Are you curious about how routers choose the best path for data transmission? Today, we will dive deeper into the world of routing tables to understand this concept. By exploring metrics and administrative distances, we can uncover the logic behind path selection. So, let’s get started!
Contents
The Role of Metrics and Administrative Distances
In a routing table, each route that isn’t local or directly connected is accompanied by two values in square brackets. The first value represents the administrative distance, while the second value denotes the metric. The metric is a measurement used by routing protocols to assess the quality of a path. It is crucial in determining the best route to a destination when multiple paths are available.
Different routing protocols calculate metrics in their own unique ways. For instance, RIP uses hop count, OSPF uses bandwidth, and EIGRP considers multiple factors such as bandwidth, delay, reliability, and load. Due to these variations, metrics can differ between protocols, which poses an interesting question – how does a router select the best route when it is learned from two different routing protocols?
To answer this, let’s examine a topology where Router A has two paths to reach the 172.16.0.0 network. One path is learned from an EIGRP neighbor, while the other is learned from an OSPF neighbor. Suppose the EIGRP metric is 3072, while the OSPF metric is 2. Does Router A choose the OSPF path due to the lower metric? Surprisingly, the answer is no.
The Role of Administrative Distance
When comparing routes from different routing protocols, the metric is not the deciding factor. Instead, routers utilize the administrative distance (AD) to determine the best routing protocol. The router selects the route with the lowest AD when multiple protocols offer the same route. By default, Cisco routers prefer EIGRP over OSPF. This preference is likely due to Cisco’s own development of EIGRP and the trust they place in it.
In our example, Router A chooses the EIGRP route for forwarding packets to the 172.16.0.0 network. We can think of the administrative distance as the router’s measure of trustworthiness for a routing protocol. Cisco routers adhere to a default set of administrative distances, as illustrated below:
- Connected route: Lowest AD
- Static route: Higher than connected routes but lower than other dynamic routing protocols
- EIGRP: AD of 90
- OSPF: AD of 110
These values may vary across different vendors. It’s essential to note that a static route overrides a dynamically learned route because the router assumes that a configured static route serves a specific purpose.
Equal Cost Multi-Path (ECMP)
In certain scenarios, routers encounter equal-cost paths when all metrics are the same. In these cases, the router installs both routes into the routing table, a concept known as equal cost multi-path (ECMP). This allows the router to distribute the traffic load across multiple links.
In our example, if there were two OSPF paths with identical metrics, the router would utilize both links, effectively sharing the load. Cisco routers default to using ECMP, but not all vendors follow the same approach. Some vendors may select only one route for the routing table unless ECMP is explicitly configured.
FAQs
Q: How do routers choose the best path?
A: Routers examine both metrics and administrative distances to select the best path. Metrics measure the quality of a path, while administrative distances determine the reliability and trustworthiness of the routing protocol.
Q: Why do Cisco routers prefer EIGRP over OSPF?
A: Cisco developed EIGRP and assigns a lower administrative distance to it by default. This preference stems from the trust Cisco places in its own routing protocol.
Q: What is equal cost multi-path (ECMP)?
A: ECMP refers to the situation where multiple routes have the same metrics. In such cases, routers install all equal-cost routes into the routing table, allowing the traffic load to be shared across multiple links.
Conclusion
Understanding how routers select the best path is vital for efficient network operation. By considering metrics, such as hop count or bandwidth, and administrative distances assigned to different routing protocols, routers make informed decisions about path selection. Remember that administrative distances play a crucial role in choosing the best route when different protocols offer the same destination. Whether it’s dynamic routing protocols, metrics, or administrative distances, building a solid foundation in these core concepts sets the stage for further exploration in the world of networking.
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