1 Routing Tables and Packet Forwarding Free Online FlashCards

What information does a routing table contain?

A routing table, or RIB, contains routes learned from connected interfaces, static configuration, and routing protocols.

What does a connected route indicate?

A connected route describes a prefix on an active, configured router interface; the router can send packets for that network directly through the interface.

What destinations match the IPv4 default prefix 0.0.0.0/0?

The 0.0.0.0/0 prefix matches any IPv4 destination that does not match a more-specific installed route.

What does a route’s next-hop address identify?

A next-hop IP address identifies the neighboring router to which a packet should be sent; it need not be the packet’s final destination.

How does a router choose between different route sources for the same prefix?

For competing routes to the same prefix, route-source preference—often called administrative distance—helps determine which route or routes are installed.

How can a protocol rank its competing routes?

When routes from the same protocol compete, that protocol’s metric helps rank them. Lower values are preferred in common Cisco implementations, but behavior can vary.

Which installed route does a router use to forward a packet?

For a destination address, the router forwards using the installed matching route with the longest prefix—the most specific match.

Which route matches 10.20.2.25: 10.20.0.0/16 or 10.20.2.0/24?

The 10.20.0.0/16 and 10.20.2.0/24 routes both match 10.20.2.25, but the /24 wins because it is more specific.

Can administrative distance make a less-specific matching route win during forwarding?

A less-specific route cannot beat a more-specific matching route during forwarding just because its administrative distance is lower.

What happens when no specific route matches a destination?

If no specific installed route matches a destination, the router uses an installed default route; without one, it cannot forward the packet.

Which packet field determines the route used for forwarding?

The router selects a route based on the packet’s destination IP address, not on which table entry has the most attractive-looking metric.

Which route handles 10.20.3.25 in the worked example?

The destination 10.20.3.25 matches 10.20.0.0/16 but not 10.20.2.0/24, so the static /16 route sends it via 192.0.2.2.