VMware ESXi and vSphere Cluster Management

EIGRP Diffusing Update Algorithm (DUAL)

Learn how EIGRP DUAL selects successors, qualifies feasible successors, prevents routing loops, and converges after route failures.

DUAL, the Diffusing Update Algorithm, is the route-calculation and convergence mechanism used by EIGRP. It evaluates routes learned from neighbors, selects loop-free paths, keeps qualified backup paths, and coordinates route recomputation when a path fails.

For a destination network, DUAL can choose and install a successor, retain other candidate paths in the EIGRP topology table, promote a feasible successor after failure, or query neighbors when no immediately safe backup exists.

What DUAL Does in EIGRP

EIGRP, or Enhanced Interior Gateway Routing Protocol, learns route advertisements from neighboring routers. DUAL uses those advertisements and their EIGRP metrics to maintain loop-free paths to remote networks.

  • It compares available paths to each destination.
  • It selects the best loop-free path and installs it in the IP routing table.
  • It retains candidate routes in the topology table.
  • It keeps qualified backup paths ready for rapid recovery.
  • It coordinates a distributed route calculation when no qualified backup is available.

DUAL operates per destination. A destination can be passive while another destination on the same router is active; “passive” and “active” do not describe the entire router or the entire EIGRP process.

The EIGRP Topology Table and Routing Table

EIGRP receives advertisements from neighbors and retains the resulting candidate-route information in its topology table. This table contains route metrics, next-hop information, successor information, and the state of each destination.

The routing table is different. It is the forwarding table used by IP to decide where packets go. Normally, DUAL installs only selected successor routes in the routing table. Other candidates can remain in the topology table without being used for forwarding.

Keeping information from multiple neighbors allows DUAL to compare paths before and after a failure. A route can therefore be valid topology information without being a usable immediate backup.

Successors and Feasible Successors

A successor is the best loop-free next hop selected by DUAL for a destination. The successor route is normally installed in the IP routing table.

A feasible successor is a prequalified loop-free backup route. If the successor fails, DUAL can promote the feasible successor locally without starting a query process for that destination.

Multiple equal-cost successors may be installed when the available paths meet EIGRP equal-cost multipath requirements. This is different from retaining a backup with a higher total metric.

Successor: Best loop-free route; normally installed in the routing table; used for ordinary forwarding and replaced after failure if another suitable route exists.

Feasible successor: Prequalified loop-free backup; usually retained in the topology table; can be promoted immediately after successor failure.

Other topology-table candidate: Learned route that may be valid but does not satisfy the backup test; not necessarily installed or immediately usable.

Passive destination: Stable destination with no outstanding DUAL computation; the destination has a settled route or has been determined unreachable.

Active destination: Destination undergoing a query-based recomputation after loss of its successor without a feasible successor.

EIGRP Metrics Used by DUAL

The two most important distance values are feasible distance and reported distance.

  • Feasible distance (FD): The local router's lowest calculated EIGRP metric to a destination. It represents the best known total distance from this router.
  • Reported distance (RD): Also called advertised distance, this is the metric a neighboring router reports for reaching the destination from that neighbor.
  • Composite metric: The calculated EIGRP path metric. By default, EIGRP uses bandwidth and delay.

Reliability and load are not used by default. They can affect the composite metric if EIGRP K-values are changed, but changing K-values requires compatible settings between neighbors.

Feasible distance: Source: local calculation. Meaning: the best total EIGRP distance currently known from this router. DUAL uses it to select the successor and to evaluate the feasibility condition.

Reported distance: Source: neighbor advertisement. Meaning: the neighbor's distance to the destination. DUAL compares it with the local FD when qualifying a feasible successor.

Composite metric: Source: EIGRP metric calculation. Meaning: total path cost based by default on bandwidth and delay. DUAL compares total metrics to rank candidates.

Feasibility condition: Source: comparison of RD and local FD. Meaning: a neighbor's RD must be less than the local FD for the route to qualify as a feasible successor.

The Feasibility Condition and Loop Prevention

The feasibility condition is:

Neighbor reported distance < local feasible distance

If a neighbor's reported distance is lower than the local router's feasible distance, that neighbor is considered closer to the destination than the local router's previously known best distance. This provides a loop-free test for using the neighbor as an immediate backup.

A lower total path metric alone is not enough. A route may have an attractive calculated metric and still fail the feasibility condition. Such a route can remain in the topology table, but DUAL does not qualify it as a feasible successor.

Example: Selecting Two Paths

Suppose a router learns network 192.168.50.0/24 through Neighbor A and Neighbor B. The path through A has the lowest total metric, so A becomes the successor. Neighbor B reports a distance lower than the local feasible distance, so B passes the feasibility condition and becomes a feasible successor.

If the path through A fails, DUAL can promote B immediately. The route remains available without querying the network for another path.

DUAL Route States

A destination is normally in the passive state. Passive means DUAL has completed its calculation and has no outstanding query-based recomputation for that destination. It does not mean that the interface is administratively passive.

A destination enters the active state when its successor is lost and no feasible successor is available. The router must ask neighbors whether an alternate path exists.

Passive: Stable per-destination state; no outstanding computation; route remains installed if reachable.

Active: Query-based per-destination computation; neighbors are being asked for alternate information; the old successor is unavailable.

Return to passive: After required replies arrive, DUAL selects a new loop-free route or determines that the destination is unreachable.

What Happens After a Route Failure

Fast Recovery with a Feasible Successor

  1. The current successor becomes unavailable.
  2. DUAL checks the topology table for a feasible successor.
  3. If one exists, DUAL promotes it locally.
  4. The replacement route is installed for forwarding.
  5. No query process is required for that destination.

This is the fastest local convergence case. The destination remains stable or returns to a stable passive state quickly.

Diffusing Computation Without a Feasible Successor

  1. The successor fails.
  2. No candidate passes the feasibility condition.
  3. DUAL changes the destination from passive to active.
  4. The router sends query packets to applicable EIGRP neighbors.
  5. Neighbors examine their own route information and return reply packets.
  6. The router waits for the required replies.
  7. DUAL selects a new loop-free successor or removes the route if no path exists.
  8. The destination returns to passive.

Convergence time depends on the topology size, the scope of the queries, neighbor responsiveness, control-plane processing, and whether an alternate route exists.

Feasible successor available: State remains stable or changes briefly; no queries are needed; backup is promoted locally.

No feasible successor, neighbors return a path: Destination becomes active; queries and replies occur; a new successor is selected and the destination returns to passive.

No feasible successor and no alternate path: Destination becomes active; queries and replies establish that it is unreachable; the route is removed and the destination returns to passive.

Missing reply or unstable neighbor: Destination can remain active too long; convergence may be delayed and the route can become stuck in active.

EIGRP Messages Used During DUAL Operation

EIGRP control-plane messages support neighbor discovery, route exchange, and reliable distributed computation.

Hello: Discovers neighbors and maintains neighbor liveliness. Stable hello exchange supports a stable adjacency.

Update: Advertises new routes or route metric changes. Updates supply information DUAL uses in its calculations.

Query: Requests alternate route information during an active computation.

Reply: Answers a query and reports whether a neighbor has a usable path or not.

Acknowledgment: Confirms receipt of applicable reliable EIGRP packets.

Reliable Transport Protocol

EIGRP uses Reliable Transport Protocol (RTP) to deliver applicable control-plane information reliably and in order. RTP is an EIGRP transport mechanism, not the unrelated application-layer protocol commonly associated with real-time media.

Successful neighbor formation and a stable adjacency are prerequisites for predictable DUAL operation. Missing replies, broken adjacencies, packet loss, or delayed control-plane processing can extend convergence.

Classless Routing, VLSM, and Summarization

EIGRP carries subnet-mask information with its route advertisements. It therefore supports classless routing and VLSM, or Variable Length Subnet Masking. Correct prefix and mask information lets DUAL compare and install routes accurately.

Manual summarization is an intentional design choice in which an administrator advertises a summary prefix on a selected interface. A well-designed summary can reduce topology-table entries and create a query boundary, limiting how far a query propagates.

Older EIGRP behavior included automatic classful summarization at major-network boundaries. Automatic summarization must be evaluated carefully in discontiguous networks, where separated portions of one classful major network are connected through another major network. A classful summary can hide more-specific subnet information and cause reachability problems.

router eigrp 100
 network 10.0.0.0 0.0.0.255
 network 192.168.1.0 0.0.0.255
 no auto-summary

Use the same autonomous system number between classic EIGRP neighbors. The exact syntax and supported features can vary by Cisco IOS platform and release.

interface GigabitEthernet0/0
 ip summary-address eigrp 100 10.10.0.0 255.255.0.0

Apply a summary only when the summary accurately represents reachable component routes. An overly broad summary can attract traffic for destinations that are not actually reachable.

Operational Verification

Verify neighbor formation before investigating DUAL route selection. Useful Cisco IOS commands include:

show ip eigrp neighbors
show ip protocols
show ip eigrp topology
show ip eigrp topology 192.168.50.0 255.255.255.0
show ip eigrp topology all-links
show ip route eigrp
show ip route 192.168.50.0
  • show ip eigrp neighbors confirms expected adjacencies and neighbor stability.
  • show ip protocols displays the EIGRP process, advertised networks, K-values, and summarization status.
  • show ip eigrp topology displays candidate routes and destination state.
  • show ip eigrp topology all-links helps reveal paths that are not currently selected.
  • show ip route eigrp confirms which EIGRP routes are installed for forwarding.

In topology output, identify the destination's passive or active state, feasible distance, reported distances, successor entries, and feasible-successor candidates. Exact formatting varies by IOS release, so interpret the labels in the output rather than relying only on spacing.

Controlled Convergence Test

  1. Capture baseline output from the neighbor, topology, and routing-table commands.
  2. Confirm that the expected successor and any feasible successor are present.
  3. Test reachability with ping 192.168.50.1 and inspect the path with traceroute 192.168.50.1.
  4. Disable or disconnect a test link in a controlled lab.
  5. Observe whether DUAL promotes a feasible successor or enters active state.
  6. Restore the link and compare route selection, state, and reachability with the baseline.

Troubleshooting DUAL and EIGRP Convergence

No EIGRP Neighbor Appears

Without a neighbor adjacency, the router cannot learn the neighbor's routes for DUAL calculations. Check subnet and Layer 2 connectivity, matching autonomous system numbers, passive-interface settings, network statements, authentication, K-values, address-family configuration, and interface state.

show ip eigrp neighbors
show ip protocols
show running-config | section router eigrp
show ip interface brief

A Topology Route Is Not an Immediate Backup

The route may fail the feasibility condition, may not have the lowest feasible distance, or may not be an equal-cost successor. Inspect the destination-specific topology entry and compare the local FD with each neighbor's RD.

show ip eigrp topology 192.168.50.0 255.255.255.0

A Destination Remains Active or Converges Slowly

Possible causes include the absence of a feasible successor, delayed query replies, an unreachable or overloaded neighbor, an unstable adjacency, or a topology that allows queries to spread too broadly.

show ip eigrp topology active
show ip eigrp neighbors
show logging

Stuck in Active (SIA) is a convergence problem in which a route remains active because required query replies are not received in time. Investigate neighbor stability, packet loss, control-plane load, and query scope. Summarization and deliberate query boundaries can reduce the risk.

An EIGRP Route Disappears After Failure

No feasible successor may have existed, and DUAL may have completed its active calculation without finding an alternate path. A summary route or filtering policy can also prevent the expected specific route from being learned.

show ip eigrp topology 192.168.50.0 255.255.255.0
show ip route 192.168.50.0

Discontiguous Subnets Have Inconsistent Reachability

Check whether automatic classful summarization is enabled or whether a manual summary is too broad or applied on an unsuitable interface.

show ip protocols
show ip route
show running-config interface GigabitEthernet0/0

Scalability and Design Considerations

  • Keep neighbor relationships stable; frequent adjacency loss can repeatedly trigger route recomputation.
  • Use accurate summarization to reduce topology information and query propagation.
  • Design query boundaries so a local failure does not require every router in a large domain to participate.
  • Provide sufficient bandwidth and processing capacity for control-plane traffic.
  • Use route filtering and summaries carefully so they do not hide legitimate destinations.
  • Test failures before production changes and observe both forwarding behavior and DUAL state.

Exam-Relevant Summary

  • DUAL is EIGRP's route-calculation and convergence algorithm.
  • The topology table stores learned candidate routes; the routing table stores selected forwarding routes.
  • The successor is the best loop-free route and is normally installed in the routing table.
  • A feasible successor is a prequalified backup that passes the feasibility condition.
  • The feasibility condition is RD < FD.
  • A lower total metric alone does not make a route a feasible successor.
  • Passive means stable for one destination; active means that destination is undergoing query-based recomputation.
  • A feasible successor enables fast local recovery without queries.
  • Without one, DUAL sends queries, waits for replies, and then selects a route or declares the destination unreachable.
  • Hello, update, query, reply, and acknowledgment messages support EIGRP operation; RTP provides reliable, ordered delivery where required.
  • EIGRP supports classless routing and VLSM.
  • Summarization can reduce query scope, while careless classful automatic summarization can cause problems in discontiguous networks.
  • SIA indicates that required query replies were not received in time.

For a related review, return to EIGRP DUAL route selection and convergence.