The correct answer is that the backup path in MPLS FRR is used to forward traffic if the primary outgoing interface fails. This is the core purpose of Fast Reroute: it pre-installs a backup next-hop into the forwarding table so that when the primary link or node goes down, traffic is switched to the backup path in sub-50ms, without waiting for routing protocol convergence. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of MPLS TE FRR and how the backup path differs from load balancing or VPN label handling—common traps include confusing FRR with ECMP or thinking the backup path strips the VPN label. Remember that FRR is purely a local repair mechanism for interface or node failure, not a load-sharing tool. A useful memory tip: “Backup is for breakage, not balance.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Exhibit
Refer to the exhibit.
Router# show mpls forwarding-table 192.168.10.0/24 detail
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
200 16015 192.168.10.0/24 1000 Gi0/0/0 10.1.1.2
MAC/Encaps: 14/18, MTU: 1500, Label Stack {16015(vcd)}
VPN label: 1234 (L3VPN)
Paths: 1 active
Backup path: via Gi0/0/1, 10.1.1.6, label 16020
Protection: FRR active, state: ready
Time since last switch: 00:01:23
Refer to the exhibit. The output shows an MPLS forwarding entry with FRR protection. What is the purpose of the backup path shown?
Refer to the exhibit.
Router# show mpls forwarding-table 192.168.10.0/24 detail
Local Outgoing Prefix Bytes Label Outgoing Next Hop
Label Label or Tunnel Id Switched interface
200 16015 192.168.10.0/24 1000 Gi0/0/0 10.1.1.2
MAC/Encaps: 14/18, MTU: 1500, Label Stack {16015(vcd)}
VPN label: 1234 (L3VPN)
Paths: 1 active
Backup path: via Gi0/0/1, 10.1.1.6, label 16020
Protection: FRR active, state: ready
Time since last switch: 00:01:23
A
To handle penultimate hop popping for the primary path.
Why wrong: PHP is separate; the backup path still uses labels.
B
To forward traffic if the primary outgoing interface fails.
The backup path activates when primary fails, ensuring fast convergence.
C
To provide a load-balancing alternative for the primary path.
Why wrong: FRR backup is not for load balancing; it's for protection.
D
To carry the VPN label separately.
Why wrong: The VPN label is already in the primary path; backup uses same label stack.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
To forward traffic if the primary outgoing interface fails.
The backup path is used when the primary next-hop fails, providing fast reroute. Option A is incorrect because VPN label is still present. Option C is incorrect because FRR does not create a load-balancing group. Option D is incorrect because backup path is not for PHP; PHP is for penultimate hop.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
To handle penultimate hop popping for the primary path.
Why it's wrong here
PHP is separate; the backup path still uses labels.
✓
To forward traffic if the primary outgoing interface fails.
Why this is correct
The backup path activates when primary fails, ensuring fast convergence.
Related concept
Static NAT maps one inside address to one outside address.
✗
To provide a load-balancing alternative for the primary path.
Why it's wrong here
FRR backup is not for load balancing; it's for protection.
✗
To carry the VPN label separately.
Why it's wrong here
The VPN label is already in the primary path; backup uses same label stack.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: To forward traffic if the primary outgoing interface fails. — The backup path is used when the primary next-hop fails, providing fast reroute. Option A is incorrect because VPN label is still present. Option C is incorrect because FRR does not create a load-balancing group. Option D is incorrect because backup path is not for PHP; PHP is for penultimate hop.
What should I do if I get this 350-501 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-501 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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