OSPF Remote Loop-Free Alternate (LFA) Fast Reroute (FRR)

Lessons Discussion
1

This topic is to discuss the following lesson:

2

Hi Rene,

Regarding the remote LFA version, when you say “P Space: these are the routers that R1 can reach without using the failed link”, do you mean that, under normal operations, these are the routers that R1 is able to reach without using the link that might fail? Otherwise, why not add R4 to the P Space?

The same question applies for the Q Space.

I’ve posted my question here because I don’t see a dedicated discussion for the remote LFA.

Thanks,
LP

3

Hello Luis

The official definition according to RFC 4790 is the following:

  The P-space of a router with respect to a protected link is the
  set of routers reachable from that specific router using the pre-
  convergence shortest paths without any of those paths (including
  equal-cost path splits) transiting that protected link.

  For example, the P-space of S with respect to link S-E is the set
  of routers that S can reach without using the protected link S-E.

So for R1, the routers that are reachable without the use of the failed link are R2 and R3 under normal circumstances . R4 is actually not reachable because it is reached via the link to R5.

The same goes for the Q-space, which includes R4 and R3, but not R2, because R2 would have gone through R1 to get to R5.

Remember that this feature is used particularly for loop topologies. The point of finding the P and Q spaces, is to find the PQ node, which is the only router that exists in both the P and the Q spaces. This is the first router along the loop that will have an OSPF route R5, and this is the router that is identified as the one with which the tunnel must be made.

I hope this has been helpful!

Laz

PS Thanks for pointing out that there was no forum topic for this lesson. It has been fixed…

2 Likes
4

Remote LFA and LFA , these are 2 separate terms …right …

can u describe little bit difference between them…??

5

Hello Narad

In short, an LFA is a next-hop route that delivers a packet to its destination without looping back.

A Loop-Free Alternate (LFA) is a concept that is used in various technologies, including OSPF, other routing protocols, as well as in MPLS. An LFA is a node other than the primary neighbor, or the primary next hop. It is a backup of sorts. Traffic is immediately directed towards the LFA after a network failure. The LFA will receive traffic and will continue to forward it without any knowledge of a failure. LFAs always use a directly connected next hop.

Remote LFA involves the use of a tunnel to a next-hop that is not directly connected, as shown in the lesson. This is the primary difference between the two.

I hope this has been helpful!

Laz

6

Here we did enable LFA FRR because as the article says we do need to enable it for remote LFA FRR.
Question:
What will happen when both LFA and remote LFA options are available.
*I am guessing that normal LFA will be used because remote LFA was introduced only to deal with situations where normal LFA option cannot be used. Kindly elaborate.

7

Hello Muhammad

Remote LFA is a feature that is required in situations where you want to enable the functionality of LFA, but the topology is such that it doesn’t function as desired.

You’ll notice from the lesson that in order for remote LFA to be enabled, you must first enable “normal” LFA. This is enabled on all routers. So if you’ve configured remote LFA, if it is not needed for the particular topology, it will revert back to “regular” LFA.

In this specific lesson, if you take a look at the output of the show ip route 6.6.6.6 command on R3, you will find that normal LFA will have been employed.

I hope this has been helpful!

Laz

8

hello Laz,

In this example we have focused on R1, How about the other routers perspective is it possible that for example R3 will use regular LFA instead of RLFA?

secondly what about Topology Independent Loop Free Alternative does it vary from Remote LFA and normal LFA?

9

Hello Ahmedlmad

In the lesson, the configuration requires that all routers in the topology be configured with the RLFA parameters. This means that all routers will attempt to use RLFA. However, there are cases where some routers may use LFA in such a topology.

In situations where a suitable RLFA path is not available (due to topology constraints, lack of PQ nodes, or other reasons), Cisco routers may automatically use LFA, if an LFA path is available and feasible. This fallback is built into the OSPF implementation to ensure network resilience. According to this Cisco Documentation, RLFA and LFA work together and in conjunction with OSPF to deliver these backup reroute paths.

Topology Independent Loop Free Alternative (TI-LFA) is a more advanced form of LFA. While the standard LFA and RLFA are dependent on the topology and may not always provide a backup path, TI-LFA provides a guaranteed backup path regardless of the topology. This is achieved by using segment routing, which allows the router to pre-determine the backup path rather than relying on the network’s current state.
More info on this can be found here:

I hope this has been helpful!

Laz

1 Like