Back to EIGRP GuideENARSI route policy

EIGRP Filtering and Redistribution

Control which routes EIGRP accepts and advertises, move selected routes between routing domains, and use tags to prevent feedback loops.

Prefix ListDistribute ListRoute MapSeed MetricTags

Policy Overview

Filtering controls visibility inside or outside EIGRP. Redistribution creates new external EIGRP routes from another source. Treat them as different operations.

Internal codeD

External codeD EX

Default external AD170

Loop markerRoute tag

1. Decide the Direction First

Policy	Effect	Question to ask
Inbound filter	Controls routes learned from a neighbor/interface	Should this router accept it?
Outbound filter	Controls routes sent toward a neighbor/interface	Should that side learn it?
Redistribution	Imports a route from another source	Which prefixes, metric, and tag?

2. Filter with a Prefix List and Distribute List

ip prefix-list BRANCHES permit 10.30.0.0/16 le 24
router eigrp 100
 distribute-list prefix BRANCHES out GigabitEthernet0/0

This example permits selected branch routes outbound. Prefix lists have an implicit deny, so confirm every route that must remain visible.

Change carefully: A filter can remove many routes immediately. Record the current topology and routing table first.

3. Add Policy with a Route Map

ip prefix-list REDIST permit 172.20.0.0/16 le 24
route-map INTO-EIGRP permit 10
 match ip address prefix-list REDIST
 set tag 220
route-map INTO-EIGRP deny 100

The route map makes the intent readable: permit only selected prefixes, mark them with tag 220, and deny everything else.

4. Supply a Complete EIGRP Seed Metric

EIGRP needs bandwidth, delay, reliability, load, and MTU for redistributed routes. Bandwidth is in kilobits per second; delay uses tens of microseconds in this command.

router eigrp 100
 redistribute ospf 10 metric 100000 100 255 1 1500 route-map INTO-EIGRP

Value	Example	Meaning
Bandwidth	100000	100 Mbps
Delay	100	1,000 microseconds
Reliability/load	255 / 1	Reliable and lightly loaded
MTU	1500	Informational metric component

5. Prevent Redistribution Feedback

When two routers redistribute between the same protocols, a route can return to its original domain. Tag routes when they enter, then deny that tag on the return path.

route-map BACK-INTO-OSPF deny 10
 match tag 220
route-map BACK-INTO-OSPF permit 100

Design principle: Filtering and tagging should describe route ownership. Do not depend only on administrative distance to prevent feedback.

6. Find Policy in Named Mode

In named EIGRP, redistribution and topology-wide policy sit under topology base for the address family.

router eigrp ENTERPRISE
 address-family ipv4 unicast autonomous-system 100
  topology base
   redistribute ospf 10 metric 100000 100 255 1 1500 route-map INTO-EIGRP
  exit-af-topology
 exit-address-family

7. Follow One Prefix End to End

Source table

Prove the route exists before redistribution.

EIGRP topology

Confirm metric, external origin, and tag.

Receiver

Confirm D EX, next hop, and reachability.

show ip route 172.20.10.0
show ip eigrp topology 172.20.10.0/24
show route-map
show ip route eigrp

8. Troubleshoot Policy Failures

Symptom	Likely cause	First check
No external route	Source absent, no seed metric, or route map denies	Source table and route-map counters
Too many routes	Broad prefix list or permit sequence	Prefix-list matches
Route returns to source domain	Missing tag filter	External route tag
Topology has route, RIB does not	Better route source or policy	Administrative distance and exact prefix

9. Guided Practice

Filter one direction

Record three learned prefixes.
Permit only two outbound.
Verify the third remains local.

Redistribute safely

Select one OSPF prefix.
Set metric and tag.
Verify D EX downstream.

Prove loop prevention

Build a second redistribution point.
Deny returning tag 220.
Verify stable ownership.

10. Frequently Asked Questions

Why does a redistributed EIGRP route show D EX?

It entered EIGRP from an external routing source rather than being native to the EIGRP domain.

Why is a seed metric required?

EIGRP needs metric components so DUAL can compare the external path with other EIGRP paths.

Can a distribute list break neighbor formation?

It normally filters routes, not Hello packets, so the neighbor can stay up while routes disappear.