1 Routing Tables and Route Selection Free Online FlashCards

What is a routing table for?

A routing table records reachable network prefixes and the information a router needs to forward packets toward them.

What does a prefix length indicate?

It indicates how many leading address bits identify the network. A longer prefix covers a smaller address range.

What information commonly appears in a route entry?

A route entry commonly includes its source or type, destination prefix, preference and metric, next hop, outgoing interface, and sometimes an age or timer.

In a Cisco-style route entry, what does O identify?

The O code indicates that the route was learned through OSPF.

In the Cisco-style entry, what does [110/20] mean?

The pair gives administrative distance first and route metric second.

How do a next hop and outgoing interface differ?

The next hop is the neighboring router to send the packet to, if applicable; the outgoing interface is the local interface used to send it.

What does a connected route represent?

It describes a network attached directly to an active router interface.

What is the purpose of an IPv4 default route?

It is a fallback route that can match any destination when no more specific route applies. Its IPv4 prefix is 0.0.0.0/0.

How can a routing protocol choose among its candidate paths?

A routing source may compare candidate paths using its own protocol-specific metric to choose the best route for a prefix.

How does Cisco administrative distance compare routes for the same prefix?

On Cisco platforms, a lower administrative distance is preferred when routes for the same prefix come from different sources. For example, the defaults are 1 for static routes and 110 for OSPF routes.

Do administrative distance and metrics choose the packet's longest-prefix match?

They help determine which route is installed for a prefix; they do not decide the packet-by-packet longest-prefix match among installed routes.

What is longest-prefix matching?

Among installed routes matching a packet's destination, the router chooses the one with the longest prefix, which covers the smallest address range.