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350-401 Practice Question: A network team is designing the underlay for an…
A network team is designing the underlay for an SD-Access fabric. The design must use a routing protocol that supports fast convergence and is commonly recommended for the fabric underlay. Which routing protocol should be used?
⚠ Common exam trap
Cisco often tests the misconception that EIGRP is the best choice for fast convergence in Cisco-centric designs, but for SD-Access underlay, the recommended protocol is IS-IS due to its open standard nature and alignment with Cisco's validated fabric architecture.
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
✓
IS-IS
IS-IS is the correct choice because it is a link-state routing protocol that provides fast convergence, is highly scalable, and is the most commonly recommended routing protocol for the underlay of an SD-Access fabric. Cisco SD-Access designs frequently use IS-IS to support the fabric's control plane and data plane requirements, leveraging its ability to handle large, flat network topologies with minimal overhead.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
IS-IS
Why this is correct
IS-IS is the recommended underlay routing protocol for Cisco SD-Access because it is a link-state IGP that scales to large campus fabrics, supports both IPv4 and IPv6 natively, and converges rapidly after link or node failures. It runs directly over Layer 2 using CLNS, making it independent of IP addressing, which is valuable during underlay bootstrap. Fabric nodes use IS-IS to build a loop-free physical topology, and it can carry host-specific routes for seamless mobility.
- ✗
RIP
Why it's wrong here
RIP is unsuitable for an SD-Access underlay because it is a distance-vector protocol that uses hop count as its only metric and discards routes beyond 15 hops. It suffers from slow convergence through periodic triggered updates and hold-down timers, and its maximum scale is far below the thousands of routes and endpoints in a fabric. Modern SD-Access demands loop-free, fast-converging, and highly scalable routing, none of which RIP can provide.
- ✗
EIGRP
Why it's wrong here
EIGRP is a Cisco proprietary advanced distance-vector protocol with fast convergence and load balancing, but it is not recommended for SD-Access underlay. Cisco's SD-Access architecture is intentionally built around open standards to support multivendor underlays, and EIGRP's proprietary nature ties the fabric to Cisco-only equipment. Moreover, EIGRP's route summarization and DUAL operation, while efficient, do not align with the underlay's role as a simple, standards-based transport for VXLAN/LISP overlays.
- ✗
BGP
Why it's wrong here
BGP is an external gateway protocol designed for routing between autonomous systems with highly policy-driven path selection, making it far more complex than needed for a campus underlay. In SD-Access, BGP is not the underlay IGP; its role is limited to overlay-related control-plane interactions (e.g., with LISP) and connecting the fabric to external networks. Using BGP as an underlay would add slow convergence, manual peering overhead, and unnecessary iBGP/eBGP complexity to the physical network.
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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