In an EIGRP based network, route redistribution may sometimes be required. In which situation is EIGRP route redistribution typically needed so that all networks can be reached correctly?

Introduction / Context:
Route redistribution is a process in which one routing protocol learns routes from another routing domain and injects them into its own domain. In EIGRP networks, redistribution becomes important when different routing protocols coexist or when different autonomous systems must share reachability information. The question here is checking whether you understand the specific scenario that requires EIGRP redistribution, rather than basic features like summarization or authentication.

Given Data / Assumptions:

• The network is using EIGRP as at least one of its routing protocols.
• There may be another routing protocol in use, such as OSPF, RIP, or static routing.
• All devices in the extended network must be able to reach all advertised networks.
• You want to identify when the redistribute command under EIGRP configuration is required.

Concept / Approach:
Redistribution is needed when routing information must cross protocol boundaries or autonomous system boundaries. For example, if one part of the network runs OSPF and another part runs EIGRP, simply enabling both protocols does not automatically share routes between them. Instead, you must configure redistribution so that routes learned by one protocol are advertised by the other. Summarization, authentication, and using multiple interfaces under a single EIGRP autonomous system do not by themselves require route redistribution. Therefore, the key idea is that redistribution is about connecting different routing domains, not about running EIGRP on additional interfaces or enabling optional security features.

Step-by-Step Solution:
1. Identify that redistribution is used to share routes between different routing processes. 2. Consider the scenario of a router running both EIGRP and OSPF. EIGRP knows some networks and OSPF knows others. 3. Without redistribution, EIGRP learned networks will not appear in the OSPF domain, and OSPF learned networks will not appear in EIGRP. 4. To fix this, you configure redistribute ospf under the EIGRP process and redistribute eigrp under the OSPF process, as appropriate. 5. Recognize that this is the classic use case described in the answer: running EIGRP alongside another protocol and needing to exchange routes.

Verification / Alternative check:
You can verify this reasoning by checking the other options. Summarization is handled by the summary-address command or auto summarization settings, not by redistribution. Running EIGRP on multiple interfaces in the same autonomous system does not require redistribution because all interfaces are part of the same EIGRP process. Enabling authentication merely secures EIGRP updates but does not create separate routing domains. If there is only one EIGRP autonomous system in the whole network, there is nothing to redistribute to, so redistribution is unnecessary. Only when EIGRP must share routes with a different routing protocol or another EIGRP AS does redistribution become necessary.

Why Other Options Are Wrong:
Option a: Summarizing EIGRP routes at an interface is done with EIGRP summary commands, not redistribution, and does not involve another protocol. Option c: Running EIGRP on multiple interfaces in the same AS is normal and does not by itself create the need for route redistribution. Option d: Authentication only affects security of routing updates, not the exchange of routes between different protocols. Option e: If there is a single EIGRP autonomous system in the entire internetwork, there is actually no additional routing domain with which to redistribute, so redistribution is not required.

Common Pitfalls:
Students often confuse redistribution with summarization and think any time multiple interfaces or subnets exist, redistribution is needed. Remember that redistribution is about sharing routes between different routing protocols or autonomous systems, not simply about having several subnets or links. Another pitfall is forgetting to configure appropriate metrics or seed metrics during redistribution, which can cause routes to appear but be unusable. Keeping the distinction clear between features like summarization, authentication, and redistribution helps avoid misconfigurations and exam mistakes. Always ask whether there are multiple routing domains; if yes, redistribution is probably involved.

Final Answer:
EIGRP route redistribution is required when you are running EIGRP and another routing protocol such as OSPF and need to exchange routes between them.