CCNA Administrative Distance Cisco Configuration Guide

Administrative Distance

Routers employ a numerical value called Administrative Distance (AD) to rank the dependability or credibility of routing information. The AD is used by the router to identify which route is the most “believable” and ought to be included to the routing table when it learns about the same target network such as different routing protocols.

Core Principles of Administrative Distance

• The “Lower is Better” Rule: The “Lower is Better” Rule States that AD is usually expressed as an integer between 0 and 255. Reliability or preference are higher when the AD value is lower. For example, if a router receives path information for the same network from both OSPF (default AD 110) and EIGRP (default AD 90), it will give preference to the EIGRP route due to its lower value.
• Local Significance: AD is exclusive to the local router and isn’t shared, publicized, or traded with other network routers.
• The 255 Value: An AD of 255 indicates an unidentified or dubious. Any route that receives this value is deemed unreliable and will never be added to the routing table.

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Administrative Distance vs. Routing Metrics

It’s critical to differentiate AD from a metric. Within a single routing protocol, a metric is a measurement (such as bandwidth or hop count) that is used to determine the optimal path. AD is used to select between many protocols, on the other hand. A router analyzes AD values in the selection process after initially searching for the longest prefix match, and it only assesses metrics if the AD values are the same.

Default AD Values (Cisco Standard)

Although different vendors (such Juniper or FortiGate) may have different default values, Cisco’s values are regarded as the industry standard. The following are typical defaults:

Manual Configuration and Practical Uses

AD settings can be manually changed by network administrators to affect routing behavior according to certain design specifications. The distance command in the sub configuration mode of the routing process is frequently used for this.

A typical real-world use case is the floating static route. Static routes that are configured with an AD higher than the primary dynamic protocol (for example, a static backup set to AD 250 while OSPF is 110) stay “invisible” and inactive. Since the backup static route is now the best option, the router immediately switches to it in the event that the primary dynamic connection fails and its route is deleted from the table.

Consider the following analogy: You are looking for a restaurant. Provide you with directions: your GPS (Connected Route), a reliable local acquaintance (Static Route), and an unidentified bystander (RIP). Because you have more faith in the GPS, you would follow it even if the passerby’s route was technically shorter. The administrative distance in this case is represented by how much you trust each individual.

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Routing metrics

When many routes to the same destination are available, routing protocols use routing metrics, which are numerical numbers, to identify the best or most optimum way for forwarding data packets. The most effective path is determined by a router using these metrics when it receives several updates for the same network under the same protocol.

Core Principles of Routing Metrics

A better, more preferred route is indicated by a lower numerical value, according to the basic rule of routing metrics. These values assist routers in load balancing, loop prevention, and network adaptation.

Metrics should be distinguished from Administrative Distance (AD).

• Routing protocols can be selected using Administrative Distance (e.g., selecting whether to trust a RIP route over an OSPF route).
• Within a single protocol, metrics are employed to determine the optimal course of action.
• A RIP hop count of three is not “better” or “worse” than an OSPF cost of three since they utilize completely different units of measurement. This is an example of how metrics from various protocols cannot be compared.

Common Types of Metrics

Different protocols compute their metrics using different criteria, or “logic,” such as:

1. Hop Count: How many routers a packet needs to go through in order to get to its destination.
   • Used by: Rest in peace.
   • Limitation: Its “blind”ness to link speed is a limitation. Since “pinhole congestion” is a drawback of high-speed fiber links, slower 56k links with fewer hops will be favored.
2. Bandwidth: The ability of a link to carry data. In principle, more bandwidth is preferable, but protocols frequently turn this into a “cost” where less is better.
3. Delay (Latency): The duration of a packet’s journey, including propagation and queuing delays, is known as its delay (latency).
4. Cost: An arbitrary number that a protocol or administrator puts on a link; it is frequently inversely correlated with bandwidth.
   • Used by: Used by IS-IS and OSPF.
5. Reliability: Determined by a link’s past reliability and mistake rate.
6. Load: Based on traffic volume, this metric indicates how busy a link is at the moment.

Metrics by Protocol

Advanced Mechanics

• Calculation Logic: Metrics can be multiplicative, concave (based on the path’s minimum value or bottleneck, such as EIGRP bandwidth), or additive (the sum of costs along a path, such as OSPF).
• Equal-Cost Multi-Path (ECMP): In order to increase efficiency, the router will usually implement load balancing, dividing traffic among those links if a protocol discovers many paths with the exact same metric.
• Viewing Metrics: You can see metrics in a Cisco routing table using the show ip route command. In the output (e.g., [90/30720]), the second number inside the brackets is the metric.

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Metaphor: Picture yourself traveling by car and use a GPS. One setting may determine the shortest distance in miles (such as RIP’s hop count), while another may determine the fastest time based on speed constraints (such as OSPF’s bandwidth-based cost). The Metric is similar to the specific data the GPS uses to determine a route. You can choose which GPS app you trust more based on the Administrative Distance if you have two separate applications that give you different directions.