Segment Routing - Topology Independent LFA

Combination of segment routing and Loop Free Alternate is known as Topology Independent LFA or Ti-LFA. The main reason for configuring LFA is to provide the protection against the destination prefixes. In case of failure of primary link, source node calculates the backup the path as described in the "Loop Free Alternate: IP Fast Reroute (FRR)". The biggest disadvantage of the existing RSVP-TE-Fast reroute, Loop Free Alternate (LFA) and remote LFA, which has seen wide adoption can't guarantee 100% coverage for all failure scenarios. As per the RFC 6571, section 4.3, simulation results proves that LFA provides average 89% coverage across various topologies.

Below are the various types of issues which can be seen with Loop Free Alternate(LFA) and Remote Loop Free Alternate(rLFA)

1. LFA has issue with topology more than 3 nodes. With 3 nodes topology it perfectly works good. If the topology has more than 3 nodes, micro loop can occur as per shown in the image.

2. Micro-Loop avoidance can be taken care by finding PQ node as per RFC 7490. XR-1 will create T-LDP session with XR-3, XR-3 has the best path to XR-5 via XR-4.

3. Now increase the metric to 100 between XR-3 and XR-4. In this case, if XR-1 creates T-LDP session XR-3, XR-3 will again finds XR-1 as best and forward the traffic back to XR-3. This will be same as of micro loop.

Implementing Segment Routing with Topology Independent LFA (TI-LFA) provides 100% loop-free guaranteed coverage against any link, SRLG and node failure. Ti-LFA protects both labelled as well as unlabelled traffic. Stay tuned for the upcoming post (Segment Routing - Ti-LFA - Adjacency Sid Protection) how Ti-LFA solves the above issues by leveraging Segment Routing.
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Analysis Of Layer 2 Design

Traditional Enterprise and Campus deployments have been designed in such a way that allows for scalability, differentiated services and high availability. However in case of traditional layer 2 network design, there are many traps and dependencies. In our daily routine, we build these small LAN networks but hard to get time to analyze it. Mainly three issues can arise in Layer 2 networks - loops, traffic drop and excessive flooding.



The design of above depicted network is very simple with below listed design considerations:-
1.One Switch Per Subnet/Per VLAN.
2.It limits the Layer 2 broadcast domain size to the size of the access switch.
3.No Layer 2 Loops.
4.Easy to troubleshoot.

But above depicted network is well suited for small networks because per VLAN scale is very limited.
Lets see how does a LAN network looks like with a traditional Layer 2 loop.



Now the above network has layer 2 loop but still the VLAN scalability is very less, in fact it is restricted up to access switch only.



Lets analyze what could be the potential issues/risks can arise in above network.
1. Frames can replicate repeatedly.
2. Large number of multicast and broadcast frames.
3. High traffic drop.
4. There might be probability of BUM (broadcast, multicast and unknown unicast) traffic.

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Remote Loop Free Alternate

In the below topology, in case of failure of link between S and D, S will forward the traffic to A and A will again forward the traffic to S. The reason is very simple the cost of next hop from A to D will be lesser than the alternate path which is A-B-C-D-E. This will create a micro loop till the topology will converge.



So answer is very clear, IP FRR is not the viable option in case of ring and square topology. Instead of LFA, Remote LFA is used when no Local LFA is available and is used to find a remote node which can reach the protected neighbor without traversing the protected link. Remote LFA is primarily used for ring and square based access topologies.

In Loop-Free Alternates, the backup routes (repair paths) are pre-computed and installed in the router as the backup for the primary paths. Once a router detects a link or adjacent node failure, it switches to the backup path to avoid traffic loss. Remote LFA allows the backup next-hop to be more than one hop away. It means that after a failure an adjacent node recognizes it and tries to find a (remote) node whose shortest path to the destination is not affected by the failed component. If such node is found then packets will be forwarded to it. Remote LFA relies on tunnels to provide additional logical links towards backup next-hops. After the remote node receives the package it sends it towards to the primary destination. Note that the tunneled traffic is restricted to shortest paths just like “normal” traffic; hence the tunnel must avoid the failure as well.

Now the question arises how to select the node which can be used to terminate the tunnel. B can’t be selected because it is ECMP for D. C can be the best suitable option to terminate the tunnel. Selection of C can be done by using P Space Q.


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How to Calculate Fast Re Route(FRR) - Repair Path

The goal of IP Fast Reroute (IP FRR) is to reduce failure reaction time to sub-50ms by using a pre-computed alternate next-hop, in the event that the currently selected primary next-hop fails, so that the alternate can be rapidly used when the failure is detected. The alternate path or repair path must be loop free path to destination in case of network failures without recomputing the network topology as per below mentioned design.



Repair path can be used for diverting live traffic flowing through the primary path protected by the repair path only if the repair path both bypasses local failure (for example, failing link) and does not cause traffic to loop. Such repair paths are called “loop-free alternatives” or LFA.

Repair path provides loop-free alternative if following criterion is met:
1. Loop Free Alternate:- Condition: D(N,D) < D(N,S) + D(S,D). Path is loop-free because N’s best path is not through local router.” Traffic sent to backup next hop is not sent back to S.
2. Downstream Path:- Condition D(N,D) < D(S,D). “Neighbor router is closer to the destination than local router.” Loop-free is guaranteed even with multiple failures (if all repair-paths are downstream path).
3. Node protection:- Condition D(N,D) < D(N,E) + D(E,D). “N's path to D must not go through E.” “The distance from the node N to the prefix via the primary next-hop is strictly greater than the optimum distance from the node N to the prefix.“

It will be more cleared with below stated example:-


Examples of Calculating Fast Re-Route


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