Introduction:
LDP is widely deployed to signal LSP due to its simple operational model.
Since Ldp establishes LSPs along IGP routed paths,its failure recovery is gated by the IGP re-convergence.this feathers perforn FRR of LDP LSPs that reatains the simple operational model of LDP.
The goal is to provide 100% coverage for all failure with recovery charateristics (link /node failures) similar to the methods in Reservation Protocol - Traffic Engineering FRR.
When FRR is configured on a node, the node pre-computes primary NHLFEs for all FECs and in addition it will pre-compute backup NHLFEs for all FECs. The backup NHLFE corresponds to the label received for the same FEC from a Loop-Free Alternate (LFA) next-hop (see also RFC5286). Both primary NHLFEs and backup NHLFEs in ALU 7X50 are programmed in the IOM/IMM which makes it possible to converge very quickly.
ISIS LFA:
Since Ldp establishes LSPs along IGP routed paths,its failure recovery is gated by the IGP re-convergence.this feathers perforn FRR of LDP LSPs that reatains the simple operational model of LDP.
The goal is to provide 100% coverage for all failure with recovery charateristics (link /node failures) similar to the methods in Reservation Protocol - Traffic Engineering FRR.
When FRR is configured on a node, the node pre-computes primary NHLFEs for all FECs and in addition it will pre-compute backup NHLFEs for all FECs. The backup NHLFE corresponds to the label received for the same FEC from a Loop-Free Alternate (LFA) next-hop (see also RFC5286). Both primary NHLFEs and backup NHLFEs in ALU 7X50 are programmed in the IOM/IMM which makes it possible to converge very quickly.
ISIS LFA:
Implementation :
The 7x50 software has implemented inequality 1 (link criterion) and inequality 3 (node criterion) of RFC5286. Similar to the Shortest Path Tree (SPT) 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 look like this :
Inequality 1 : [SP(backup NHR, D) < {SP(backup NHR, S) + SP(S, D)}]
Inequality 3 : [SP(backup NHR, D) < {SP(backup NHR, PN) + SP(PN, D)}]
With ‘SP’ = ‘shortest IGP metric path’, ‘NHR’ = ‘next-hop router’,’D’ = ‘destination’, ‘S’ = ‘source node or upstream node doing the actual LFA next-hop computation’ and ‘PN’ = ‘protected node’. Inequality 3 rule is stricter than inequality 1 rule. if the formulas does not exist the prefix did not protected.
Configuration :
Configuration :
The IGP CLI configuration will look like this :
*A:7450-lab3# configure router isis loopfree-alternate
After enabling LFA inside the IGP context, FRR needs to be enabled within the LDP context :
*A:7450-lab3# configure router ldp fast-reroute
Several show commands are possible to display LFA information:
* ‘show router isis statistics’ gives you an idea on the number of LFA runs on a specific node.
*‘show router isis lfa-coverage’ will perform a mathematical calculation between the number of nodes and IPv4/IPv6 routes in the network versus present LFA next-hop protections: * ‘show router isis statistics’ gives you an idea on the number of LFA runs on a specific node.
*A:7450-lab3# show router isis lfa-coverage
===============================================================================
LFA Coverage
===============================================================================
Topology Level Node IPv4 IPv6
-------------------------------------------------------------------------------
IPV4 Unicast L1 0/0(0%) 9/9(100%) 0/0(0%)
IPV4 Unicast L2 4/4(100%) 9/9(100%) 0/0(0%)
===============================================================================
* ‘show router isis spf lfa detail’ gives you a reference to LFA protection type (link or node).
* ' show router isis routes alternative' give you the next hop backup
* 'show router ldp bindings active prefix 172.30.1.1/32' we can see the backup lable for the fec
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