Section 16: Routing Fundamentals

What is a routing table?

Consists of directly connected networks, static routes, and dynamically learned routes — lists best next hop(s) to destination networks

What is a default route?

Catch-all route for traffic that doesn't match any more specific route

What command views the routing table?

show ip route

What does a "C" route indicate in show ip route?

Connected Route — directly connected network

What does an "L" route indicate in show ip route

Local Route — /32 address configured on the router's own interface

Local routes always have a [...] mask

/32

Which has the longest prefix match: /16, /24, or /28?

/28 — more specific mask = longer prefix match

When multiple equal cost routes exist for same destination, router

adds all to routing table and load balances

Static route command syntax

ip route <network> <mask> <next-hop>

What requirement must be met for the next-hop address used in a static route?

It must be directly reachable — i.e., reachable via a directly connected interface. You can't point a static route at an address that itself requires another route to reach.

What condition must a set of subnets meet in order to be summarized into a single route?

They must fall within a contiguous range.

When a destination matches multiple routes in the routing table with different prefix lengths, which route wins?

The route with the longest prefix match (the most specific route).

What does a router do when it has multiple routes to the exact same destination network and prefix length?

It load-balances traffic across the next hops.

What is the destination network and mask used to configure a default static route?

0.0.0.0 0.0.0.0 (the "quad-zero" route).

What is a default route labeled as in the output of show ip route?

Gateway of last resort.

How are dynamic routing protocols classified, and which protocols fall into each category?

IGPs (used within an autonomous system) — Distance Vector: RIP; Advanced Distance Vector: EIGRP; Link State: OSPF, IS-IS. EGPs (used between autonomous systems) — Path Vector: BGP.

What are the default administrative distances (low to high) for: connected, static, eBGP, EIGRP, OSPF, IS-IS, RIP?

Connected = 0, Static = 1, eBGP = 20, EIGRP = 90, OSPF = 110, IS-IS = 115, RIP = 120.

When a router has routes to the same destination from two different sources, which one does it prefer — lower or higher administrative distance?

Lower AD is preferred (more trusted).

Why is it best practice to configure loopback interfaces as passive interfaces on a routing protocol?

Loopbacks don't need to form neighbor relationships, so there's no reason to send routing protocol traffic out of them.

What metric does OSPF use, and what is it based on by default?

Cost, automatically calculated from interface bandwidth.

Why might RIP and OSPF choose different paths through the same physical topology?

RIP optimizes for fewest hops regardless of bandwidth/speed; OSPF optimizes for lowest cumulative cost (bandwidth-based), so RIP can pick a slower, shorter-hop path OSPF would avoid.

What is the metric of RIP

Hop Count

What is it called when two routers running the same IGP find each other and form a relationship?

An adjacency.

Does longest prefix match or administrative distance get evaluated first when choosing between routes to overlapping destinations?

Longest prefix match — it's applied before AD is ever considered. AD only breaks ties when prefix length is identical.

What is a floating static route, and how is it configured?

A backup static route made to only be used when the primary route fails, by manually setting its administrative distance higher than the primary route's.