Cisco CCNP Enterprise (ENCOR 350-401) · lesson 3 of 12 · 35 min
EIGRP Review: The Hybrid Protocol
EIGRP (Enhanced Interior Gateway Routing Protocol) is Cisco’s advanced distance-vector protocol. It combines the simplicity of distance-vector with some link-state characteristics — it sends partial updates only when topology changes occur, maintains a topology table (not just a routing table), and uses a sophisticated loop-prevention algorithm called DUAL.
EIGRP Metric Calculation
EIGRP’s composite metric formula looks intimidating but the exam almost always uses the simplified version with only bandwidth and delay (K1=1, K3=1, K2=K4=K5=0): Simplified EIGRP Metric Formula Metric = 256 × (10,000,000 / minimum-bandwidth-kbps + cumulative-delay-in-tens-of-microseconds) The minimum bandwidth is the lowest bandwidth on the entire path (bottleneck). The cumulative delay is the sum of all interface delays along the path. Both are reported in the topology table as the Feasible Distance (FD) .
DUAL: How Loop-Free Backup Paths Work
DUAL is what makes EIGRP special. When a successor route fails, EIGRP checks the topology table for a feasible successor. If one exists, it is promoted to successor instantly — no query process, no waiting. This is called a local computation and takes milliseconds. If no feasible successor exists, EIGRP enters Active state and sends Query packets to all neighbors asking if they have a path to the destination. Routers that receive a Query check their own topology table and reply. The router waits for all replies before recalculating. This is the diffusing computation . Think of DUAL like a fire escape plan. The Feasible Successor is the pre-approved alternate exit — you can use it immediately without asking anyone. If that exit is also blocked, you have to ask everyone in the building (Active state / Query process) before you can find a new way out. SIA (Stuck in Active) is a critical troubleshooting topic. It usually means a query propagated too far (unbounded query domain) or a neighbor is overloaded. Fix with route summarization to limit query scope, or stub routers to prevent queries from propagating to stub sites.
EIGRP Stub Routers
An EIGRP stub router tells its neighbors “don’t send me Queries — I have no alternate paths.” This is configured on spoke routers in hub-and-spoke topologies. It prevents queries from propagating to sites that can’t possibly have alternate paths, dramatically reducing SIA risk.
EIGRP Variance: Unequal-Cost Load Balancing
EIGRP’s most unique feature is its ability to load-balance across unequal-cost paths using the variance multiplier. No other IGP can do this. The rule: a route is eligible for unequal-cost load balancing if its metric is less than or equal to (variance × FD of the successor). The route must also be a feasible successor (satisfy the FC). Variance Example Successor FD = 1000. Variance = 2. Any feasible successor with FD ≤ 2000 will be included in load balancing. Traffic is distributed proportionally — a path with FD 1000 gets twice the traffic of a path with FD 2000. Variance only works with feasible successors — a path that doesn’t satisfy the FC cannot be used for load balancing even if variance would include it. This is a common exam trap.
EIGRP Named Mode vs Classic Mode
Modern EIGRP uses named mode (address-family configuration) which supports both IPv4 and IPv6 in a single process and provides more granular per-interface configuration. Classic mode is still tested but named mode is the current standard.
Flashcard Drill
Feasible Distance (FD) The best total metric from this router to the destination — the metric of the installed successor route. Reported Distance (RD) The metric a neighbor advertises for reaching a destination — the neighbor’s own FD. Feasibility Condition RD < local FD — a neighbor’s reported distance must be strictly less than our current best metric to guarantee loop-freedom. Feasible Successor A backup next-hop that satisfies the FC — can be promoted to successor instantly without a query. Active state EIGRP is querying neighbors for a new path — no feasible successor was available after the successor failed. Stuck in Active (SIA) A neighbor didn’t reply to a Query in time — neighbor relationship is reset. Usually caused by unbounded queries or overloaded routers. EIGRP stub Tells neighbors not to send Queries — prevents SIA in hub-and-spoke. Stub advertises only connected/summary by default. Variance EIGRP multiplier for unequal-cost load balancing — paths with FD ≤ variance × successor FD are included (must be feasible successors). DUAL Diffusing Update Algorithm — EIGRP’s loop-prevention algorithm. Uses FD/RD/FC to find loop-free backup paths instantly.
Knowledge Check — 12 Questions
EIGRP DUAL, Feasible Successor & Variance
What condition must a neighbor satisfy to be a Feasible Successor?
- RD > FD
- RD = FD
- RD < FD
- RD ≤ FD
Answer: RD < FD
The Feasibility Condition requires the neighbor’s Reported Distance to be strictly less than the local Feasible Distance. This guarantees the backup path is loop-free.
When a successor fails and a feasible successor exists, what does EIGRP do?
- Sends queries to all neighbors
- Runs SPF algorithm
- Promotes the feasible successor instantly — no query needed
- Waits for Hello timer to expire
Answer: Promotes the feasible successor instantly — no query needed
A feasible successor is pre-validated as loop-free. EIGRP promotes it to successor immediately — this is a local computation taking milliseconds.
What does EIGRP variance 3 mean?
- Only 3 equal-cost paths allowed
- Paths with FD up to 3× the successor FD are eligible for load balancing
- The metric is divided by 3
- Maximum 3 hops allowed
Answer: Paths with FD up to 3× the successor FD are eligible for load balancing
Variance multiplies the successor’s FD. Any feasible successor with FD ≤ variance × successor FD is included in unequal-cost load balancing.
A path that does not satisfy the Feasibility Condition can still be used for variance-based load balancing.
- True
- False
Answer: False
Variance only works with feasible successors. A path that doesn’t satisfy the FC cannot be used for load balancing regardless of variance setting.
What is Stuck in Active (SIA) and what typically causes it?
- A route that never changes — caused by static configuration
- A neighbor that didn’t reply to a Query in time — caused by unbounded queries or overloaded routers
- A route with infinite metric — caused by split horizon
- A passive interface — caused by misconfiguration
Answer: A neighbor that didn’t reply to a Query in time — caused by unbounded queries or overloaded routers
SIA occurs when a Query propagates to a neighbor that can’t respond in time (default 3 min). The neighbor relationship is reset. Fix with stub routers or summarization.
Which EIGRP stub option prevents the stub router from advertising any routes?
- eigrp stub connected
- eigrp stub summary
- eigrp stub receive-only
- eigrp stub static
Answer: eigrp stub receive-only
receive-only makes the router a pure stub — it receives routes but advertises nothing. Useful for sites that only consume routes.
EIGRP’s loop-prevention algorithm is called _____.
Answer: DUAL
DUAL (Diffusing Update Algorithm) uses Feasible Distance, Reported Distance, and the Feasibility Condition to guarantee loop-free paths.
What is the default EIGRP variance value?
- 0
- 1
- 2
- 4
Answer: 1
Variance 1 means only equal-cost paths are used for load balancing. This is the default.
EIGRP uses which transport mechanism to ensure reliable delivery of updates?
- TCP
- UDP
- RTP (Reliable Transport Protocol)
- OSPF flooding
Answer: RTP (Reliable Transport Protocol)
EIGRP uses its own Reliable Transport Protocol (RTP) to ensure ordered, reliable delivery of packets like Updates, Queries, and Replies.
In EIGRP metric calculation, which bandwidth value is used for a multi-hop path?
- Average bandwidth of all links
- Maximum bandwidth on the path
- Minimum bandwidth on the path (bottleneck)
- Bandwidth of the first link
Answer: Minimum bandwidth on the path (bottleneck)
EIGRP uses the minimum (lowest) bandwidth along the entire path — the bottleneck link. This is combined with cumulative delay to form the composite metric.
What is the administrative distance of an EIGRP internal route?
- 90
- 110
- 120
- 170
Answer: 90
EIGRP internal routes have AD 90. External EIGRP routes (redistributed from other protocols) have AD 170.
EIGRP stub routers prevent Query packets from being sent to them, reducing SIA risk.
- True
- False
Answer: True
When a router is configured as EIGRP stub, its neighbors mark it as stub and do not send Query packets to it — this limits query scope and prevents SIA.
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