Implementing and Operating Cisco Enterprise Network Core Technologies (ENCOR) v1.2InfrastructureHard
A data center network uses multiple Layer 3 switches for routing between VLANs. Engineers observe that traffic between certain VLANs is experiencing suboptimal forwarding paths, specifically that packets are being forwarded to a router that is not the most optimal path, even though a better path exists. All switches are running OSPF. Which feature, if misconfigured or missing, could lead to this suboptimal Layer 3 forwarding issue?
- AEqual-cost multi-path (ECMP) routing
- BBidirectional Forwarding Detection (BFD)
- CIP SLA tracking for link failures
- DSpanning Tree Protocol (STP) convergence
Show answer & explanationAnswer & explanation
Correct answer: A. Equal-cost multi-path (ECMP) routing
If multiple equal-cost paths exist to a destination, ECMP allows routers to use all of them, distributing traffic and preventing suboptimal forwarding by utilizing the best available paths. If ECMP is not configured or is misconfigured, traffic might stick to a single path even when other equally good or better paths exist, leading to suboptimal forwarding. While OSPF can learn multiple equal-cost paths, the router needs to be configured to actually use them for forwarding.
Why the other options are wrong
- B. BFD quickly detects link failures to speed up routing protocol convergence, but doesn't dictate how multiple optimal paths are utilized.
- C. IP SLA tracks link health for failover, but doesn't influence which path is chosen as 'most optimal' when multiple paths are available and healthy.
- D. STP operates at Layer 2 and prevents loops; it doesn't directly influence Layer 3 suboptimal routing paths.
Equal-Cost Multi-Path (ECMP)
A routing strategy where next-hop packet forwarding to a single destination can occur over multiple best paths that have equal cost metric.
- Improves bandwidth utilization and provides redundancy.
- Relies on hashing algorithms to distribute traffic across paths.
- Requires the routing protocol to find multiple paths with identical costs.
Memory trick: Suboptimal Path? Check if ECMP is Missing from the Map.