0 - Fast Reroute and Segment Routing QCM - Noah Van Horenbeke - TFE

Question 1: MPLS vs SRv6: Where is the path encoded?

In Segment Routing, the source router encodes the path as a list of segments. What is the fundamental difference between SR-MPLS and SRv6 in terms of how this information is carried?

SR-MPLS requires RSVP-TE signaling and SRv6 does not

Both use MPLS labels, but SRv6 labels are 128-bit wide

SR-MPLS uses IPv6 extension headers and SRv6 uses a label stack

SR-MPLS uses a label stack in the packet header and SRv6 encodes segments in an IPv6 extension header

Question 2: Loop-Free Alternate condition

A router S wants to use neighbor N as a Loop-Free Alternate (LFA) to reach destination D after the link S→D fails.

Which condition must hold?

Distance(N, D) < Distance(S, D)

Distance(N, S) < Distance(N, D)

Distance(S, N) + Distance(N, D) = Distance(S, D)

Distance(N, D) < Distance(N, S) + Distance(S, D)

Question 3: LFA coverage on a square topology

Consider the following symmetric topology where all link costs are 10.

R1 ── R2
│      │
R4 ── R3

R1 wants to protect the link R1→R2. Can R4 act as an LFA for R1 to reach R2?

Apply the LFA condition: Distance(R4, R2) < Distance(R4, R1) + Distance(R1, R2)

Yes, R4 is a neighbor of R1, so it is always a valid LFA

No, the condition is not satisfied: Distance(R4, R2) = 20, which is not strictly less than Distance(R4, R1) + Distance(R1, R2) = 20

Yes, R4 satisfies the LFA condition because Distance(R4, R2) = 20 < 10 + 10 = 20

No, LFA only works on point-to-point links, not on ring topologies

Question 4: TI-LFA vs classic LFA

Which of the following statements are true about Topology-Independent LFA (TI-LFA) compared to classic LFA?

TI-LFA achieves 100% link protection coverage regardless of topology

Classic LFA may leave some links unprotected in certain topologies

TI-LFA requires RSVP-TE to establish protection tunnels

TI-LFA uses Segment Routing to steer packets along the post-convergence path

TI-LFA can be implemented with either SR-MPLS or SRv6 as the data plane

Question 5: P-space and Q-space

In the context of TI-LFA for protecting link R1→R2, which of the following best defines the Q-space?

R1 ── R2
│      │
R4 ── R3

The set of routers reachable from R1 without using the link R1→R2

The set of routers adjacent to R1

The set of routers on the shortest path from R1 to R2

The set of routers that can reach R2 without using the link R1→R2

Question 6: Diagnosing LFA/TI-LFA with FRR commands

After enabling isis fast-reroute ti-lfa on all interfaces in FRR, you want to verify that TI-LFA backup paths are correctly computed. Which commands are useful for this verification?

show ipv6 route isis

show isis route prefix-sid

show bgp summary

show isis fast-reroute summary

show mpls table

Question 7: Node protection vs link protection

What is the key difference between link protection and node protection in TI-LFA?

Node protection requires RSVP-TE and link protection does not

Link protection uses adjacency segments and node protection uses only node segments

Link protection is only available with SR-MPLS and node protection requires SRv6

Node protection computes the post-convergence path assuming both the link and the next-hop router have failed, while link protection only assumes the link is down

Question 8: Why load MPLS kernel modules?

On a standard Linux system, you configure FRR with IS-IS and Segment Routing. MPLS label forwarding does not work out of the box. Why, and what must you do first?

MPLS only works if IPv4 is also configured on every interface

MPLS is disabled in FRR by default, enable it with mpls on in vtysh

The Linux kernel does not load MPLS support automatically, you must load mpls_router and mpls_iptunnel with modprobe, and enable label processing per interface via sysctl

Question 9: LFA coverage count

Hint Add answer

Same symmetric square topology, all costs 10.

How many of the 8 directed links (4 bidirectional × 2 directions) can be protected by classic LFA (x1) and TI-LFA (x2)?

Write you answer with this format: x1,x2

Format example : expression: 5x+3 equation: x=y+3 inequation: x<2y+1

Question 10: Link Failure

In this topology, after the link failure, wich path the Source will use to reach the Destination.

2,3,4,5,6

2,5,6

2,4,5,6

2,4,6

Question 11: Node Failure

In this topology, after the node failure, wich path the Source will use to reach the Destination.

2,4,5,6

2,4,6

2,5,6

2,3,4,6

Question 12: LFA Limitations

Consider the topology below and assume that link failure.

With a classic LFA mechanism:

This topology provides full LFA protection for destination D

TI-LFA would still be able to protect this destination

Router 2 can use router 4 as a loop-free backup next-hop toward D

Packets forwarded to router 4 may loop back to router 2

Author(s)	Noah Van Horenbeke
Deadline	No deadline
Submission limit	No limitation

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