Designing Safe Route Redistribution for CCNP ENARSI

Why Redistribution Is Powerful and Dangerous

Route redistribution lets you share reachability information between different routing protocols—OSPF to EIGRP, EIGRP to BGP and so on. It is a powerful tool, but also a frequent source of network outages and exam traps. In CCNP ENARSI you are expected to understand the design behind redistribution, not just the commands.

Poorly planned redistribution can cause loops, suboptimal routing and black holes that are hard to troubleshoot. Safe designs use clear boundaries, filters and tags so that routes do not bounce endlessly between protocols.

Step 1 – Define Roles and Direction of Redistribution

Before configuring anything, decide which protocol plays the primary role and which is temporary, legacy or limited to certain domains. Ask yourself:

• Which protocol should own the core or backbone of the network?
• Where exactly do I need prefixes from the other protocol?
• Do I really need two-way redistribution or is one-way enough?

In many real designs, redistribution is performed in one direction only and at very specific points. This keeps domains independent and simplifies troubleshooting. Two-way redistribution is possible but must be controlled carefully to avoid loops.

Step 2 – Use Filters and Tags, Not Blind Trust

Safe redistribution is never “redistribute everything and hope for the best”. Instead, you:

• Use prefix lists or route maps to limit which routes are exported.
• Apply tags to redistributed routes so they can be recognized later.
• Block routes from re-entering their original domain based on those tags.

Tagging is a common ENARSI topic. When OSPF learns a route from EIGRP and then tries to send it back into EIGRP, tags allow you to detect and drop this loop. In the exam, watch for tasks mentioning “prevent routing loops between domains” or “use route tags appropriately”.

Step 3 – Take Control of Metrics and Administrative Distance

Routing protocols use different metrics. When you redistribute, you often need to set metrics explicitly, rather than relying on defaults:

• EIGRP requires a composite metric; using a default-metric command is common.
• OSPF external routes need type (E1/E2) and cost decisions.
• RIP may require careful hop-count settings to avoid unreachable routes.

On top of metrics, remember that administrative distance (AD) decides which protocol wins when two paths to the same destination exist. Be careful when modifying AD; it can cause a route from one protocol to silently override a better path from another.

Step 4 – Verify and Troubleshoot Systematically

In both the lab and the real world, never assume redistribution is working just because the command is present. Verify:

• Which routes appear in each routing table, and with what metrics and tags.
• Whether traffic follows the expected path using traceroute and ping.
• That routes do not oscillate or disappear when a neighbor flaps.

Use show ip route, show ip protocols and protocol-specific verification commands. In an ENARSI exam scenario, a few carefully chosen show commands are often enough to reveal misconfigurations in metrics, filters or tags.

Article Details

• Level: CCNP ENARSI
• Topic: Route Redistribution
• Audience: Enterprise network engineers

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