Understanding Administrative Distance

 Administrative Distance

Administrative distance (AD) is a numerical value assigned to different routing protocols that determine which route a router will choose when multiple paths are available to the same destination, essentially acting as a trust level for the source of routing information, with a lower AD value signifying a more trusted source and thus being preferred by the router; essentially, when a router receives routes from multiple protocols for the same destination, the route with the lowest AD will be chosen to forward traffic.

Key points about administrative distance:

• Trust level: AD's primary function is to indicate the trustworthiness of a routing protocol, with a lower value representing a more reliable source of routing information.
• Router decision-making: When a router receives routes for the same destination from multiple protocols, it compares their AD values and selects the route with the lowest AD to install in its routing table.
• Configuration: Most network devices, like Cisco routers, have a default AD value for each routing protocol, but network administrators can manually adjust these values to prioritize specific routes based on their network design.
• Range: Administrative distance values typically range from 0 (most trusted) to 255 (least trusted).

Example scenario:

• Imagine a network where both OSPF and RIP are running.
• On a Cisco router, OSPF usually has a default AD of 110, while RIP has a default AD of 120.
• If the router receives routes to the same destination from both protocols, it will choose the OSPF route because it has a lower AD, indicating a more reliable path.

Administrative Distance chart:

Important considerations:

Static routes: Static routes often have a very low AD, making them a good choice for critical connections where you want to force traffic along a specific path.

Directly connected networks: Directly connected networks usually have the lowest AD value and are considered the most reliable.

Impact on network design: Properly configuring AD values is crucial for network stability and performance, as it allows administrators to control traffic flow based on the trustworthiness of different routing sources.

This post is covered in Network+.

Understanding EIGRP: A Comprehensive Guide to Enhanced Interior Gateway Routing Protocol

 EIGRP

EIGRP, which stands for Enhanced Interior Gateway Routing Protocol, is a dynamic routing protocol used in network environments to efficiently share routing information between routers, allowing data to traverse different network topologies by determining the best path to reach a destination, considered a hybrid protocol combining aspects of both distance-vector and link-state routing algorithms; it is known for its fast convergence, scalability, and efficient bandwidth utilization, making it a popular choice for large networks.

Key features of EIGRP:

• Neighbor Discovery: EIGRP establishes neighbor relationships with other routers by exchanging "Hello" packets on shared networks, which is crucial for routing information exchange.
• DUAL Finite State Machine: A core component of EIGRP that calculates the best path to a destination using a complex algorithm, considering factors like bandwidth, delay, and reliability.
• Reliable Transport Protocol: Ensures reliable delivery of routing updates between neighbors using a mechanism to retransmit lost packets.
• Fast Convergence: EIGRP quickly adapts to network changes by rapidly recalculating routes when topology updates occur, minimizing network disruptions.

How EIGRP works:

1. Neighbor Discovery: Routers send "Hello" packets to discover potential neighbors on connected networks.

2. Route Updates: Once neighbors are established, routers exchange routing updates containing information about reachable networks and their associated metrics.

3. DUAL Calculation: The DUAL algorithm within each router calculates the best path to a destination by considering the received routing updates and factors like bandwidth and delay.

4. Route Propagation: The calculated best routes are then propagated to other neighbors, allowing the entire network to converge on the optimal paths.

5. EIGRP uses IP protocol 88

Important EIGRP Concepts:

• Autonomous System (AS): A collection of routers that share routing information within the same EIGRP domain.
• Feasible Distance: The metric used by EIGRP to determine the best path to a destination, considering the cost to reach a neighbor and the advertised distance from that neighbor to the destination.
• Feasible Successor: EIGRP also uses the term "feasible successor," which refers to a loop-free backup route that is not necessarily the best route.
• Split Horizon: A mechanism that prevents loops by preventing a router from advertising a route back to the neighbor from which it received the route.
• Query Process: When a router receives a route update with a better metric for a known network, it initiates a query process to gather more information from neighbors to verify the new path.

Advantages of EIGRP:

• Fast Convergence: EIGRP quickly reacts to network changes due to its DUAL algorithm.
• Scalability: Can efficiently manage large networks with many routers.
• Load Balancing: Supports load balancing by using multiple equal-cost paths.

Disadvantages of EIGRP:

• Complexity: EIGRP configuration can be more intricate than other routing protocols.
• Proprietary: Primarily implemented on Cisco devices, which can limit interoperability with other vendor routers.

This post is covered in Network+.