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Segment Routing TI-LFA

fabricplane June 15, 2020
Segment Routing

Before Segment Routing was introduced, IP FRR and Loop-Free Alternates (LFAs) were not widely deployed. In these implementations the level of coverage depended on the network topology. When SR combined with FRR techniques, Fast Reroute can deliver 100% network coverage regardless of the topology. That is called TI-LFA (Topology Independent – Loop Free Alternate)

Here are some functions and benefits of TI-LFA;

  • It is IGP automated. It is very simple to understand and deploy. No need to run and manage additional protocol.
  • It can work and provide less than 50ms switch-over in any topology.
  • TI-LFA finds and calculates the post-convergence shortest path to every possible destination.
  • It can provide link protection, node protection and SRLG protection.

There are some certain rules for FRR to work.

Here is the formula;

Assume;

S = Source Router

D = Destination Router

N = Alternate Next Hop Router

E = Primary Next Hop Router

If Total Cost of N to D < Total cost of S to N + Total Cost of S to D FRR will install the backup route. Otherwise it will not be calculated as backup not to create micro loops in the network. SR TI-LFA is IGP automated, we don’t need to worry about this formula.

Let’s examine a few topology to understand how TI-LFA with a basic example. In the following example, IP FRR may also work as well as TI-LFA.

Case Study

According to below topology;

Cost between Node-E and Node-D is 10 and all other links have a cost of 1.

Prefix SID of Node-D: 16004

Prefix SID of Node-E: 16005

Without TI-LFA enables, Node-A will have the following entry in its SR forwarding table;

Incoming Active Segment: NULL

Operation: PUSH 16005 (PHP is enable by default)

Egress Interface: The interface towards to Node-B

If we enable TI-LFA Link protection on Node-A, the following entries will be installed to the SR forwarding table; (And it is IGP automated)

Incoming Active Segment: NULL

Operation: PUSH 16005 (Prefix-SID of Node-E)

Egress Interface: The interface towards to Node-B

TI-LFA Backup: PUSH 16005, Interface towards to Node-C
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