multiarea for which I have to enable expand-loose-hop option in every MPLS router. According to the description from Juniper page “ it allows an LSP to traverse multiple OSPF areas within a service provider’s network.” Also according to juniper docs, if you configure an interarea LSP, you must set inter-domain option.

Execute the followings in all routers (P,PE)

Additionaly set inter-domain and fast-reroute option in your LSPs on your PE routers

In the outputs below look for the “FastReroute desired” keyword.

In order to show you what an amazing technology this fast reroute is, I have removed previous primary and secondary paths.

I let the PE router to choose OSPF route for its exit interface towards network and it chose J34 ( as the next hop.

I initiated ping from J36 device and in the mean time I disabled interface between J34-J30 which is along the path of LSP

You can see that only one DOT is visible. Fastreroute detoured the traffic immediately. You can see it below from detour down and up messages.

I hope I could give some information about fast reroute in this post. In the next post, I will possibly set up a BGP L3VPN connection.

Click here for the 4th post

Stay tuned!

2 thoughts on “MPLS/RSVP configuration & troubleshooting #3

  1. Steve

    The best explanation i have ever found online. Can you please tell me from where did you get JUNOS images to set up your own lab? Thanks again for this useful info.


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