Troubleshoot redistribution between any routing protocols or routing sources

📘CCNP Enterprise – ENARSI (300-410)

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1. What is Route Redistribution?

Route redistribution is the process of sharing routes learned from one routing protocol into another routing protocol or routing source.

• In real IT networks, organizations often run multiple routing protocols for different parts of the network: for example, OSPF in the data center and EIGRP in the branch offices.
• Redistribution ensures that all routers in different protocol domains can learn about each other’s networks.

Key Concept: Redistribution is needed because routing protocols don’t automatically share information with each other.

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2. When do you need Redistribution?

Some common scenarios:

1. Different routing protocols in different parts of a network
   • Data center: OSPF
   • Branch offices: EIGRP
   • Redistribute routes so OSPF and EIGRP routers know each other’s networks.
2. Connecting static routes with dynamic routing protocols
   • You may have static routes to certain servers. Redistributing static routes allows dynamic routers to reach them.
3. Connecting BGP with an internal routing protocol
   • Internet routes learned via BGP might need to be shared with internal OSPF or EIGRP routers.

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3. How Redistribution Works

When you redistribute:

1. The router takes routes from Protocol A.
2. Injects them into Protocol B with a new metric (cost).
3. Protocol B routers treat these routes like any other route they learn, but with some differences:
   • Protocol B may apply default metrics.
   • Redistributed routes may have tags to prevent loops.

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4. Basic Redistribution Commands (Cisco Syntax)

Let’s look at an example:

Scenario:

• OSPF in the backbone (Area 0)
• EIGRP in a branch office (AS 100)
• We want OSPF routes to be known by EIGRP and vice versa.

Steps:

Redistribute OSPF into EIGRP

router eigrp 100
  redistribute ospf 1 metric 10000 100 255 1 1500

Redistribute EIGRP into OSPF

router ospf 1
  redistribute eigrp 100 subnets

Explanation of Metric:

• Different protocols use different metrics (OSPF uses cost, EIGRP uses bandwidth/delay).
• When redistributing, you must provide a compatible metric; otherwise, the routes may not propagate correctly.

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5. Common Issues When Redistributing

a) Missing Metric

• If you redistribute routes without specifying a metric, some protocols will ignore the redistributed routes.
• Example: Redistributing EIGRP into OSPF without subnets or a metric will cause the routes to not appear in OSPF.

b) Routing Loops

• Redistribution can cause loops if Protocol A redistributes to Protocol B, and Protocol B redistributes back to Protocol A.
• Solution: Use route tagging to mark redistributed routes so they aren’t re-redistributed.

c) Default Metrics

• Each routing protocol expects different metrics.
• If a default metric is too high, the route may not be used.

d) Filtering

• Sometimes, you don’t want to redistribute all routes.
• Use route-maps or distribute-lists to control which routes are redistributed.

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6. Preventing Loops with Tags

Route Tagging:

• Assign a tag to redistributed routes to identify them.
• When a route comes back into the original protocol, routers can ignore routes with the tag.

Example:

route-map OSPF-to-EIGRP permit 10
  set tag 100

router eigrp 100
  redistribute ospf 1 metric 10000 100 255 1 1500 route-map OSPF-to-EIGRP

• This ensures redistributed OSPF routes to EIGRP won’t get re-redistributed back into OSPF.

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7. Advanced Redistribution Concepts

1. Subnets Keyword
   • In OSPF redistribution, always use subnets to include all subnetted networks.
   redistribute eigrp 100 subnets
2. Route Maps
   • Use route maps to filter or modify attributes during redistribution.
   • Can set metrics, tags, or even deny certain routes.
3. Administrative Distance
   • Redistributed routes inherit the default AD of the receiving protocol.
   • Sometimes AD must be adjusted to control which route is preferred.

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8. Example Exam Question Scenarios

• Scenario 1: OSPF routes are not appearing in EIGRP.
  • Likely cause: Missing metric during redistribution.
• Scenario 2: Network loops after redistribution.
  • Likely cause: Routes being redistributed back to the original protocol without tagging.
• Scenario 3: Only some routes are redistributed.
  • Likely cause: Route-map or distribute-list filtering.

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9. Best Practices for Redistribution

1. Always set a metric when redistributing between different protocols.
2. Use route tags to prevent loops.
3. Use route maps to filter and control which routes are redistributed.
4. Check for AD conflicts to ensure the right route is preferred.
5. Document which routes are redistributed to simplify troubleshooting.

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10. Troubleshooting Commands

Check redistributed routes in the routing table

show ip route

Check redistribution configuration

show running-config | section redistribute

Debug redistribution (for lab environments)

debug ip routing
debug ip ospf events
debug ip eigrp packets

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✅ Summary (Exam-Focused)

• Redistribution shares routes between protocols or sources.
• Must configure metrics, subnets, and optionally tags.
• Common problems: loops, missing routes, wrong metrics, AD conflicts.
• Use route maps and tags to filter and prevent loops.
• Verify with show ip route and debug commands.

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