300-420 Practice Questions

The design should summarize routes from the distribution layer toward the core and configure access-layer switches as EIGRP stubs. EIGRP query scope is a major design consideration in large routed campus networks. When a route is lost, EIGRP may send queries to neighbors to find an alternate path. If the network is not bounded properly, these queries can spread widely, slow convergence, and create stuck-in-active risk. Configuring the access layer as EIGRP stub tells upstream routers that the access device is not a transit path to other networks, so unnecessary queries are not sent into the access layer. Summarizing at the distribution layer toward the core hides access-link instability and provides a smaller, more stable routing table to the rest of the network. Summarizing at the access layer is usually less useful when many VLANs are aggregated at distribution. Configuring the core as stub is inappropriate because the core should provide transit. Reference topics: EIGRP stub, route summarization, query boundary, campus hierarchy, DUAL convergence.

Route summarization and EIGRP stub routing are the two correct design techniques for a stable and scalable EIGRP deployment. Summarization reduces the number of prefixes advertised upstream, conserves bandwidth and memory, hides route flaps behind the summary boundary, and reduces the size of the EIGRP topology table. Stub routing prevents branch or access devices from being used as transit routers and limits the scope of EIGRP queries. This is critical because uncontrolled EIGRP query propagation can increase convergence time and processing load, especially in larger hierarchical networks. Prefix lists and distribute lists can filter routes, but filtering alone does not provide the same query-boundary and topology-hiding benefits as summarization and stubs. Static redistribution increases complexity and can create loop or policy risks if not controlled with tags and metrics. The question specifically asks to conserve bandwidth, memory, and CPU, prevent suboptimal routing, and avoid unnecessary queries; summarization and stub routing are the direct Cisco design answers. The architect should summarize at hierarchy boundaries and configure access or branch routers as EIGRP stubs where they should not provide transit. Reference topics: EIGRP summarization, EIGRP stub routing, query scope, hierarchical routing.

The OSPF design should include LSA/SPF timer tuning and summarization with appropriate stub-area design. Cisco OSPF convergence is affected by failure detection, LSA generation, flooding, SPF scheduling, and routing-table installation. Tuning LSA and SPF throttling timers helps the network react quickly to genuine failures while avoiding excessive CPU churn during repeated flaps. This is especially important when voice and video traffic are sensitive to convergence delay. Manual route summarization at area boundaries reduces the number of detailed routes advertised between areas and limits the blast radius of link failures. Configuring nonbackbone areas as stub networks where appropriate also reduces external LSA propagation and lowers router resource use. Reducing DR and BDR election frequency is not a primary design tool for convergence, and increasing hello and hold timers would slow failure detection rather than improve it. Advertising default routes can be useful, but it does not by itself contain link flap impact. Reference topics: OSPF area design, LSA throttling, SPF throttling, route summarization, stub areas, convergence optimization.