Dynamic Routing Notes
Networking Notes for High School Students
Guiding Question
How does dynamic routing enhance network adaptability, and what factors influence a router’s decision when selecting the best route?
Learning Goals
- Explain the purpose and function of dynamic routing
- Compare static routing and dynamic routing
- Identify the main features of BGP, EIGRP, and OSPF
- Explain how routers choose the best route
1. What Is Dynamic Routing?
Dynamic routing is when routers automatically figure out the best path for data to travel.
Instead of a network administrator manually entering every possible route, routers share information with each other and update their routing tables automatically.
Dynamic routing is especially helpful in large networks, growing networks, and networks that change often.
2. How Dynamic Routing Works
Routers in a dynamic network communicate with each other by sharing route information.
They use this information to keep their routing tables up to date.
A routing table is a list of known networks and the best path to reach them.
When the network changes, routers automatically recalculate the best route. This automatic adjustment is called convergence.
Convergence
Convergence is the process of all routers updating and agreeing on the best available paths after a network change.
- A link goes down
- A router fails
- A new router is added
- A faster path becomes available
3. Static Routing vs. Dynamic Routing
Static Routing
With static routing, routes are entered manually by a network administrator.
Advantages:
- Simple to understand
- Predictable
- Uses fewer router resources
Disadvantages:
- Does not automatically adjust to changes
- Requires manual updates
- Not practical for large networks
Dynamic Routing
With dynamic routing, routers automatically update routing tables.
Advantages:
- Adapts to network changes
- Useful for large networks
- Reduces day-to-day manual work
- Can provide backup paths if one link fails
Disadvantages:
- Uses more CPU and memory
- Can take more work to set up at first
- May create security risks if routing updates are not protected
4. Why Dynamic Routing Is Important
Dynamic routing makes a network more adaptable.
That means the network can respond to problems or changes without a person having to fix every route manually.
- Finding new routes when links fail
- Supporting large and growing networks
- Reducing human error
- Making networks more reliable
5. Dynamic Routing Protocols
Routers use dynamic routing protocols to communicate. A routing protocol is a set of rules routers use to exchange route information.
The main protocols in these notes are:
6. Border Gateway Protocol (BGP)
BGP stands for Border Gateway Protocol.
It is the main protocol used to route traffic across the Internet.
- Type: Exterior Gateway Protocol (EGP)
- Main use: Used between different autonomous systems (ASes)
- Method: Path vector
- Strength: Handles extremely large networks
- Weakness: Slow convergence
An autonomous system is a large network or group of networks under one organization’s control.
7. Enhanced Interior Gateway Routing Protocol (EIGRP)
EIGRP stands for Enhanced Interior Gateway Routing Protocol.
- Type: Interior Gateway Protocol (IGP), hybrid protocol
- Main use: Used within an organization’s internal network
- Metric: Bandwidth, delay, load, and reliability
- Strengths: Fast, flexible, easy to configure
- Supports: VLSM and CIDR
- Special feature: Unequal-cost load balancing
8. Open Shortest Path First (OSPF)
OSPF stands for Open Shortest Path First.
- Type: Interior Gateway Protocol (IGP), link-state protocol
- Main use: Routes data inside large internal networks
- Method: Shares link-state information to build a full network map
- Metric: Based on link speed
- Strengths: Widely supported, works across many vendors
- Structure: Uses a hierarchical design
9. Comparing BGP, EIGRP, and OSPF
| Protocol |
Type |
Main Use |
Strength |
Weakness / Note |
| BGP |
EGP |
Between large networks / Internet |
Handles huge networks |
Slow convergence |
| EIGRP |
IGP / Hybrid |
Inside organizations |
Fast and flexible |
Often used in Cisco environments |
| OSPF |
IGP / Link-State |
Inside large organizations |
Widely supported |
Builds a full network map |
10. How Routers Choose the Best Route
Routers may learn more than one route to the same destination. When that happens, the router must decide which route is best.
Routers compare routes using several factors:
- Administrative Distance (AD)
- Prefix Length
- Routing Metrics
11. Administrative Distance (AD)
Administrative Distance tells how trusted a route source is.
Lower AD = More Trusted
| Route Type |
Administrative Distance |
| Direct | 0 |
| Static | 1 |
| BGP | 20 |
| EIGRP | 90 |
| OSPF | 110 |
12. Prefix Length
A router also looks at prefix length.
Longer prefix = more specific route = better match
Example:
- /24 is more specific
- /16 is less specific
13. Routing Metrics
A metric is a value used by a routing protocol to decide which path is better.
Examples of things a metric may consider:
- Link speed
- Number of hops
- Delay
- Load
- Reliability
Lower metric = better route
14. Route Selection Example
Destination: 10.10.20.0/24
Available routes:
- Static route (AD 1)
- EIGRP route (AD 90)
- OSPF route (AD 110)
The router chooses the static route first because it has the lowest Administrative Distance.
If the static route is no longer available, the router would then prefer EIGRP over OSPF.
15. Benefits of Dynamic Routing
- Self-updating: Changes are handled automatically
- Scalable: Works well in large or growing networks
- Less manual work: Reduces the need for constant human updates
- Redundancy: Another path can be used if one fails
16. Drawbacks of Dynamic Routing
- More resource-heavy: Uses CPU and memory
- Initial setup can be harder
- Security risks: Fake routing updates could cause problems if the network is not protected
17. Key Vocabulary
- Dynamic Routing: Routers automatically learn and update routes
- Static Routing: Routes are manually entered by an administrator
- Routing Table: A list of routes a router knows about
- Convergence: The process of routers updating after a change and agreeing on the best paths
- Protocol: A set of rules used for communication
- BGP: A routing protocol used between major networks on the Internet
- EIGRP: A fast internal routing protocol often used in Cisco environments
- OSPF: A link-state internal routing protocol that builds a map of the network
- Administrative Distance: A number showing how trusted a route source is
- Prefix Length: Shows how specific a network route is
- Metric: A value used to compare routes and choose the best path
- Autonomous System: A large network or group of networks managed by one organization
18. Quick Summary
- Dynamic routing allows routers to update routes automatically.
- It is better than static routing for large or changing networks.
- BGP is used on the Internet between autonomous systems.
- EIGRP is fast and flexible, especially in Cisco networks.
- OSPF is widely supported and works well in large internal networks.
- Routers choose routes based on Administrative Distance, prefix length, and metrics.
- Lower AD and lower metrics are usually better.
- Longer prefix length means a more specific match.
19. Review Questions
- What is the main purpose of dynamic routing?
- What is convergence?
- How is static routing different from dynamic routing?
- What is BGP mainly used for?
- What makes EIGRP fast and flexible?
- How does OSPF help routers make decisions?
- What does Administrative Distance tell us?
- Why is a /24 route preferred over a /16 route?
- What are two benefits of dynamic routing?
- What are two drawbacks of dynamic routing?