In Chapter 7, we explore Enhanced Interior Gateway Routing Protocol (EIGRP), a Cisco proprietary protocol that combines features from distance vector and link-state protocols, making it a hybrid routing protocol. EIGRP is widely used in Cisco networks due to its fast convergence, scalability, and flexibility. This chapter covers EIGRP terminology, topology tables, packet types, and key metrics, along with its advanced features like unequal-cost load balancing.
Diff between EIGRP & OSPF
📝 EIGRP Terminology
Before we dive deeper into EIGRP, let’s define some key terms:
- Successor: The primary route to a destination with the lowest metric.
- Feasible Successor: A backup route that meets the feasibility condition (i.e., it has a lower advertised distance than the current successor).
- Advertised Distance (AD): The cost from a neighboring router to a destination.
- Feasible Distance (FD): The total metric (cost) from the local router to a destination, calculated by adding the metric from the local router to a neighbor and the AD from that neighbor.
These terms are essential for understanding how EIGRP chooses and maintains routes.
🌐 EIGRP Topology Table
EIGRP maintains a topology table that holds information on all learned routes, including the feasible successors. Unlike the routing table, which only contains active routes, the topology table contains all possible routes, allowing EIGRP to quickly converge in the event of a primary route failure.
To view the EIGRP topology table, use:
show ip eigrp topology
Tip: This command is essential for troubleshooting and verifying EIGRP path selection and backup routes.
📦 EIGRP Packet Types
EIGRP uses five main packet types to communicate between routers and maintain its topology. Here’s a breakdown:
- Hello Packets 👋: Used for neighbor discovery and to establish adjacencies. Sent periodically to maintain connectivity.
- Update Packets 📬: Carry routing information to update neighboring routers about network changes.
- Query Packets ❓: Sent when a route is lost and no feasible successor is available. Routers query neighbors for an alternative route.
- Reply Packets 💬: Responses to query packets, providing information about alternative routes.
- Request: Used to get specific information from one or more neighbors
These packets allow EIGRP to efficiently exchange routing information and maintain the network topology.
🔄 EIGRP Attribute Propagation and Metrics
EIGRP uses a composite metric formula that considers bandwidth, delay, reliability, load, and MTU, giving network administrators fine-grained control over route selection. The default metric calculation uses only bandwidth and delay, but additional metrics can be included as needed.
EIGRP Metric Calculation:
EIGRP uses the following formula for its composite metric:
Metric = [K1 * Bandwidth + (K3 * Delay)] * 256- (10**7 / Lowest BW(kbps) + Delay / 10 ) * 256
Where:
- Bandwidth is the minimum bandwidth along the path.
- Delay is the cumulative delay along the path.
To configure EIGRP metrics, adjust the K-values as needed:
Router(config-router)# metric weights <tos> <K1> <K2> <K3> <K4> <K5>
The default K-values are K1=1 and K3=1, with all others set to 0, meaning EIGRP only considers bandwidth and delay by default.
⚖️ EIGRP Unequal-Cost Load Balancing
One of EIGRP’s unique features is unequal-cost load balancing, which allows traffic to be distributed across multiple paths with different metrics. This capability is useful for leveraging available bandwidth and enhancing network resilience.
Enabling Unequal-Cost Load Balancing:
To enable unequal-cost load balancing, use the variance command. The variance multiplier determines which backup routes can be used for load balancing based on their feasibility distance.
Router(config-router)# variance <multiplier>
For example, setting variance 2 allows EIGRP to use paths with a metric up to twice as high as the successor route for load balancing.
⏱️ EIGRP Convergence
EIGRP’s convergence time is fast, thanks to its topology table and use of feasible successors. Upon detecting a primary route failure, EIGRP immediately switches to a feasible successor if one exists, minimizing downtime. If no feasible successor is available, EIGRP initiates a query process to find an alternative path.
Quick Recovery: By keeping feasible successors in the topology table, EIGRP can recover from link failures rapidly, making it suitable for mission-critical networks.
🔍 Active Route State
In EIGRP, a route enters the Active state when the primary route fails, and the router is searching for a replacement. During this state:
- The router sends query packets to neighboring routers to find an alternative path.
- The route remains active until a reply is received or until a feasible successor is identified.
EIGRP’s use of the active route state allows for a controlled search for backup routes, contributing to its stability and efficiency.
📊 EIGRP Wide Metrics Formula
With newer Cisco devices, EIGRP supports a wide metric formula, which is beneficial for high-speed networks (e.g., gigabit and 10-gigabit links). This formula uses larger metric values and provides greater granularity in route selection.
The wide metric formula accommodates modern network speeds, making EIGRP more flexible and future-proof in environments with diverse link capacities.
📋 Summary
Chapter 8 highlights EIGRP’s unique characteristics and advanced capabilities, making it an ideal protocol for scalable, resilient networks. Here’s a recap:
- EIGRP Terminology: Understanding successors, feasible successors, and distances is essential for interpreting EIGRP’s route selection.
- Topology Table: Stores all learned routes, including backup paths, allowing EIGRP to converge quickly.
- Packet Types: EIGRP uses Hello, Update, Query, Reply, and Acknowledgment packets to maintain adjacencies and manage topology changes.
- Attribute Propagation and Metrics: EIGRP calculates metrics based on bandwidth and delay, but it can include additional factors for more control.
- Unequal-Cost Load Balancing: Use the variance command to enable load balancing over multiple paths with different metrics.
- Convergence: EIGRP’s rapid convergence and efficient query mechanism ensure minimal disruption during network changes.
- Active Route State: Provides a controlled way for EIGRP to find alternative paths when primary routes are lost.
In the next chapter, we’ll explore OSPF (Open Shortest Path First), another dynamic routing protocol that provides an alternative to EIGRP, particularly in multi-vendor environments. Stay tuned for more insights!
