Route redistribution is the process of injecting routes from one routing protocol into another, and it's a critical skill for the CCNA 200-301 exam (objective 3.7) because real-world networks rarely run a single routing protocol. Mergers, acquisitions, and multi-vendor environments force network engineers to connect OSPF, EIGRP, and static routes—and without proper redistribution, traffic blackholes or routing loops will destroy your network. This chapter gives you the mechanistic understanding and IOS CLI commands to configure and troubleshoot redistribution like a senior engineer.
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Imagine you're the network architect for a massive international conference where two separate groups of delegates speak different languages: Group A speaks only Spanish, Group B speaks only Mandarin. Each group has its own internal communication system (routing protocol) that works perfectly within their own group—they share messages, know who's who, and can find any member of their own group instantly. But when a delegate from Group A needs to send a message to a delegate in Group B, they can't directly communicate because the languages (protocols) are incompatible. This is where you, the translator (redistribution point), come in. You are bilingual: you understand both Spanish and Mandarin. Your job is to listen to all messages from Group A, translate them into Mandarin, and then broadcast the translated messages to Group B—and do the same in reverse. However, there's a catch: the two groups have different ways of describing how far away something is. Group A measures distance in kilometers (OSPF cost), while Group B measures in steps (EIGRP metric). When you translate a message from Group A to Group B, you must assign a default step count (seed metric) because kilometers don't convert neatly to steps. If you assign a step count that is too low, Group B might think the destination is very close and start sending all their messages through you, overloading you (suboptimal routing). If you assign a step count too high, Group B might think the destination is too far and ignore it (route not installed). Worse, if you translate messages from Group B back into Spanish and then re-translate them into Mandarin again, you could create an infinite loop of translations (routing loop). To prevent this, you must apply a rule: "Never translate a message that I originally translated myself"—this is the administrative distance and route tagging mechanism. In networking, the translator is a router running both protocols, the seed metric is the default cost assigned to redistributed routes, and the loop prevention is achieved through administrative distance adjustments and route tagging.
What is Route Redistribution and Why Does It Exist?
Route redistribution is the mechanism by which routes learned via one routing protocol (or source, like static or connected) are advertised into another routing protocol. In the CCNA 200-301 exam, redistribution is tested under objective 3.7 (Configure and verify route redistribution) and is a common scenario in troubleshooting questions. Real enterprise networks often have multiple routing domains due to mergers, departmental boundaries, or gradual migrations. Without redistribution, each domain would be isolated, and devices in different domains couldn't communicate. Redistribution bridges these domains, but it introduces complexity: different protocols use different metrics, administrative distances, and loop-prevention mechanisms.
How Redistribution Works Step by Step
When a router redistributes routes, it takes routes from the source protocol's routing table and injects them into the destination protocol's routing process. The router then advertises these routes to its neighbors in the destination protocol. However, the router only redistributes the best routes (those installed in the routing table) from the source protocol. The redistribution process involves:
Route Selection: The router examines its routing table for routes learned via the source protocol (or static/connected). For each route, it checks if the route is a candidate for redistribution (not already learned via the destination protocol to avoid loops).
Metric Translation: The source protocol's metric (e.g., OSPF cost) is meaningless in the destination protocol (e.g., EIGRP composite metric). Therefore, a default metric (seed metric) must be assigned. If not configured, the router may use a default value (e.g., 0 for EIGRP, which is unreachable) or fail to redistribute.
Route Advertisement: The redistributed routes are advertised to neighbors in the destination protocol as if they were internal routes (or external routes with a special flag). For OSPF, redistributed routes become Type 5 External LSAs (for E1/E2) or Type 7 (for NSSA). For EIGRP, they are marked as external routes with an administrative distance of 170.
Loop Prevention: Redistribution can cause routing loops if routes are redistributed back into the source protocol. To prevent this, routers use route tagging (tagging redistributed routes with a number) and administrative distance adjustments. By default, routes learned from a protocol have a lower administrative distance than redistributed routes, so the original protocol's routes are preferred.
Key States, Timers, and Defaults
- Seed Metric: The metric assigned to redistributed routes. Defaults vary: - EIGRP: No default; must be configured. If not set, routes are not redistributed (metric 0 is considered infinite). - OSPF: Default metric type is E2 with cost 20 (except BGP routes, cost 1). - RIP: Default metric is 0 (unreachable); must be set to a value between 1 and 15. - IS-IS: Default metric is 0.
- Administrative Distance: Default AD for redistributed routes: - EIGRP external: 170 (internal is 90) - OSPF external: 110 (same as internal? No, OSPF internal is 110, external is also 110 by default, but the route type (E1/E2) affects path selection) - RIP: 120 (redistributed routes have the same AD as RIP-learned)
Route Tag: A 32-bit value attached to redistributed routes. Default is 0. Can be set manually to identify the origin of routes for filtering.
IOS CLI Verification Commands with Example Output
To verify redistribution, use:
- show ip route – view routes with their origin code (e.g., 'O E2' for OSPF external type 2)
- show ip protocols – displays redistribution configuration
- show ip ospf database external – view OSPF Type 5 LSAs
- show ip eigrp topology – see EIGRP topology including external routes
Example: After configuring redistribution from EIGRP into OSPF:
R1# show ip route ospf
10.0.0.0/24 is subnetted, 1 subnets
O E2 10.1.1.0 [110/20] via 192.168.1.2, 00:00:05, GigabitEthernet0/0R1# show ip protocols
*** IP Routing is NSF aware ***
Routing Protocol is "ospf 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Router ID 1.1.1.1
Redistributing: eigrp 100
...Interaction with Related Protocols
Redistribution interacts with route filtering (distribute-lists, prefix-lists, route-maps), administrative distance, and metric calculations. For example, a route-map can set the metric or tag during redistribution. Also, redistribution of connected routes is often used to advertise directly attached networks into a dynamic protocol. When redistributing static routes, the default metric must be set unless the static route is already in the routing table with a valid metric (which it isn't).
OSPF vs EIGRP: Redistribution between OSPF and EIGRP is common in exam scenarios. OSPF uses cost (bandwidth-based) while EIGRP uses composite metric (bandwidth, delay, load, reliability). Setting a seed metric is mandatory for EIGRP.
RIP vs OSPF: RIP uses hop count (max 15), so the seed metric must be <=15. OSPF external routes can have a cost of 20 by default.
Static into Dynamic: Static routes have AD 1, so they are preferred over dynamic routes. When redistributing static into OSPF, the seed metric is 20 if not specified.
Identify redistribution boundaries
Determine which routers will perform redistribution. Typically, a router that runs both protocols (e.g., OSPF and EIGRP) is the redistribution point. In the CCNA exam, you'll often have a single router connecting two routing domains. Ensure that the router has interfaces in both domains and that both routing protocols are configured and operational. Use `show ip protocols` to verify that both protocols are running and have neighbors.
Configure the seed metric
Set a default metric for redistributed routes in the destination protocol. For EIGRP, use the `default-metric` command under router eigrp: `R1(config-router)# default-metric 10000 100 255 1 1500` (bandwidth, delay, reliability, load, MTU). For OSPF, you can set the default metric with `default-metric cost` (e.g., `default-metric 30`). If you don't set a default metric, EIGRP will not redistribute routes (metric 0 is considered infinite), and OSPF will use a default cost of 20 (except BGP routes, cost 1).
Apply redistribution command
Enter the configuration mode for the destination protocol and use the `redistribute` command. Syntax: `redistribute source-protocol [source-options] [metric metric-value] [metric-type type-value] [route-map map-name]`. Example: `R1(config-router)# redistribute eigrp 100 metric 30 metric-type 1 subnets`. The `subnets` keyword is required for OSPF to redistribute subnet routes (without it, only classful routes are redistributed). For EIGRP, use `redistribute ospf 1 metric 10000 100 255 1 1500`. Verify with `show ip protocols`.
Verify redistributed routes
Check the routing table on the redistribution router and on other routers in the destination domain. Use `show ip route` and look for routes with the appropriate code (e.g., `O E2` for OSPF external type 2). Example output: `O E2 10.1.1.0/24 [110/20] via 192.168.1.2, 00:00:05, GigabitEthernet0/0`. Also verify on a remote router in the destination domain that the route is present. If not, check that the redistribution router is advertising the route (use `show ip ospf database external` for OSPF).
Adjust administrative distance if needed
By default, redistributed routes may have a higher AD than internal routes of the same protocol, which can cause suboptimal routing or loops. For example, EIGRP external routes have AD 170, while internal have AD 90. If you want the redistributed routes to be preferred over other routes, you can change the AD with the `distance` command. Example: `R1(config-router)# distance 150` (sets AD for all OSPF routes to 150). However, be cautious: lowering AD can create loops. Use route tagging and filtering to control redistribution.
Implement route filtering with route-maps
In production, you rarely redistribute all routes. Use route-maps to filter specific routes, set metrics, or set tags. Example: `R1(config)# route-map EIGRP-to-OSPF permit 10` then `R1(config-route-map)# match ip address prefix-list EIGRP-ROUTES` and `R1(config-route-map)# set metric 50`. Then apply: `R1(config-router)# redistribute eigrp 100 route-map EIGRP-to-OSPF subnets`. Verify with `show route-map` and check that only matched routes appear in the routing table.
In a typical enterprise merger scenario, Company A runs OSPF and Company B runs EIGRP. The network engineer must connect the two networks without causing a routing loop. The engineer configures a border router with both protocols, sets seed metrics carefully (e.g., EIGRP external metric of 10000/100/255/1/1500 to represent a moderate cost), and uses route-maps to filter out private or overlapping subnets. They also set route tags (e.g., tag 100 for routes from Company A) to identify the origin, and then on the other side, they filter out routes with that tag to prevent re-redistribution. This prevents mutual redistribution loops. Common misconfigurations include forgetting the subnets keyword in OSPF (causing only classful routes to be redistributed), not setting a default metric for EIGRP (routes not redistributed), and accidentally redistributing back into the source protocol (routing loop). Performance considerations: redistribution adds CPU load on the border router because it must process all routes from both protocols. In large networks (thousands of routes), route summarization before redistribution reduces the number of advertised routes. Another scenario is a dual-homed connection to an ISP: the enterprise redistributes a default route from BGP into OSPF, but must ensure the default route does not get redistributed back into BGP. This is done by setting a route tag and filtering on the BGP side. Misconfiguration can cause the ISP to learn the enterprise's internal routes, leading to blackholing or loops. In production, always use route-maps to control redistribution, and test in a lab before deploying.
The CCNA 200-301 exam tests route redistribution under objective 3.7, typically in multiple-choice or simulation questions. You must know the default seed metrics, the subnets keyword for OSPF, and the fact that EIGRP requires a metric to be set. Common wrong answers: (1) Thinking that OSPF automatically assigns a seed metric of 1 for all redistributed routes—wrong, default is 20 except BGP (1). (2) Believing that EIGRP will use a default metric of 10000 if not configured—wrong, EIGRP does not redistribute without a metric; the command default-metric or a metric in the redistribute command is mandatory. (3) Assuming that redistributed routes have the same AD as internal routes of the destination protocol—for OSPF, both internal and external have AD 110, but for EIGRP, external is 170 vs internal 90. (4) Forgetting the subnets keyword in OSPF redistribution—this is a classic trap; without it, only classful routes (e.g., 10.0.0.0/8) are redistributed, not subnets. Decision rule: In any redistribution question, first check if a default metric is set for EIGRP; if not, routes are not redistributed. For OSPF, look for the subnets keyword. Also, remember that route-maps can filter and set metrics; a question may ask which routes are redistributed based on a route-map configuration. Study the output of show ip protocols and show ip route to identify redistributed routes.
Route redistribution injects routes from one routing protocol into another; it is configured under the destination protocol with the `redistribute` command.
EIGRP requires a seed metric to be set; without it, routes are not redistributed. Use `default-metric` or specify metric in the `redistribute` command.
OSPF default seed metric is 20 for most routes (except BGP routes, which get 1). Use the `subnets` keyword to redistribute subnet routes.
Redistributed routes in OSPF appear as Type 5 External LSAs (E1 or E2) or Type 7 (NSSA). In EIGRP, they are external routes with AD 170.
Route tagging and route-maps are used to prevent redistribution loops and control which routes are redistributed.
Administrative distance for redistributed routes: EIGRP external = 170, OSPF external = 110, RIP = 120.
Always verify redistribution with `show ip route`, `show ip protocols`, and `show ip ospf database external` (for OSPF).
These come up on the exam all the time. Here's how to tell them apart.
Redistribution into OSPF
Default seed metric is 20 (except BGP: 1).
Requires `subnets` keyword to redistribute subnet routes.
Routes become Type 5 External LSAs (E1/E2) or Type 7 (NSSA).
AD remains 110 (same as internal).
Can use `metric-type` to set E1 or E2.
Redistribution into EIGRP
No default seed metric; must be configured.
No `subnets` keyword needed (EIGRP is classless).
Routes become external routes with AD 170.
AD is 170 (higher than internal 90).
Metric is a composite of bandwidth, delay, reliability, load, MTU.
Mistake
OSPF automatically assigns a seed metric of 1 to all redistributed routes.
Correct
OSPF default seed metric is 20 for most redistributed routes (e.g., from EIGRP, RIP, static) and 1 for BGP routes. You can change it with the `metric` option in the `redistribute` command or `default-metric`.
Candidates often confuse OSPF default metric with the default administrative distance or assume it's always 1.
Mistake
EIGRP will redistribute routes with a default metric of 10000 if no metric is specified.
Correct
EIGRP does not have a default seed metric; if neither `default-metric` nor a metric in the `redistribute` command is configured, EIGRP will not redistribute any routes (metric 0 is considered infinite).
Some candidates think EIGRP uses a reasonable default metric like OSPF, but EIGRP requires explicit metric configuration.
Mistake
The `subnets` keyword is optional and only needed for classless routing; without it, OSPF still redistributes subnets.
Correct
Without the `subnets` keyword, OSPF redistributes only the classful network (e.g., 10.0.0.0/8) even if subnets exist. You must include `subnets` to redistribute subnet routes.
Candidates may overlook this keyword because they assume OSPF is classless by default, but the redistribution command has this quirk.
Mistake
Redistributed routes have the same administrative distance as routes learned natively within the protocol.
Correct
For OSPF, both internal and external routes have AD 110, so they are the same. For EIGRP, external routes have AD 170, while internal have AD 90. For RIP, all routes have AD 120.
Candidates may generalize that redistributed routes always have higher AD, but OSPF is an exception.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
The default seed metric for OSPF is 20 for most redistributed routes (e.g., from EIGRP, RIP, static, connected) and 1 for BGP routes. You can override this with the `metric` option in the `redistribute` command or the `default-metric` command under router ospf. This is a common exam point—remember that BGP gets a special default metric of 1.
EIGRP uses a composite metric based on bandwidth, delay, reliability, load, and MTU. Other routing protocols (like OSPF, RIP, or static routes) do not have these parameters. Therefore, EIGRP cannot derive a meaningful metric from the source protocol. If you do not set a metric (via `default-metric` or in the `redistribute` command), EIGRP assigns a metric of 0, which is considered infinite, so the routes are not installed in the routing table. Always configure a metric when redistributing into EIGRP.
OSPF external type 1 (E1) routes add the internal cost to the external metric, so the total cost increases as the route propagates through the OSPF domain. Type 2 (E2) routes use only the external metric, ignoring internal costs. By default, redistributed routes are type 2. E1 is preferred when you want the path selection to consider the internal topology. In redistribution, you can set the metric type with the `metric-type` keyword.
To prevent loops, use route tagging and filtering. Tag redistributed routes with a unique value (e.g., tag 100) using a route-map. On other routers that perform redistribution, filter out routes with that tag using a route-map or distribute-list. Also, adjust administrative distances so that routes learned from the source protocol are preferred over redistributed routes. Never redistribute both ways without proper filtering.
The `subnets` keyword tells OSPF to redistribute all subnet routes from the source protocol. Without it, OSPF only redistributes the classful network address (e.g., 10.0.0.0/8) even if the source has subnets like 10.1.1.0/24. This is a common misconfiguration that causes missing routes in the OSPF domain. Always include `subnets` when redistributing into OSPF unless you specifically want only classful routes.
Yes, you can redistribute connected routes using the `redistribute connected` command under the routing protocol. This is often used to advertise directly attached networks into a dynamic routing protocol without configuring the network statement for each interface. You can also set a metric and use route-maps to filter which connected routes are redistributed.
In OSPF, both internal and external routes have an administrative distance of 110. In EIGRP, internal routes have AD 90, while external routes (redistributed) have AD 170. This difference means that if a route is learned both via EIGRP internally and via redistribution from another protocol, the internal route is preferred.
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