Routing, Forwarding, and the Internet Control Plane
What Routers Do
Forwarding vs routing
Routing is the process of discovering paths through a network, while forwarding is the act of sending packets out the correct interface based on the routing table.
Control plane and data plane
The control plane computes routes; the data plane moves packets at line speed using the forwarding table.
Route selection
Routers usually prefer the most specific matching prefix, then compare administrative distance, metric, and policy. The routing table may contain connected, static, and dynamic routes.
Common Routing Concepts
| Concept | Meaning |
|---|---|
| Default route | Fallback route used when no more specific match exists |
| Next hop | Immediate router used to reach a destination |
| Metric | Cost used to compare routes |
| Administrative distance | Preference between routing sources |
What is the longest-prefix match rule used for?
When multiple routes match, the route with the most specific prefix is chosen.
Correct answer: Selecting the most specific route for a destination
What is the difference between a static route and a dynamic route?
Static routes are simple and predictable; dynamic routes adapt to network changes.
Correct answer: A static route is manually configured, while a dynamic route is learned through a routing protocol
Interior and Exterior Routing
Routing protocol roles
Interior Gateway Protocols
- Used within one organization or autonomous system
- Examples include OSPF and IS-IS
- Optimize internal path selection
Exterior Gateway Protocols
- Used between autonomous systems
- BGP is the main example
- Support policy-based interdomain routing
Which protocol is primarily used to exchange routes between autonomous systems on the internet?
BGP is the inter-domain routing protocol that enables Internet-wide route exchange.
Correct answer: BGP
Diagnosing a routing failure
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1
Step 1: Confirm the host has the correct IP, mask, and gateway.
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2
Step 2: Verify the destination is in or beyond the expected route table.
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3
Step 3: Check the next hop and interface status.
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4
Step 4: Test connectivity hop by hop using traceroute or similar tools.