Learn how routers use routing-table entries to decide whether a destination is local, which next hop to use, and how competing routes are handled.
What you will be able to do
- Explain how a router uses a routing table to choose forwarding information for an IP destination.
- Identify the interface and next-hop information needed for a reachable destination prefix.
- Distinguish direct delivery, forwarding through another router, static routes, and default routes.
- Determine why a more specific matching route can matter when several entries match.
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1. The routing decision
A router receives a packet with an IP destination. It consults its routing table to decide where that packet should go next.
The table connects destination prefixes with forwarding information. This lets the router turn an IP destination into an action rather than guessing a path.
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2. What a route entry provides
A reachable destination prefix needs, at minimum, an interface identifier and next-hop information. Together, these values tell the router where to send the packet next.
The interface identifies the router’s path out. The next-hop value identifies the receiving device, or identifies the final destination when delivery is direct.
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3. Direct delivery versus forwarding
The router first determines whether the destination is reachable directly through one of its connected paths. If not, it forwards the packet toward another router.
For example, a packet for an adjacent host can be sent directly through the appropriate interface. A packet beyond that local reach needs another router as its next hop.
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4. When the destination is the next hop
For an adjacent destination, the next-hop IP address is the packet’s destination address. No separate intermediate router is needed for that final delivery.
This differs from forwarded traffic. In forwarded traffic, the packet keeps its ultimate destination, while the router sends it toward a different next-hop device.
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5. Static and default routes
A static route is an explicitly configured instruction for reaching a particular destination. It defines the next hop and makes traffic follow the selected path.
A default route provides a broader fallback. It sends packets toward destinations that do not already have a matching entry in the routing table.
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6. Several entries can match
A destination may match more than one entry. This can happen when a table contains a default route, a subnet route, and a host route.
CIDR permits destination prefixes to have subset relationships. A smaller covered range can therefore sit inside a larger covered range, creating overlapping matches.
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7. Path count and traffic sharing
When multiple paths reach the same destination, the traffic-share min command installs only paths with the minimum metric. This limits the installed choices to the lowest-metric paths.
Multi-interface load splitting controls traffic that travels across multiple interfaces toward one destination. These mechanisms address different parts of path selection and distribution.
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8. When a route disappears
A static route depends on usable forwarding information. If its interface goes down, the route is removed from the IP routing table.
A configured forwarding device can also lose its valid next hop. In that case, the associated static route is removed as well.
Network 127 is a special case. A router should not forward packets addressed to that network, except through a loopback interface.
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9. Recap before Practice and Prove
A routing table helps a router choose the next forwarding step for an IP destination. Its entries provide an interface and next-hop information.
The router distinguishes direct delivery from forwarding through another router. For an adjacent destination, the destination address also serves as the next-hop IP address.
Static routes specify a chosen path, while a default route covers destinations without another matching entry. Several entries may match when route ranges overlap.
When paths share a destination, minimum-metric selection and multi-interface load splitting can affect installed paths and traffic distribution.
Routes can disappear when their interface or next hop becomes unusable. Use these ideas in Practice, then explain the complete selection process in Prove.