200-301 - IP Connectivity (25% of the exam) - Section 3.1
Interpret the components of the routing table and determine how a router makes a forwarding decision by default using longest prefix match, administrative distance, and routing protocol metric.
Read a routing table by its parts - routing protocol code, prefix, network mask, next hop, administrative distance, metric, and gateway of last resort - and trace which entry a destination matches. Apply the default forwarding logic: the most specific (longest) prefix match wins first, administrative distance breaks ties between sources for the same prefix, and the protocol metric breaks ties within one source.
Routing table componentsLongest prefix matchAdministrative distanceRouting protocol metricGateway of last resort
Practice question for this objective
lock_openFree sampleIP Connectivitymedium
A router has a full routing table but receives a packet destined for a public address that matches no specific entry. The output shows the line: Gateway of last resort is 203.0.113.1 to network 0.0.0.0. What does the gateway of last resort tell the router to do with this packet?
ADrop the packet and send an ICMP unreachable, because a gateway of last resort means routing has failed.
BFlood the packet out every interface, because the destination is unknown and must be discovered.
CBuffer the packet until a more specific route is learned for the destination network.
DForward the packet using the default route toward 203.0.113.1, because no more specific prefix matches the destination.check_circle Correct
The gateway of last resort is the next hop for the default route, used when no more specific prefix matches the destination. The gateway of last resort marks where packets go when longest prefix match finds no specific entry. It is the next hop of the 0.0.0.0/0 default route, so a packet with no better match is forwarded toward 203.0.113.1 rather than being dropped, flooded, or buffered.
Why A is wrong: A configured gateway of last resort is the opposite of failure: it provides a catch-all next hop. The router only drops and signals unreachable when there is no default route at all.
Why B is wrong: Routers never flood unknown unicasts; that is switch behaviour for unknown MAC addresses. A router with a default route forwards the packet to the single gateway of last resort.
Why C is wrong: A router does not hold packets waiting for a better route. If a default route exists it forwards immediately to the gateway of last resort rather than buffering.
Why D is correct: The gateway of last resort identifies the next hop for the default route 0.0.0.0/0. When longest prefix match finds no more specific entry, the router uses this default and forwards toward 203.0.113.1.
Answers that look right on this material and are not. Each one is a distractor from a different question in the 200-301 bank for this domain.
Packets are dropped if no specific route matches, because the gateway of last resort blocks unknown destinations for security
errorWhy it is wrong: The gateway of last resort exists to forward unmatched traffic, not to drop it; a router only drops packets to unknown destinations when no default route is set.
The OSPF route to 10.1.0.0/16, because OSPF has a lower administrative distance than EIGRP
errorWhy it is wrong: Administrative distance is tempting here, but it only breaks ties between routes to the SAME prefix; with different prefix lengths the longest match is chosen first, so the /16 is never compared by AD.
The OSPF 10.1.4.0/24 route, because OSPF has a lower administrative distance than RIP and iBGP.
errorWhy it is wrong: This applies administrative distance first, but AD only breaks ties between sources offering the SAME prefix length; here the /26 is more specific, so longest prefix match selects it before AD is ever consulted.