EIGRP DUAL Algorithm - Practical Successor and Feasibility Guide

DUAL at a Glance

DUAL stands for Diffusing Update Algorithm. It uses information from EIGRP neighbors to select a best path while preventing routing loops during both normal operation and failure recovery.

Best pathSuccessor

Safe backupFeasible successor

Stable routePassive

Searching routeActive

Simple meaning: DUAL first asks, “What is my best path?” It then asks, “Which backup has already proved that it cannot loop through me?”

1. Build the DUAL Mental Model

R1 can reach one destination through three neighbors. DUAL compares the complete metric through each neighbor, but it also remembers the distance that each neighbor reports from itself to the destination.

R1Making the decisionNeeds 10.40.0.0/24

Three choicesR2, R3, and R4Different total and reported metrics

Destination10.40.0.0/24Advertised by the neighbors

DUAL chooses one successor and may keep another neighbor as a feasible successor.

The successor is used now. A feasible successor is stored as a safe backup but is not necessarily installed in the routing table unless equal-cost or variance rules also allow it.

2. Understand the Four Core Terms

Term	Plain-English meaning	Where to look
Reported distance (RD)	The neighbor's own metric from itself to the destination. It is also called advertised distance.	The second value in an EIGRP topology metric pair.
Feasible distance (FD)	The local router's best known total metric to the destination for the current DUAL calculation.	The topology entry header and successor metric.
Successor	The neighbor providing the lowest accepted total metric and the path used for forwarding.	EIGRP topology and IP routing tables.
Feasible successor (FS)	A neighbor whose RD proves a loop-free alternate path under the feasibility condition.	`show ip eigrp topology all-links`.

Read metric pairs as local/neighbor: In (4096/2048), 4096 is R1's total distance through that neighbor and 2048 is the neighbor's reported distance.

3. Apply the Feasibility Condition

A neighbor qualifies as a feasible successor when its reported distance is strictly lower than the current feasible distance:

Feasibility condition: Neighbor RD < current successor FD

Candidate	Total metric through neighbor	Neighbor RD	Compare with FD 3072	Result
Via R2	3072	2816	Current best path	Successor
Via R3	4096	2048	2048 < 3072	Feasible successor
Via R4	3584	3328	3328 is not < 3072	Not a feasible successor

R4 has a lower total metric than R3, but it still fails the safety test because its RD is too high. Feasibility proves loop freedom; it does not rank performance.

Common exam trap: Compare the candidate's RD with the successor's FD. Do not compare total metric with total metric, and remember that “less than” does not include equality.

4. Choose the Successor

DUAL evaluates the paths learned from valid EIGRP neighbors. The path with the lowest composite metric becomes the successor when it is usable.

1

Learn paths

Updates provide destinations and metric components.

2

Calculate totals

R1 calculates its complete distance through each neighbor.

3

Select successor

The lowest accepted total becomes the best path.

4

Test backups

Each remaining RD is checked against the FD.

In the example, R2 is the successor with total metric 3072. R3 is kept as a feasible successor. R4 remains known but is not trusted as a precomputed loop-free backup.

5. Recover Quickly with a Feasible Successor

If the R2 path fails, DUAL can install R3 immediately because R3 already passed the feasibility condition. The route does not need to enter an Active search.

1

Successor fails

R1 removes R2 as the current next hop.

2

Backup is ready

R3 is already proven loop-free.

3

Install R3

Forwarding changes without a diffusing Query.

Why EIGRP can converge quickly: The safe backup decision was made before the failure, not while traffic was already disrupted.

6. Follow an Active Search When No Backup Qualifies

If the successor fails and no feasible successor exists, DUAL cannot safely trust an alternate from current information. The route becomes Active and the router sends Queries to eligible neighbors.

1

Best path fails

No proven backup is available.

2

Send Queries

R1 asks neighbors for another path.

3

Collect Replies

Every required neighbor must answer.

4

Return Passive

DUAL selects a new path or declares the route unreachable.

A path that failed the feasibility condition can still be selected after the diffusing calculation finishes. It was not rejected forever; it simply was not safe to install without new information.

7. Compare Passive and Active States

State	Meaning	Healthy interpretation
Passive	DUAL is not currently searching for the route.	Normal stable state. Passive does not mean disabled.
Active	DUAL sent Queries and is waiting for required Replies.	Can be brief during convergence; investigate if it lasts or spreads widely.

Memory aid: Passive is peaceful. Active is asking.

8. Control How Far Queries Travel

A large Query domain makes more routers participate in failure recovery. Good topology and routing boundaries reduce unnecessary work.

EIGRP stub

Marks a branch as non-transit so upstream routers avoid unnecessary Queries toward it.

Open stub routing

Summarization

Hides component topology and can stop detailed Query propagation at the boundary.

Open summarization

Stable hierarchy

Keeps slow branches and unreliable links away from unnecessary transit decisions.

Stub and summarization do not repair packet loss. They reduce the number of routers involved when a route must be recomputed.

9. Understand Stuck-in-Active and SIA Packets

A route is Stuck-in-Active when the expected Reply process does not complete within the allowed time. The underlying problem is usually a delayed or broken Reply path, not the word “Active” itself.

Transport problem: Loss, errors, congestion, or one-way reachability delays reliable packets.
Resource problem: A busy router cannot process or answer the Query promptly.
Design problem: A long Query chain crosses too many routers or slow links.
Instability: Repeated route changes keep extending the work.

SIA-Query and SIA-Reply packets provide progress information during a long calculation. They help preserve the adjacency when a neighbor is still working, but repeated SIA traffic is still a reason to investigate.

Continue with the focused EIGRP SIA and Query Boundaries lesson for a hop-by-hop repair workflow.

10. Read DUAL Evidence in Cisco IOS

Start with one prefix. Read its topology entry before scanning the entire table.

Focused verification

show ip eigrp topology 10.40.0.0/24
show ip eigrp topology all-links
show ip eigrp topology active
show ip eigrp neighbors
show ip route 10.40.0.0
show ip cef 10.40.0.0/24

Example topology evidence

P 10.40.0.0/24, 1 successors, FD is 3072
 via 192.168.12.2 (3072/2816), Gi0/0
 via 192.168.13.3 (4096/2048), Gi0/1
 via 192.168.14.4 (3584/3328), Gi0/2

Output clue	Interpretation
`P`	The route is Passive and stable.
`1 successors`	One current best next hop exists.
`FD is 3072`	The current feasible distance used in the safety comparison.
`(4096/2048)`	Total distance through R3 is 4096; R3 reports 2048.

Production safety: Use debug commands only when show commands are not enough. Filter and stop them promptly with undebug all.

11. Troubleshoot by the DUAL Decision Point

Symptom	Meaning	First useful check
No successor	No usable EIGRP path is installed.	Neighbors, topology entry, route source, and policy.
Alternate is known but not an FS	Its RD fails the feasibility condition.	Compare candidate RD with current FD.
FS exists but is not in the RIB	It is a backup, not necessarily an installed forwarding path.	Successor count, variance, and maximum paths.
Route briefly Active	No FS was available and DUAL diffused a search.	Query and Reply direction for the exact prefix.
Route stays Active	A required Reply is delayed or missing.	`show ip eigrp topology active` and the named waiting peer.
Many prefixes become Active	Wider transport, stability, or topology problem.	Link health, queues, CPU, summaries, and stubs.

Name one affected prefix and record its current state.
Identify the successor, FD, and every candidate RD.
If Active, find the neighbor whose Reply is still outstanding.
Move one hop toward that neighbor and repeat the same evidence check.

12. Guided DUAL Failure Labs

Run these tasks in an isolated three- or four-router topology. Save the healthy output before causing a failure.

Task 1: Identify the Roles

Choose one learned prefix.
Record each total metric and RD.
Name the successor.
Apply RD < FD to every alternate.
Predict each feasible successor.

Expected result: Your prediction matches show ip eigrp topology all-links.

Task 2: Fail a Path with an FS

Confirm a feasible successor exists.
Save route and CEF output.
Shut the successor link.
Watch the backup become successor.
Confirm the route stays Passive.

Expected result: The backup installs without a diffusing Query.

Task 3: Trigger an Active Search

Use a prefix with no feasible successor.
Shut its successor link.
Observe the Active state.
Identify Query and Reply peers.
Confirm the final Passive state.

Expected result: You connect packet exchange to the route state.

Task 4: Reduce Query Scope

Record which routers receive the Query.
Restore the baseline.
Add an appropriate stub or summary boundary.
Repeat the failure.
Compare the Query domain.

Expected result: Unnecessary routers no longer join the search.

13. Practice ENARSI-Style Decisions

Scenario	Correct interpretation
Candidate total is lower, but its RD is above the current FD	It may look attractive, but it is not a feasible successor.
Candidate RD equals the current FD	It fails because the rule is strictly less than.
A feasible successor is absent from the routing table	Normal; an FS can remain only in the topology table.
A route shows Passive	Normal stable state, not a disabled route.
No FS exists after the successor fails	The route becomes Active and DUAL sends Queries.
A path fails feasibility before failure but is selected after Replies	Valid; the diffusing calculation supplied fresh loop-free information.

14. Frequently Asked Questions

Is a feasible successor always installed in the routing table?

No. It is a proven loop-free backup in the topology table. Installation also depends on path cost, equal-cost rules, variance, and maximum paths.

Is reported distance the same as administrative distance?

No. Reported distance is the metric a neighbor advertises to a destination. Administrative distance ranks information learned from different routing sources.

Does the lowest alternate metric always become the feasible successor?

No. The candidate must first satisfy RD < current FD. Feasibility is a loop-safety test, not a speed ranking.

Does Passive mean EIGRP stopped working?

No. Passive is the normal stable state for a route. It means DUAL is not currently waiting for Replies.

Can a non-feasible path ever be used?

Yes. If the successor fails, a diffusing Query process can produce fresh information that allows DUAL to select that path safely.

What causes a route to become Active?

The current successor failed and no feasible successor was ready, so DUAL must ask eligible neighbors for another path.

What is the first command for an Active route problem?

Use show ip eigrp topology active to identify the exact prefix and the neighbor from which a Reply is still required.

How do stub routing and summaries help DUAL?

They reduce the Query domain by preventing searches from reaching routers that cannot provide a useful alternate path.

EIGRP DUAL: Choose a Path and Recover Safely