FRR is configured on a network. What occurs when the headend router on the path is alerted to a link failure over IGP?
FRR is configured on a network. What occurs when the headend router on the path is alerted to a link failure over IGP?
When Fast Reroute (FRR) is configured on a network, it uses pre-established backup LSPs (Label Switched Paths) to handle failures quickly. Upon being alerted to a link failure via IGP, the headend router uses these presignaled LSPs to bypass the failure point. This rapid switch to the backup path ensures minimal disruption in network traffic and maintains continuity until the primary path is restored or a new path is established.
It should be B: MPLS Fast Reroute (FRR) is a fast recovery local protection technique that protects TE LSPs from link, shared risk link group (SRLG), and node failure. One or more TE LSPs (called backup LSPs) are preestablished to protect against the failure of a link, node, or SRLG. If there is a failure, each protected TE LSP traversing the failed resource is rerouted onto the appropriate backup tunnels.
Answer: B https://www.cisco.com/en/US/docs/ios/12_0st/12_0st10/feature/guide/fastrout.html Fast Reroute provides link protection to LSPs. This enables all traffic carried by LSPs that traverse a failed link to be rerouted around the failure. The reroute decision is completely controlled locally by the router interfacing the failed link. The headend of the tunnel is also notified of the link failure through the IGP or through RSVP; the headend then attempts to establish a new LSP that bypasses the failure.
Answer: B From cisco SPCORE online materials: The FRR mechanism allows data to continue to flow while the headend router attempts to establish a new end-to-end LSP that bypasses the failure.
Um tunnel TE é pré-sinalizado com FRR para proteger o LSP. B está correta.
Answer A is Correct. https://networklessons.com/mpls/mpls-traffic-engineering-te-fast-reroute-frr
Agree answer A. Answer B is wrong because the Headend is not involved in repair unless it is failure of a direct link or NHOP. Head End may recieve an initial PathErr error. Then the LSP fixes itself by using a pre-signalled backup path at the PLR node, and sends traffic toward the MP. The HeadEnd will then still receive RESV messages. Network lessons covers it. I vote for A
However, rethinking how the question is worded, The question says that Headend is alerted to a link failure over IGP. So this seems to mean an error from a direct IGP connection. In that case, the head end is itself the PLR and will use its own pre-signalled LSP to bypass the failure point. I change my vote to B.
For me is B. FRR is always pre-signaled, backup tunnel and FRR are totally two different things. https://www.ciscopress.com/articles/article.asp?p=520184&seqNum=4 " FRR uses a local protection approach that relies on a presignaled backup TE LSP to reroute traffic in case of a failure. The node immediately next to the failure is responsible for rerouting the traffic and is the headend of the backup TE LSP."
I think B as FRR is using pre-established to use directly after fail detection. All other options mention try setting up a new path after failure.
B is wrong: The headend router is alerted via IGP, so it is not a local failure and no FRR is happening on a presignaled LSP. If there is a backup path, it will switch to it. If there is only one path and it is not constrained over the failed element, then it will be recalculated.
From Cisco training (no link) if it even helps? The important fact to remember is that when using fast reroute, the protection kicks in as soon as the protected interface fails, which in turn forces the head end to recalculate a new path for the dynamic Cisco MPLS TE tunnel. In that brief period of time, between when the head-end of tunnel 1 realizes that the path is invalid and when the new path is calculated, the fast reroute happens via a predefined tunnel. Fast Reroute prevents end to end traffic loss during the path recalculation. When the headend of the tunnel is notified of the path failure through the IGP or through RSVP, it attempts to establish a new LSP. The headend is notified of the failure through the IGP and through RSVP. The headend then attempts to establish a new LSP that bypasses the failure (LSP rerouting).
None of the question have the best wording but I believe "A” is the is the most accurate option. The node that detects the failure switch de traffic to the already established alternate path without intervention of headend router. https://www.cisco.com/en/US/docs/ios/12_0st/12_0st10/feature/guide/fastrout.html
I think D based on the document below A - NO because "fast switching" not relevant or mentioned B - NO because there is no mention about the headend router using a presignaled LSP C - NO because "It should not pass through the element it protects". D - YES because "It should intersect with a primary tunnel at a minimum of two nodes: point of local repair (PLR) and merge point (MP). The PLR should be the headend LSR of the backup tunnel, and the MP should be the tailend LSR of the backup tunnel. The PLR is where FRR is triggered when a link, node, or SRLG failure occurs." https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_te_path_protect/configuration/12-2sy/mp-te-path-protect-12-2sy-book/mp-te-inter-as-te.html#:~:text=or%20tunnel%20destination.-,Fast%20Reroute,-MPLS%20Fast%20Reroute
This question is not formulated ok. I agree with D(70%), but A also can be right. Can anyone share another opinion?
From CISCO doc : LSPs that are actively using backup tunnels are not considered for promotion. If an LSP is actively using a backup tunnel and a better backup tunnel becomes available, the active LSP is not switched to the better backup tunnel.------A is wrong , for me "D" is the correct answer https://www.cisco.com/c/en/us/td/docs/routers/asr920/configuration/guide/mpls/mp-te-path-protect-xe-3s-asr920-book/mp-te-frr-node-prot.html
Agreed, it makes sense
on a second thought, "D. Backup tunnel is established and intersects with the primary tunnel at the headend." but for FRR, backup tunnels are pre-establisted even before a link failure. Therefore option B seems a better option.