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Question 400
A packet arrives for 203.0.113.9 and no installed route, connected network, host route or default matches that address. Which router action is appropriate instead of guessing an egress?
- Forward to the lowest numeric local IPv4 interface when no route exists
- Drop the packet and generally signal unreachable as applicable
- Fabricate an ARP entry for the destination to force delivery
- Use OSPF’s administrative distance to choose a route even though none is installed
Explanation
Correct Answer
B:Without any matching forwarding entry the router has no valid next hop.
Incorrect Answers
A:Router interfaces are not arbitrary fallback paths; without a route there is no valid next-hop selection.
C:ARP can resolve an on-link next hop but cannot replace a missing routing-table match to an off-link destination.
D:Administrative distance ranks available same-prefix candidates, and cannot produce a route from an empty match set.
Question 401
An IOS router has a valid installed static route to 10.20.0.0/16 via on-link 192.0.2.9. The route appears in show ip route, but show ip cef adjacency reports an incomplete next-hop and no ARP reply arrives from 192.0.2.9. Which forwarding conclusion is supported?
- RIB installation proves that all traffic can be sent on Ethernet even without a next-hop MAC
- The router can still deliver the packet to a remote destination by ARPing for 10.20.5.8 across the WAN
- An unresolved next-hop Layer 2 adjacency can prevent successful packet delivery despite the installed IP route
- The router must automatically replace the next hop with its own LAN MAC without changing the route
Explanation
Correct Answer
C:CEF needs a valid adjacency containing Layer 2 forwarding information; persistent unresolved ARP can cause punt/drop behavior until it is resolved.
Incorrect Answers
A:A route in the RIB establishes a path selection, not a complete Ethernet rewrite; the unresolved adjacency can still block actual data delivery.
B:ARP operates for the on-link next-hop address, not ordinarily the routed remote host reachable beyond that neighbor.
D:Substituting its own MAC would create self-delivery rather than reaching the unavailable adjacent next-hop router.
Question 402
An IPv4 router must forward a packet larger than its outbound IP MTU. The packet has its Don’t Fragment flag set and the outgoing interface cannot carry the datagram intact. Which handling is correct for standard IPv4 forwarding?
- Fragment it anyway because the router ignores the IPv4 DF flag when paths are congested
- Silently increase the Ethernet frame MTU without coordination with its peer
- Discard the oversized packet and, when permitted, return ICMP Destination Unreachable fragmentation-needed information
- Reassemble the entire TCP application stream into smaller UDP messages before relaying it
Explanation
Correct Answer
C:The router cannot forward the DF-marked oversized packet, so dropping and signaling the needed smaller MTU enables Path MTU Discovery.
Incorrect Answers
A:The DF bit prohibits in-path IPv4 fragmentation; the forwarding router must not override that condition to split the datagram.
B:A routing decision cannot unilaterally increase physical link MTU acceptance on another device or change the given interface limit.
D:A router does not convert or reassemble TCP application transport data into UDP merely to fit a lower IP MTU.
Question 403
Two candidate OSPF paths to the same prefix have different accumulated costs while all route-type preferences match. Which OSPF numerical metric should be compared?
- Use administrative distance 110 to distinguish the two paths within the same OSPF source
- Use the OSPF metric or cost
- Compare the destination IP addresses numerically, ignoring the identical prefix
- Use Ethernet MAC address learning as the IGP best-path metric
Explanation
Correct Answer
B:OSPF computes protocol-specific route cost for equal destination prefixes.
Incorrect Answers
A:Both candidates have the same route source and AD, so their accumulated OSPF costs rank them.
C:Both entries are for the same destination prefix; numerical address order offers no preference.
D:Layer 2 MAC learning does not compute OSPF shortest-path cost.
Question 404
A router has an installed OSPF route for 10.20.30.0/24 with administrative distance 110. The engineer needs a reachable floating-static backup for that exact prefix that remains inactive until OSPF fails. Which static route administrative distance meets this requirement?
- Set the floating static route administrative distance to 90
- Set the floating static route administrative distance to 1
- Set the floating static route administrative distance to 200
- Install a /32 host route instead of the /24
Explanation
Correct Answer
C:AD 200 is higher than the competing OSPF 110 and therefore leaves the resolved static standby until the preferred route disappears.
Incorrect Answers
A:AD 90 would outrank OSPF AD 110 for the same prefix and cause the static to install prematurely.
B:A default static AD of 1 also wins against OSPF rather than remaining dormant.
D:Changing prefix length to /32 alters destination scope rather than making a floating /24 backup.
Question 405
Traffic goes to an unexpected egress even though the running configuration looks correct. Which operational route and next-hop resolution information must be read to learn the active forwarding choice?
- Inspect only the running-config route statements and assume each is installed
- Ping the final destination but do not inspect routing/adjacency information
- Inspect the installed route and next-hop resolution
- Read switch STP root priority rather than the router’s active RIB and adjacency
Explanation
Correct Answer
C:Forwarding uses the installed route after source-selection and next-hop resolution.
Incorrect Answers
A:A configured floating static or disconnected route may be absent from the active RIB/FIB.
B:A reachability probe signals success/failure but does not identify which installed route and egress were chosen.
D:STP root selection governs Layer 2 tree topology, not the IP next-hop decision requested.
Question 406
A destination 192.0.2.5 matches both installed 192.0.2.0/24 via A and 192.0.2.4/30 via B. Which prefix is the longest matching route for that host?
- Select 192.0.2.0/24 because its prefix starts on a lower IP address
- Select the default route when two specific installed routes already cover .5
- Choose the route with lower protocol AD despite its wider /24 prefix
- Use the /30 route
Explanation
Correct Answer
D:The longer /30 prefix is a more-specific match than the /24 aggregate.
Incorrect Answers
A:Prefix base numerical magnitude does not outrank the more specific 192.0.2.4/30 match.
B:The default is least specific and not consulted instead of a matching /30.
C:AD is source selection prior to forwarding; a more-specific installed /30 wins regardless of a /24 source distance.
Question 407
A router learns the identical IPv4 prefix 10.77.0.0/16 through OSPF (default AD 110) and RIPv2 (default AD 120). Both next hops are reachable; no custom distance is configured. Which source is preferred in the installed routing table?
- RIPv2, because a higher numerical administrative distance is more preferred
- OSPF, because AD 110 is lower than RIP AD 120 for the same prefix
- Both automatically as equal-cost paths despite different administrative distances
- Neither; route-source comparison is impossible when the prefix lengths are identical
Explanation
Correct Answer
B:Cisco defaults to OSPF distance 110 and RIP distance 120, so the OSPF route is the eligible preferred installed source.
Incorrect Answers
A:Lower rather than higher administrative distance wins between eligible sources for the same destination prefix.
C:Different administrative distances prevent simple equal-cost installation of the two competing same-prefix candidates.
D:Identical prefix length is precisely the case where the router compares eligible route-source distance during RIB installation.
Question 408
Within the same OSPF process, a router has two valid external routes to precisely 10.77.0.0/16: one O E1 route with calculated total cost 90 and one O E2 route with advertised external cost 20. There is no intra-area or interarea route for this prefix. Which OSPF external route type normally wins the same-prefix path-type preference?
- The O E2 route because its external metric 20 is numerically smaller than E1 cost 90
- Both routes always load-share because OSPF ignores type-1 versus type-2 classification
- The O E2 route because type 2 outranks type 1 inside a single OSPF process
- The O E1 route because OSPF path preference ranks external type 1 ahead of external type 2
Explanation
Correct Answer
D:With no intra/interarea competitor, O E1 wins over O E2 for the same prefix in the same OSPF process; 90 and 20 are different metric types and do not reverse this route-class order.
Incorrect Answers
A:OSPF does not compare E1 additive cost and E2 external cost as like-for-like metrics to override the route-type preference.
B:Different external route types for one prefix are not automatically equal-cost paths simply because both use the OSPF protocol.
C:Cisco documentation describes external type 1 as preferred ahead of external type 2 when both are valid for the same prefix in one process.
Question 409
An IPv4 transit router receives a packet with TTL 64 and finds a valid route to its destination. The packet is forwarded through a normal IPv4 interface without tunneling. How should its IPv4 header change at this hop?
- The router increases TTL to 65 so downstream routers can recognize a successfully forwarded hop
- The router decrements TTL to 63 and updates the IPv4 header checksum for the modified header
- The router leaves TTL unchanged while updating only the destination IPv4 address
- The router resets TTL to 255 and preserves the original header checksum
Explanation
Correct Answer
B:Every transit router decrements the IPv4 TTL by one when forwarding, so 64 becomes 63. The IPv4 header checksum is updated to remain valid after the header field changes.
Incorrect Answers
A:TTL counts down at each forwarding hop and must not increase, or packets could continue around routing loops without expiring.
C:Destination-based routing uses the destination address to choose the next hop; it does not rewrite the destination address as part of ordinary IP forwarding.
D:A transit router does not restore the initial TTL to 255, and a changed TTL would require a valid updated header checksum.
Question 410
After an interface shutdown, users report packets leaving on a different uplink. Which *current routing-table and egress-interface* inspection can prove the active installed path actually changed?
- Inspect the saved startup-config without checking current RIB or egress
- Inspect current show ip route and outgoing interface
- Look only at interface descriptions to infer the active forwarding route
- Check the switch MAC table rather than router route/egress state
Explanation
Correct Answer
B:An interface state change can remove connected routes and their dependent forwarding paths.
Incorrect Answers
A:Startup configuration may not reflect current link state or selected installed routes after a shutdown.
C:Interface descriptions are administrative text and cannot prove which route or adjacency forwards user packets.
D:MAC learning may reveal hosts but not how the router’s current destination-prefix decision changed.
Cisco Certified Network Associate (CCNA) (200-301) Practice Tests
