2.1 Explain characteristics of routing technologies
Route Selection
📘CompTIA Network+ (N10-009)
What is a Metric?
In networking, a metric is a value used by a router to decide the best path to send data from one network to another. When a router has multiple routes to the same destination, it uses the metric to compare those routes. The route with the lowest metric is usually chosen as the best path.
Think of a metric as a “score” assigned to each route. The lower the score, the better the route for sending data.
Why Metrics Matter
Routers don’t just send data randomly—they must choose the most efficient path to reduce:
- Delay – How long it takes for data to reach its destination
- Congestion – How busy a network path is
- Cost – Resources used to send data
By using metrics, routers can automatically select paths that are faster, more reliable, or cheaper in terms of resources.
How Metrics Are Calculated
Different routing protocols calculate metrics differently. Here are some examples:
| Routing Protocol | How Metric is Calculated | Example |
|---|---|---|
| RIP (Routing Information Protocol) | Uses hop count (number of routers a packet passes through) | A route through 3 routers has a metric of 3, and through 5 routers has a metric of 5 → router chooses 3-hop route |
| OSPF (Open Shortest Path First) | Uses cost, usually based on bandwidth | Higher bandwidth interfaces have lower cost. A 1 Gbps link might have a cost of 1, a 100 Mbps link a cost of 10 → router chooses 1 Gbps link |
| EIGRP (Enhanced Interior Gateway Routing Protocol) | Uses a composite metric that can include bandwidth, delay, load, and reliability | Router evaluates multiple factors to calculate the best route |
| BGP (Border Gateway Protocol) | Uses path attributes like AS path length, policy, and next-hop IP | Chooses the route with the shortest AS path or preferred policy |
Common Metric Types in IT Networks
- Hop Count – Number of routers (hops) between source and destination.
- Simpler metric, but may not reflect speed or quality of the path.
- Bandwidth – The speed of a network link.
- Higher bandwidth → lower metric → preferred route.
- Delay – Time it takes for a packet to travel across a path.
- Lower delay → lower metric → faster route.
- Load – Current traffic on a network path.
- Less busy path → lower metric → less chance of congestion.
- Reliability – How stable the path is.
- More reliable path → lower metric → less risk of dropped packets.
How Routers Use Metrics to Make Decisions
- Router receives multiple routes to the same network.
- It looks at the metric associated with each route.
- Router selects the route with the lowest metric.
- That route becomes the best path in the routing table.
Example:
A router has two paths to the network 192.168.10.0/24:
- Path A: metric 5 (100 Mbps, low delay)
- Path B: metric 10 (10 Mbps, higher delay)
The router will choose Path A because 5 < 10.
Key Exam Points for Network+
- Metric: Value used to determine the best route.
- Lower metric = preferred path.
- Metrics vary by routing protocol (RIP, OSPF, EIGRP, BGP).
- Factors affecting metrics: hop count, bandwidth, delay, load, reliability.
- Routers use metrics to populate the routing table with the best routes.
Quick Memory Tip for Exams
Think: “Routers always pick the path with the lowest ‘score.’”
- RIP → hop count
- OSPF → cost (bandwidth-based)
- EIGRP → composite metric (bandwidth, delay, load, reliability)
- BGP → policy and AS path
Metrics are critical for dynamic routing, ensuring routers make intelligent decisions automatically. On the exam, you may see questions asking:
- “Which route will a router choose?” → compare metrics.
- “What is a metric?” → lowest-cost value used for route selection.
- “Which routing protocols use hop count?” → RIP.
- “Which routing protocol uses cost based on bandwidth?” → OSPF.
