Administrative Distance & RIB Practice Questions

Answers and explanations appear when you work through the set above.

1. For the same prefix, a router has a valid eBGP path and a valid OSPF intra-area path, both with default distances. Which one is installed?

   • The OSPF path, since eBGP's default distance (20) is only used when no IGP path exists
   • The eBGP path, because its default administrative distance of 20 beats OSPF's default of 110
   • Both are installed as equal-cost paths
   • The path is chosen by whichever protocol converged first

2. A router has a static route and a learned OSPF route to the same destination network, and the static route was configured without an explicit distance. Which wins?

   • The OSPF route, because static routes always defer to any dynamic protocol
   • The static route, because its default administrative distance of 1 is lower than OSPF's default of 110
   • Neither is installed until the administrator manually breaks the tie
   • The one with the shorter AS path

3. A route to 10.5.0.0/16 is marked best inside the BGP table, but `show ip route` shows an OSPF route installed for that same prefix instead. What is the most likely explanation?

   • BGP's best-path algorithm must have failed to pick a winner
   • OSPF administrative distance always overrides BGP by design, regardless of iBGP or eBGP
   • A route can be the best path within the BGP table yet still lose the RIB decision, because RIB installation is governed by administrative distance across sources, not by which protocol's internal algorithm chose it
   • BGP routes learned over an eBGP session are never installed in the RIB

4. For the same destination, a router holds a valid external EIGRP path and a valid iBGP path, both at default distance. Which is installed in the RIB?

   • The iBGP path, because its default distance of 200 beats external EIGRP's default of 170
   • The external EIGRP path, because its default distance of 170 beats iBGP's default of 200
   • Both, because EIGRP external and iBGP routes are treated as equal-cost by default
   • Whichever route has fewer AS-path hops

5. A route map used to filter redistributed routes has only one sequence: `permit 10` matching `ip address prefix-list PL-CORE`. A candidate route fails to match PL-CORE. What happens to that route?

   • It is permitted anyway, because sequence 10 is a permit statement
   • It is denied, because route maps carry an implicit deny for anything that falls through every configured sequence
   • It is automatically re-evaluated against the running routing table
   • It is permitted but none of its attributes are modified

6. A single route-map sequence contains two separate match statements: `match ip address prefix-list LIST-A LIST-B` and `match tag 40`. What must a candidate route satisfy to match this sequence?

   • It must match LIST-A and LIST-B and tag 40 all at once
   • It must match either LIST-A or LIST-B (multiple values on one match line are ORed), and it must also satisfy the separate tag 40 match, since distinct match statements in one sequence are ANDed together
   • It only needs to satisfy any one of the three conditions
   • Only the first match statement in the sequence is evaluated

7. R3 has a route to 172.20.0.0/16 in its BGP table, but that route never reaches the RIB because a lower-AD IGP entry for the exact same prefix already exists. R3 is also configured to redistribute BGP into OSPF. What should happen to this prefix?

   • It is redistributed anyway, since it is the best path within the BGP table itself
   • It is not redistributed from BGP, because standard redistribution only pulls from routes that are actually installed in the RIB for that protocol, and this BGP entry lost the RIB race to the IGP route
   • Both the BGP and IGP paths are redistributed as equal-cost OSPF externals
   • It is redistributed only when BGP synchronization is turned on

8. Given `ip prefix-list PL10 permit 10.1.1.0/24` with no `ge` or `le` keywords, does the exact route 10.1.1.0/24 match this entry?

   • No, because prefix lists with no ge/le only match more specific subnets
   • Yes, because a prefix-list entry with no ge/le keywords matches only that exact network and mask combination
   • It depends entirely on which routing protocol originated the route
   • No, because route maps cannot reference prefix lists directly

9. Given `ip prefix-list PLDEFAULT permit 0.0.0.0/0`, does this entry match the candidate route 192.0.2.0/24?

   • Yes, because 0.0.0.0/0 without ge/le is treated as matching everything
   • No, because with no ge/le keywords the entry matches only the default route 0.0.0.0/0 itself, not any other prefix
   • Yes, but only if the route arrived over an eBGP session
   • It depends on whether the router has synchronization enabled

10. Given `ip prefix-list PLALL permit 0.0.0.0/0 le 32`, does the candidate route 198.51.100.0/27 match?

    • No, because le 32 only extends matching to host routes, not to /27 networks
    • Yes, because 0.0.0.0/0 le 32 covers every valid IPv4 prefix length from /0 through /32, including this /27
    • No, prefix lists cannot use ge/le with a /0 base network
    • Only if a route map also references this prefix list

11. Given `ip prefix-list PLRANGE permit 192.168.0.0/20 ge 24 le 28`, does the candidate route 192.168.7.0/24 match?

    • No, because /24 falls outside the configured range
    • Yes, because 192.168.7.0/24 falls within the 192.168.0.0/20 parent block and its mask length of 24 sits inside the allowed ge 24 through le 28 range
    • No, ge and le cannot be combined on the same prefix-list line
    • Yes, but only for routes learned via EIGRP

12. Given `ip prefix-list PLRANGE permit 192.168.0.0/20 ge 24 le 28`, does the candidate route 192.168.8.0/29 match?

    • Yes, because the base network 192.168.0.0/20 is present regardless of mask length
    • No, because a /29 mask is longer than the le 28 upper bound, so it falls outside the permitted mask range even though the network itself is covered by the parent block
    • Yes, because /29 masks are always treated as host routes and automatically permitted
    • The line is invalid, so nothing can match it

13. Given `ip prefix-list PLWIDE permit 172.16.0.0/12 ge 24`, does the candidate route 172.16.64.0/18 match?

    • The line is invalid because ge is used without a matching le
    • Yes, because the base network 172.16.0.0/12 always matches every subordinate prefix
    • No, because a /18 mask is shorter than the ge 24 floor, so it does not satisfy the minimum prefix-length requirement even though it falls inside the parent range
    • Yes, but only if the route also matches an access list

14. R2 redistributes OSPF into EIGRP with a route map permitting only tagged routes, but no redistributed prefixes ever appear in the EIGRP topology table, even though the route map and redistribute command both look correct. What should be checked first?

    • Whether an EIGRP seed metric (bandwidth, delay, reliability, load, MTU, or a default-metric statement) has been configured for the redistributed routes
    • Whether an OSPF virtual link is missing somewhere in the topology
    • Whether a BGP community string was applied by mistake
    • Whether a static default route exists on R2

15. Two routers each redistribute in opposite directions between OSPF and EIGRP: R1 redistributes OSPF into EIGRP, and R2 (elsewhere in the network) redistributes EIGRP into OSPF. You want to guarantee that a route originally learned from OSPF at R1 never loops back into OSPF at R2. Which design is most deterministic?

    • Raise the OSPF hello and dead timers so convergence slows down
    • Apply a route tag to the route during the first redistribution event and explicitly deny that tag on the return redistribution point
    • Disable EIGRP split horizon on every interface in the topology
    • Assign new OSPF router IDs on both boundary routers

16. R1 redistributes OSPF routes into EIGRP. Later, a different router downstream redistributes EIGRP into BGP. Can you assume the original OSPF prefixes are now treated in BGP exactly as if they had been redistributed straight from OSPF into BGP?

    • Yes, redistribution effects are transitive across multiple hops of redistribution
    • No, redistribution is not transitive; each redistribution point only acts on the routes actually present in its own source protocol's table at that moment, with whatever metric and tag values resulted from the prior hop
    • Yes, but only if both routers use the same autonomous system number
    • No, because BGP can never accept routes that originated as OSPF

17. A route-map sequence used for PBR reads `route-map PBR-MAP deny 10` and matches source subnet 10.10.10.0/24 with a `set ip next-hop` clause attached. What happens to packets sourced from that subnet?

    • They are dropped outright by the deny action
    • They are excluded from policy routing and instead forwarded using the normal routing table lookup, because a deny sequence in a PBR route map exempts matching traffic from the set clauses rather than dropping it
    • They are policy-routed using the configured next hop, since the set clause still applies
    • They are policy-routed only when the normal route to the destination is down

18. An engineer wants PBR to affect traffic that the router itself generates, such as locally sourced pings or Telnet sessions, not just transit traffic. Which feature accomplishes this?

    • Applying `ip policy route-map` inbound on every interface
    • Configuring a local policy route map under global configuration so router-generated packets are evaluated against it
    • Adding a distribute list under the routing protocol process
    • Applying a control-plane service policy

19. An EIGRP interface summary is configured, and all of the component subnets covered by that summary are up and present in the topology table. Why does a discard route to Null0 appear locally on the summarizing router?

    • To force all summarized traffic toward the default route
    • To safely drop traffic destined for an address that falls within the summary's range but does not correspond to any actual, more-specific component route, preventing a routing loop
    • To trigger an EIGRP query for every component prefix on every update
    • To convert the summary from an internal route into an external EIGRP route

20. A router learns the same destination prefix through a static route pointing at a next hop, and also through OSPF, and the static route was configured with an explicit distance of 200 (a floating static). Which route wins?

    • The static route, because static routes always take precedence
    • The OSPF route, because a floating static's explicit distance of 200 is higher than OSPF's default 110, so OSPF wins under normal conditions and the static only becomes active as a backup if OSPF withdraws the route
    • Both routes are installed and load-balanced
    • Whichever route was configured most recently

21. An OSPF-learned route exists in the LSDB, but a directly connected interface that is not enabled for OSPF covers that exact same prefix and mask, so the connected route wins the RIB. This router is configured to redistribute OSPF into EIGRP. What is the practical concern?

    • The router IDs must be changed so the prefix is treated as locally originated
    • The OSPF path is not the RIB-installed source for that prefix (the connected route is), so it is not a normal candidate for redistribution out of the OSPF process, and the connected route would need its own redistribute statement instead
    • EIGRP split horizon must be disabled everywhere before this can work
    • Hello timers must be adjusted before checking any policy

22. Which command most directly shows whether an inbound or outbound distribute list is filtering routes on a specific routing process, along with other high-level process settings like redistribution sources and administrative distance?

    • show ip route
    • show ip protocols
    • show ip arp
    • show ip interface brief

23. Given `ip prefix-list PLNARROW permit 10.0.0.0/8 ge 16 le 24`, does the candidate route 10.44.16.0/28 match?

    • Yes, because the base 10.0.0.0/8 network is present in the route
    • No, because a /28 mask exceeds the le 24 ceiling, placing it outside the permitted mask-length range even though the address falls inside 10.0.0.0/8
    • Yes, because ge and le ranges only restrict the network portion, not the mask
    • The entry is invalid because ge and le cannot be combined

24. For the same destination prefix, a router has both a directly connected interface route and a static route pointing out that same interface. Which one is installed in the RIB?

    • The static route, because static routes are always more specific
    • The connected route, because directly connected interfaces carry the lowest possible administrative distance (0), beating any static route configured for the same prefix
    • Both, since connected and static routes are treated as equal cost
    • The static route, if it was configured more recently