300-410 Infrastructure Services Practice Question
A network engineer is configuring a Cisco IOS router to support MPLS Traffic Engineering (TE). The engineer has enabled MPLS TE globally and on the interfaces, and has configured a TE tunnel. However, the tunnel is not coming up. The engineer verifies that the IGP (OSPF) is advertising TE information. Which additional configuration is required to establish the TE tunnel?
⚠ Common exam trap
The trap here is assuming that IGP TE extensions alone are sufficient for TE tunnel establishment, overlooking the need for RSVP signaling.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable RSVP on the interfaces along the path.
For MPLS TE tunnels to be established, RSVP must be enabled on all interfaces that the tunnel traverses. RSVP handles the signaling and resource reservation. While the IGP advertises TE information, without RSVP the headend cannot signal the path. Therefore, enabling RSVP on the relevant interfaces is the required additional configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure the tunnel mode to mpls traffic-eng on all routers.
Why it's wrong here
The tunnel mode mpls traffic-eng command is configured only on the tunnel interface of the headend router. It is not required on transit routers. Transit routers only need to have MPLS TE and RSVP enabled on their interfaces. Configuring tunnel mode on all routers is incorrect and does not address the issue of the tunnel not coming up.
- ✗
Enable LDP on the TE tunnel interface.
Why it's wrong here
LDP is used for label distribution in MPLS, but TE tunnels use RSVP for signaling and label distribution. Enabling LDP on the TE tunnel interface is not a standard practice and would not help establish the tunnel. In fact, it could cause conflicts. The correct protocol for TE signaling is RSVP, not LDP.
- ✓
Enable RSVP on the interfaces along the path.
Why this is correct
RSVP is the signaling protocol used by MPLS TE to reserve resources and establish label-switched paths. Even if the IGP advertises TE information, without RSVP enabled on the interfaces, the TE tunnel cannot signal the path and reserve bandwidth. Enabling RSVP on all interfaces along the intended path is essential for the tunnel to come up.
- ✗
Configure CSPF on all routers in the path.
Why it's wrong here
CSPF is a path computation algorithm used by the headend router to calculate the best path for a TE tunnel based on constraints. It is not configured on all routers; only the headend router uses CSPF. Enabling CSPF on transit routers is unnecessary and does not affect the tunnel establishment. The missing piece is likely RSVP signaling.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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