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EIGRP - Key concepts

 EIGRP (Enhanced Interior Gateway Routing Protocol)

Key Concepts

  • EIGRP: An advanced distance-vector routing protocol, originally Cisco proprietary but later standardized (RFC 7868). It’s more efficient than traditional distance-vector protocols, offering fast convergence, reduced bandwidth usage, and robust loop avoidance mechanisms.

Adjacencies

  • Adjacencies: EIGRP forms neighbor relationships using Hello packets. It supports seamless connectivity across all Layer 2 protocols and operates in both LAN and WAN environments.
  • Neighbor Table: Tracks EIGRP neighbors, including details like uptime, hold time, and retransmission timeout (RTO).
  • Passive Interface: Prevents EIGRP from sending Hello packets on specific interfaces while still advertising the network connected to that interface.

Best Path Selection

  • DUAL Algorithm: EIGRP uses the Diffusing Update Algorithm (DUAL) to find the best path, ensuring loop-free routing.
    • Successor: The best path to a destination.
    • Feasible Successor: A backup path that can be used if the successor fails, provided it satisfies the Feasibility Condition (FC).
    • Feasible Distance (FD): The total metric for the best path from the local router’s perspective.
    • Reported Distance (RD): The metric reported by the neighboring router.

Metrics

  • Classic Metrics: EIGRP uses a composite metric based on bandwidth, delay, load, reliability, and MTU. However, only bandwidth and delay are used by default.
    • Formula: Metric = 256 * ((K1 * Bandwidth) + (K3 * Delay)).
    • K-Values: Adjust the weighting for each metric component.
  • Wide Metrics: Introduced for high-speed networks, supporting interfaces up to 655 Tbps by scaling bandwidth and delay metrics.

Packet Types

EIGRP uses five key packet types for communication:

  1. Hello: For discovering and maintaining neighbor relationships.
  2. Update: Used to advertise route changes to neighbors.
  3. Ack: Acknowledges receipt of update packets.
  4. Query: Sent when a route is lost, asking neighbors for alternative routes.
  5. Reply: Response to a query, either with a valid route or indicating no route.

Load Balancing

  • Equal-Cost Load Balancing: EIGRP automatically balances traffic across multiple equal-cost paths.
  • Unequal-Cost Load Balancing: Achieved using the variance command, allowing traffic to be distributed across paths with different metrics.
    • The variance multiplier determines how much worse a route can be compared to the best route and still be used for load balancing.

Named Mode

  • EIGRP Named Mode: Simplifies EIGRP configuration by allowing IPv4, IPv6, and VRF instances to be managed under a single EIGRP process. This mode supports wide metrics and introduces new features like IPv6 support and dynamic protocol adjustments.
    • Auto-Summary: Disabled by default in named mode.

Route Summarization and Redistribution

  • Route Summarization: Reduces routing table size and limits the scope of queries by summarizing networks at the boundary of areas or autonomous systems.
  • Redistribution: EIGRP supports redistribution of routes from other routing protocols (OSPF, BGP, RIP). Metrics can be manually adjusted during redistribution to ensure proper path selection.

Stuck in Active (SIA)

  • SIA: Occurs when a router does not receive replies to its queries within the designated timeout (typically three minutes). This could be caused by network congestion, bad links, or excessive redundancy. Summarization and query scoping help prevent SIAs.

EIGRP Timers

  • Hello and Hold Timers: Control how often Hello packets are sent and how long the router waits before declaring a neighbor dead. Hello timer is typically set to 5 seconds on LANs and 60 seconds on slower WAN links, with the hold timer set to three times the Hello timer.

Stub Routing

  • Stub Routers: Used to reduce query propagation by marking routers as stubs. Stub routers only advertise connected and summary routes, and they do not propagate queries further into the network. This is particularly useful in hub-and-spoke topologies.
    • Stub options include receive-only, connected, summary, static, and redistributed routes.

Query Propagation Boundaries

  • Query Scoping: Queries can propagate throughout the network when a route is lost. Summarization and stubs help limit the spread of queries, improving network stability and reducing convergence time.

Fast Reroute (FRR)

  • Loop-Free Alternates (LFA): EIGRP supports fast reroute by precomputing backup paths (feasible successors) that can be immediately used in case of failure, ensuring quick convergence without recalculation.

Authentication

  • Authentication: EIGRP supports MD5 and SHA-based authentication methods, ensuring that only trusted routers can participate in EIGRP adjacencies. Authentication can be configured per-interface or globally for the entire autonomous system.

Graceful Shutdown

  • Graceful Shutdown: EIGRP supports a graceful shutdown mechanism, allowing a router to inform neighbors when it is going down, reducing the chances of network instability.

Troubleshooting EIGRP

Common issues include:

  • Mismatched AS numbers: Neighbors must be in the same autonomous system.
  • K-value Mismatch: K-values must be identical on all routers to form adjacencies.
  • Passive Interface: If configured, the interface won’t send or receive Hello packets, preventing adjacency formation.
  • Subnet Mismatch: Neighbors must share the same subnet for Hellos to be exchanged.
  • Authentication Failures: Key ID and key string must match across neighbors.
  • Access Control Lists (ACLs): Ensure ACLs aren’t blocking EIGRP packets, especially inbound on interfaces.

Conclusion

EIGRP is a highly scalable, robust, and efficient routing protocol with features such as fast convergence, load balancing, and flexible authentication. It is ideal for both small and large enterprise networks, offering fine control over routing behavior and minimal bandwidth usage during convergence. Summarization, stub routing, and FRR further enhance its efficiency, especially in complex topologies.
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