MPLS LDP FRR Using ISIS As IGP

We would be describing the MPLS LDP FRR utilizing ISIS as the IGP. You could gain more details regarding the same by the courses offered at a reliable institute, like the SPOTO.

Applicability:

MPLS Label Distribution Protocol Fast Re-Route (LDP FRR) is supported on all 7x50 platforms including the 7710 SR and 7710 SR c-4/12 or the 7750 SR c-4/12. This feature is supported on all IOM/IMMs and MDA/CMA types that support network interfaces from 9.0R4 and higher. This feature was tested on release 12.0.R5. There are no pre-requisites for this configuration.

Summary:

LDP FRR which would be improving convergence in case of a single link or single node failure in the network. Convergence times will be in the order of 10s of milliseconds. This is important to some application services, like VoIP, which are considered to be sensitive to traffic loss when running over the MPLS network.

Without FRR, link and/or node failures inside an MPLS LDP network would be resulting in traffic loss in the order of 100s of milliseconds. The reason for that is that LDP depends on the convergence of the underlying IGP (IS-IS sending LSPs in this case). After IGP convergence, LDP itself is going to be needed to compute new primary next-hop Label Forwarding Entries (NHLFEs) for all affected Forwarding Equivalence Classes (FECs). Finally, the different Label Forwarding Information Bases (LFIBs) are updated.

When FRR is configured on a node, the node pre-computes primary NHLFEs for all FECs and in addtion, it will pre-compute backup NHLFEs for all FECs. The backup NHLFE corresponds to the label which would be received for the same FEC from a Loop-Free Alternate (LFA) next-hop. Both primary NHLFEs and backup NHLFEs are programmed in the IOM/IMM which makes it possible to converge very quickly.

Implementation

The 7x50 software would be implemented Inequality 1 (link criterion) and Inequality 3 (node criterion) of RFC 5286. Similar to the SPT (Shortest Path Tree) computation that is part of standard link-state routing functionality, also the LFA next-hop computation is based on the IGP metric.

The underlying LFA formulas would be appearing in the following format:

Inequality 1: [SP(backup NHR, D) less than {SP(backup NHR, S) plus SP(S, D)}]

Inequality 3: [SP(backup NHR, D) less than {SP(backup NHR, PN) plus SP(PN, D)}]

With ‘SP’ is equal to ‘shortest IGP metric path’, ‘NHR’ is equal to ‘next-hop router’, ’D’ is equal to ‘destination’, ‘S’ is equal to ‘source node or upstream node doing the actual LFA next-hop computation’ and ‘PN’ is equal to ‘protected node’. The inequality 3 rule is considered to be stricter than the inequality 1 rule.

Configuration

This section provides information to configure:

•    Configuring the IP/MPLS network.

•    Enabling LDP FRR and verifying with show commands.

•    Enable a synchronization timer between IGP and LDP protocol.

•    Data path verification using a Layer 2/VLL service.

Conclusion

In production MPLS networks where FRR would be needed to be deployed, a tradeoff must be made between RSVP-TE FRR versus LDP FRR. The two main advantages of using LDP FRR compared to RSVP FRR are simple configuration and LFA next-hop calculation is a local decision, which means no interoperability issues when working in a multi-vendor environment. The main disadvantage of using LDP FRR is that LFA next-hop calculation has to deal with the source-route paradigm, inequality formulas exclude a path going over the original source router.

So, now you have observed the MPLS LDP FRR by utilizing ISIS as IGP, but that’s not all as you know that. Thus if you wish to have more details, you could gain the preparation courses which you could gain at the SPOTO.