In networking, a metric is a value used by routing protocols to determine the best path for data packets to reach their destination. When multiple routes exist to the same network, routers use metrics to compare and select the optimal path. Lower metric values typically indicate preferred routes.
…In networking, a metric is a value used by routing protocols to determine the best path for data packets to reach their destination. When multiple routes exist to the same network, routers use metrics to compare and select the optimal path. Lower metric values typically indicate preferred routes.
Different routing protocols use various factors to calculate metrics:
**RIP (Routing Information Protocol)** uses hop count as its metric. Each router a packet must traverse counts as one hop, with a maximum of 15 hops. A route with fewer hops is considered better.
**OSPF (Open Shortest Path First)** uses cost as its metric, which is calculated based on bandwidth. The formula is Reference Bandwidth divided by Interface Bandwidth. Higher bandwidth links result in lower costs, making them more desirable paths.
**EIGRP (Enhanced Interior Gateway Routing Protocol)** uses a composite metric that considers bandwidth, delay, reliability, and load. By default, only bandwidth and delay are used in the calculation, providing a more sophisticated path selection mechanism.
When a router has multiple routes to the same destination learned through the same routing protocol, the route with the lowest metric is installed in the routing table. If two routes have equal metrics, load balancing may occur.
Its important to understand that metrics are only comparable within the same routing protocol. You cannot compare an OSPF cost value to a RIP hop count. When routes from different protocols exist, Administrative Distance determines which protocols route is preferred.
Metrics play a crucial role in network convergence and traffic engineering. Network administrators can manipulate metrics to influence traffic flow, such as preferring certain links over others or distributing traffic across multiple paths. Understanding how metrics work is essential for troubleshooting routing issues and optimizing network performance in enterprise environments.
Understanding Routing Metrics in CCNA
Why is Routing Metric Important?
Routing metrics are fundamental to how routers make decisions about the best path to reach a destination network. When multiple routes exist to the same destination, the router must have a way to determine which path is optimal. The metric serves as this decision-making criteria, making it essential knowledge for any network professional.
What is a Routing Metric?
A routing metric is a numerical value that routing protocols use to determine the best path to a destination. Lower metric values typically indicate preferred routes. Different routing protocols use different metrics to calculate the best path:
• RIP (Routing Information Protocol) - Uses hop count as its metric. Each router the packet must pass through counts as one hop. Maximum is 15 hops; 16 is considered unreachable.
• OSPF (Open Shortest Path First) - Uses cost based on bandwidth. The formula is: Cost = Reference Bandwidth / Interface Bandwidth. Default reference bandwidth is 100 Mbps.
• EIGRP (Enhanced Interior Gateway Routing Protocol) - Uses a composite metric based on bandwidth and delay by default. It can also consider reliability and load.
How Routing Metrics Work
When a router receives routing updates or calculates paths, it compares metric values for routes to the same destination. The route with the lowest metric is installed in the routing table and used for forwarding traffic.
If two routes have equal metrics, the router can perform load balancing across both paths. This is called equal-cost load balancing.
The metric is recalculated as routing updates propagate through the network. For example, in RIP, each router adds 1 to the hop count before advertising the route to neighbors.
Key Metric Values to Remember
• RIP maximum hop count: 15 (16 = unreachable) • OSPF default reference bandwidth: 100,000 Kbps (100 Mbps) • OSPF cost for 100 Mbps link: 1 • OSPF cost for 10 Mbps link: 10 • Administrative Distance is NOT the same as metric
Exam Tips: Answering Questions on Metric
Tip 1: Always identify which routing protocol the question references first. The metric type varies by protocol.
Tip 2: Remember that lower metrics are always preferred. This is a universal rule across all routing protocols.
Tip 3: Do not confuse Administrative Distance (AD) with metric. AD is used to choose between different routing protocols, while metric is used within the same protocol.
Tip 4: For OSPF cost calculations, know the formula: Cost = Reference Bandwidth / Interface Bandwidth. Practice calculating costs for common bandwidths.
Tip 5: When questions show multiple paths, add up the total metric for each complete path, then select the one with the lowest total.
Tip 6: Watch for questions about modifying metrics. OSPF cost can be manually configured on interfaces, and RIP hop count cannot be modified but can be filtered.
Tip 7: Equal-cost paths result in load balancing. If asked what happens when metrics are equal, remember traffic is distributed across those paths.
Tip 8: For simulation questions, use commands like show ip route to view metrics in brackets. The format is [AD/Metric].