50 Expert Q&As + 15-Question Interactive Quiz

Diffusing Update Algorithm — EIGRP's path calculation engine that guarantees loop-free routing.

Successor — the path with the lowest Cumulative Distance to the destination.

Feasible Successor — a path that satisfies the Feasibility Condition.

Feasible Distance — the best (lowest) cumulative metric seen to a destination.

Reported Distance — the metric a neighbor advertises to reach a destination.

Cumulative Distance — the total metric from this router to the destination via a specific neighbor.

RD < FD — the neighbor's reported distance must be less than the local feasible distance.

Active — the router is querying neighbors to find an alternate path.

Passive — the stable state where the route is forwarding normally.

Stuck-In-Active — a failure state when a neighbor does not reply to a Query within the Active timer.

3 minutes — how long DUAL waits for a Query reply before declaring SIA.

Query — sent via multicast 224.0.0.10 to all neighbors.

Negative Reply — the neighbor responds with delay infinity.

Stub Routing — prevents the router from forwarding queries to other stub routers.

Summarization — summary routes cause queries for specific subnets to stop at the boundary.

A Successor is the primary path with the lowest Cumulative Distance and is installed in the routing table, while a Feasible Successor is a loop-free backup path kept in the topology table, ready to become the successor instantly if the primary fails.

The Feasibility Condition (RD < FD) mathematically guarantees that a neighbor is closer to the destination than the local router, which prevents routing loops because the neighbor cannot route traffic back through the local router to reach the same destination.

The router sends multicast Queries to all neighbors asking if they have a valid path to the destination, and the route remains Active until every neighbor replies or the Active Timer expires and triggers a SIA event.

Because each router independently uses its own Hello and Hold timers, and the Hold timer only needs to be long enough to receive the neighbor's Hello before the adjacency expires.

The Active Timer (default 3 minutes) defines how long a router will wait for Query replies before declaring the neighbor Stuck-In-Active and tearing down the adjacency.

The SIA-Query is sent at half the Active Timer (1.5 minutes) to confirm the neighbor is still alive and working on the reply, extending the deadline if the neighbor responds.

Because the alternate path fails the Feasibility Condition, meaning its Reported Distance is greater than or equal to the Feasible Distance, so it cannot be used as a loop-free backup.

A Positive Reply indicates the neighbor has a valid path to the destination and includes the alternate route, while a Negative Reply indicates the neighbor has no path and reports the destination with delay infinity.

Stub routing confines query propagation to the edge of the network by ensuring that stub routers do not forward queries, preventing cascading SIA events across a large topology.

A boundary router advertises a summary route instead of individual subnets, so queries for specific subnets stop at the boundary because downstream routers only know the summary, not the specific destinations.

The Null0 route (AD 5) prevents routing loops and blackholes by discarding traffic for specific subnets that are no longer available within the summary range.

An offset-list inflates the Reported Distance on a specific path, and if the inflated RD exceeds the Feasible Distance, the path fails the Feasibility Condition and can no longer serve as a Feasible Successor.

A Passive route has a valid successor and is stable, while an Active route has lost its successor and no Feasible Successor exists, so the router is querying neighbors to find an alternate path.

Delay is a logical metric used only by routing protocols and can be safely adjusted without impacting physical link behavior, while bandwidth changes affect QoS policies, queue behavior, and upper-layer application calculations.

A query storm propagates queries across the entire topology in a matter of seconds, causing CPU spikes, delayed convergence, and possible SIA events that result in cascading route failures.

Immediate Steps:

show ip eigrp topology active
show ip eigrp events | include SIA|Query

Check which neighbor has not replied to the Query. Verify the neighbor is reachable using ping and show ip eigrp neighbors. If the neighbor is slow to reply due to a large topology, the Active Timer may need to be raised — but the real fix is usually query confinement via stub routing or summarization.

Root Cause: An unstable underlying link or a path with no Feasible Successor.

Diagnostic:

show ip eigrp topology | include Active
show log | include UPDOWN|NBRCHANGE
show interfaces | include flap|error|reset

Fix: Stabilize the physical link. If the link is stable but the route still flaps, add a Feasible Successor by tuning metrics or adding an alternate path.

Yes. A branch router at the network edge should be configured as an EIGRP stub to prevent query propagation from the core.

router eigrp 1234
 eigrp stub connected summary

Use connected summary if the branch advertises local subnets. Use receive-only if the branch is a pure consumer with no local subnets to advertise.

Possible Causes:

• The Feasible Successor is via the same physical interface that failed
• The Feasible Successor lost reachability at the same time
• A recent metric change invalidated the FS just before the failure

Diagnostic: Check show ip eigrp topology all-links and correlate timestamps with show log. Verify that the FS is reachable.

Recovery Steps:

show ip eigrp topology all-links | include OFFSET_DEST
router eigrp 1234
 no offset-list OFFSET_DEST in XXX Gi0/2

! Or reduce the offset so RD stays < FD
router eigrp 1234
 offset-list OFFSET_DEST in 5000 Gi0/2

After removing or reducing the offset, verify the FS returns with show ip eigrp topology.

Meaning: A route has been in Active state for 1.5 minutes (half the Active Timer). The router is sending an SIA-Query to confirm the neighbor is alive.

Actions:

1. Check show ip eigrp topology active to see which route and neighbor
2. Check show ip eigrp neighbors for the neighbor state
3. Verify the neighbor CPU and link are stable
4. If this repeats, deploy query confinement (stub + summarization)

No. The Feasibility Condition is strictly RD < FD. Since RD (156160) equals FD (156160), the condition fails and the backup path cannot be a Feasible Successor.

Implication: If the primary path fails, the route enters Active state and the router will send Queries to find a path that satisfies the condition.

Safe Testing Approach:

1. Schedule a maintenance window
2. Use a non-critical edge router as the test point
3. Shut down the interface on the test router only
4. Monitor with show ip eigrp topology active and debug eigrp packets
5. Restore the interface within the window

Do NOT: Test on a core router during business hours. A single SIA event can cascade across the network.

Meaning: The route has lost its successor, and the FD cannot be computed because no valid path exists. The route is likely in Active state or about to be removed.

Diagnostic:

show ip eigrp topology all-links | include Inaccessible
show ip eigrp events | include Lost|Active

Fix: Restore the failed link or add an alternate path. If the destination is genuinely gone, the route will be removed automatically.

Likely Causes:

• The specific subnets are not actually advertised upstream
• A filter (distribute-list or prefix-list) is blocking the specific subnets
• The summary mask is incorrect (does not encompass the intended subnets)

Verification:

show ip route | include 10.16
show ip eigrp topology | include 10.16

Fix: Correct the summary mask or remove the blocking filter.

Yes — but only with unequal-cost load balancing using the variance command.

router eigrp 1234
 variance 2

The candidate path must satisfy: FD × variance ≥ candidate metric. If the metric difference is too large, no single variance value can bring the path into the routing table.

Immediate Actions (in order):

1. Check show ip eigrp neighbors — are the expected neighbors up?
2. Check show ip eigrp topology all-links — is the route Active?
3. Check show log — look for recent NBRCHANGE or SIA messages
4. Check physical interfaces: show interfaces | include error|flap
5. If neighbors are up, look for a filter (distribute-list) blocking the advertisement

Top Three Design Patterns:

1. Stub routing on all branch routers (eigrp stub connected summary)
2. Summarization at the distribution/core boundary (ip summary-address eigrp)
3. Hierarchical topology with no more than 3–4 EIGRP tiers deep

These three patterns reduce the query domain from hundreds of routers to a handful, eliminating SIA risk.

No. A path in the topology table may be a Successor, Feasible Successor, or an Alternate. Only paths that satisfy RD < FD qualify as Feasible Successors. Paths in show ip eigrp topology all-links that fail this condition are Alternates and cannot be used for instant failover.

Possible Causes:

• The delay change affected both paths equally
• The change was not large enough to alter the path ordering
• K-values may not include delay (non-default metric weights)
• Another path still has a lower cumulative metric

Verify:

show ip protocols | include K-values
show ip eigrp topology | include FD

Most Likely Cause: The neighbor is part of a large query domain and the reply is delayed because the neighbor must wait for its own downstream queries to complete.

Fix: This is a design problem, not a device problem. Add stub routing or summarization to break the query chain.

DUAL uses the Active Timer (default 3 minutes). If no positive reply arrives within this window, DUAL:

1. Sends an SIA-Query at 1.5 minutes
2. If the neighbor replies to SIA-Query, extends the deadline
3. If no reply at 3 minutes, declares the neighbor Stuck-In-Active
4. Tears down the neighbor adjacency and removes all routes via that neighbor

Partially. When summarization is configured correctly, EIGRP automatically installs a Null0 discard route with AD 5. If the summary is manually removed or the configuration is deleted, the Null0 route disappears and traffic to subnets inside the summary range is silently dropped or looped.

Fix: Re-apply the summarization command. The Null0 route will be reinstalled automatically.

Approach:

! Baseline the current counters
show ip eigrp traffic

! Wait 5 minutes

! Re-check counters and calculate delta
show ip eigrp traffic

! Sample output shows Query and Reply counters
! Compare before vs after to compute query rate

For ongoing monitoring, use SNMP polling of the same OIDs.

Redesign Strategy:

1. Reduce the topology to a 3-tier hierarchical design (Core → Distribution → Access)
2. Configure stub on all access-tier routers
3. Configure summarization at the distribution layer
4. Ensure every access subnet has a Feasible Successor where possible
5. Use offset-lists carefully — avoid inflating metrics past FD

Result: The query domain is confined to the core and distribution layers, eliminating the query storm even during multiple simultaneous failures.