A routing protocol metric is a value used by routing protocols to determine the best path to a destination network when multiple routes exist. This metric helps routers make intelligent decisions about which route to place in the routing table and ultimately use for forwarding packets.
Different r…A routing protocol metric is a value used by routing protocols to determine the best path to a destination network when multiple routes exist. This metric helps routers make intelligent decisions about which route to place in the routing table and ultimately use for forwarding packets.
Different routing protocols use different types of metrics to evaluate paths. RIP (Routing Information Protocol) uses hop count as its metric, where each router a packet must traverse counts as one hop. The maximum hop count in RIP is 15, with 16 considered unreachable. This simple metric does not account for bandwidth differences between links.
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 have lower costs, making them preferred paths. The default reference bandwidth is 100 Mbps, though this can be adjusted for networks with faster links.
EIGRP (Enhanced Interior Gateway Routing Protocol) uses a composite metric that can include bandwidth, delay, load, and reliability. By default, only bandwidth and delay are used in the calculation. This provides a more comprehensive evaluation of path quality compared to simple hop count.
When a router learns multiple routes to the same destination through the same routing protocol, it compares metrics to select the best path. The route with the lowest metric value is considered optimal and installed in the routing table. If multiple routes have equal metrics, some protocols support load balancing across those paths.
Understanding metrics is essential for network administrators because it affects traffic flow patterns across the network. Administrators can manipulate metrics to influence path selection, implement traffic engineering, or create backup routes. Proper metric configuration ensures efficient network performance and optimal resource utilization across all available paths in the infrastructure.
Routing Protocol Metric and Forwarding Decisions
Why This Topic Is Important
Understanding routing protocol metrics is fundamental to the CCNA exam and real-world networking. Metrics determine which path a router selects when multiple routes exist to the same destination. This knowledge is essential for troubleshooting routing issues, optimizing network performance, and predicting router behavior in exam scenarios.
What Is a Routing Protocol Metric?
A metric is a value used by routing protocols to determine the best path to a destination network. When a router learns multiple routes to the same destination, it uses the metric to select the preferred route. Lower metric values indicate better paths in most routing protocols.
Each routing protocol uses different factors to calculate its metric:
• RIP (Routing Information Protocol): Uses hop count - the number of routers a packet must pass through. Maximum is 15 hops; 16 is considered unreachable.
• OSPF (Open Shortest Path First): Uses cost - calculated based on interface 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. Can also include reliability and load if configured.
How Metric-Based Forwarding Decisions Work
When a router receives routing updates or calculates paths, it follows this process:
1. The router learns multiple paths to the same destination network 2. It calculates the metric for each path using the routing protocol's formula 3. The path with the lowest metric is installed in the routing table 4. If multiple paths have equal metrics, load balancing may occur 5. Backup paths may be kept in the topology table for failover
Administrative Distance vs. Metric
It is critical to understand the difference:
• Administrative Distance (AD): Used to choose between routes learned from different routing protocols • Metric: Used to choose between routes learned from the same routing protocol
The router first compares AD values. Only when AD values are equal does the metric become the deciding factor.
Exam Tips: Answering Questions on Routing Protocol Metric
Tip 1: Always identify which routing protocol is being used before analyzing the metric. Each protocol calculates metrics differently.
Tip 2: Remember that lower metrics are preferred. When comparing routes, the route with the smallest metric value wins.
Tip 3: For OSPF questions, know how to calculate cost. If a question mentions a 1 Gbps link with default reference bandwidth (100 Mbps), the cost equals 1. For faster links, you may need to adjust the reference bandwidth.
Tip 4: For RIP questions, count the hops. Each router traversed adds one to the hop count. The maximum usable hop count is 15.
Tip 5: Watch for questions that mix AD and metric concepts. If routes come from different protocols, AD decides. If routes come from the same protocol, metric decides.
Tip 6: When equal-cost paths exist, understand that load balancing can occur. EIGRP can also perform unequal-cost load balancing with variance configured.
Tip 7: Pay attention to interface bandwidth values in questions. OSPF and EIGRP both factor bandwidth into their calculations.
Tip 8: In troubleshooting scenarios, use show ip route to view the metric in brackets after the AD value. Format is [AD/Metric].