Layer 3 Technologies

A network engineer configures BFD on a Layer 3 point-to-point link between R1 and R2 to speed up EIGRP convergence. The interface commands 'bfd interval 300 min_rx 300 multiplier 3' are applied on both routers, and the interface is up. However, when the link fails on an intermediate transport device, EIGRP still waits for its hold timer to expire before removing the neighbor. Running 'show bfd neighbors' on R1 shows no BFD sessions established. Which action will most likely enable BFD-triggered fast failover for EIGRP?

An engineer configures a route map on R1 to tag routes learned from a customer eBGP peer with community 65000:100 so an upstream provider can filter them. After applying the policy, the neighbor R2 (the upstream) reports that none of the received prefixes carry the community. The relevant configuration on R1 is: route-map SET-COMM permit 10 match ip address prefix-list CUST-NETS set community 65000:100 ! router bgp 65000 neighbor 10.1.1.2 remote-as 65001 neighbor 10.1.1.2 route-map SET-COMM out The prefixes are being advertised to R2 correctly, but without the community value. What is the most likely cause?

Two routers, R1 and R2, connect an EIGRP domain (AS 100) to an OSPF domain. Both routers perform mutual redistribution between EIGRP and OSPF. Network 10.50.0.0/24 originates natively in EIGRP. Users report intermittent reachability to 10.50.0.0/24 from the OSPF domain, and R2's routing table shows 10.50.0.0/24 pointing back into OSPF instead of via its native EIGRP path. The redistribution is configured with default settings and no route tagging or filtering is applied. What is the MOST likely cause?

A network engineer configures mutual redistribution between BGP (AS 65100) and EIGRP (AS 10) on two border routers, R1 and R2, that both connect to the same BGP and EIGRP domains. External EIGRP routes learned from BGP on R1 are being redistributed back into BGP on R2, then re-advertised and reinjected into EIGRP, causing intermittent routing loops. The engineer wants to prevent routes that originated in BGP from being redistributed back into BGP. Which configuration on both routers best prevents this loop?

A network engineer redistributes BGP-learned routes into OSPF on an ASBR using the command 'redistribute bgp 65001 subnets'. Users report that when there are two ASBRs advertising the same external prefixes with different internal path costs to reach them, traffic does not always take the path with the lowest total cost to the ASBR. The engineer confirms both ASBRs are advertising the routes. What is the MOST likely reason the internal cost to the ASBR is being ignored in path selection?

A network engineer configures policy-based routing on R2 to forward all HTTP traffic sourced from 10.1.1.0/24 out a backup ISP link. The route-map is applied inbound on the LAN interface and contains: 'match ip address HTTP_ACL' and 'set ip next-hop 203.0.113.5'. The primary default route via 198.51.100.1 remains active. After applying the policy, 'show route-map' shows zero policy matches even though users on 10.1.1.0/24 are actively browsing HTTP sites. The ACL correctly permits tcp any eq 80. What is the most likely reason PBR is not matching the traffic?

A dual-homed enterprise router learns the prefix 172.20.50.0/24 from three sources simultaneously: an eBGP peer at a partner site, an internal OSPF neighbor, and an EIGRP-learned route redistributed from a branch. All three routes have identical prefix length and are valid. After troubleshooting a traffic path complaint, you notice the router installs the eBGP-learned route in the RIB even though the OSPF path is topologically shorter. Assuming default administrative distances, why does the router prefer the eBGP route?

A network engineer reports that R3 is missing several specific subnets of 172.16.0.0/16 learned via EIGRP AS 100. The topology uses discontiguous subnets of 172.16.0.0/16 separated by a 10.0.0.0/8 backbone. On R1 and R2, 'show ip protocols' shows 'auto-summary' is in effect. Both R1 and R2 advertise the classful summary 172.16.0.0/16 toward each other across the 10.x backbone. What is the most likely cause of R3's missing routes, and what corrects it?

A network engineer is troubleshooting an EIGRP adjacency failure between R1 and R2 in AS 100. Both routers have matching AS numbers and are directly connected on the same subnet. The engineer runs 'show logging' on R1 and sees the message: '%DUAL-5-NBRCHANGE: EIGRP-IPv4 100: Neighbor 10.1.1.2 (GigabitEthernet0/0) is down: K-value mismatch'. Interfaces are up and pings succeed. What is the most likely cause?

A network engineer migrates R1 from EIGRP classic mode to EIGRP named mode. After the migration, R1 forms a stable adjacency with R2 (still in classic mode) but the neighbor relationship with R3 (also classic mode) repeatedly flaps with the log message '%DUAL-5-NBRCHANGE: ... K-value mismatch'. On R1 named mode, the engineer configured 'metric weights 0 1 1 0 0 0 0' under the address-family topology base. R2 and R3 both use default K-values except R3, which was previously configured with 'metric weights 0 1 0 0 0 0 0'. What is the root cause of the flapping adjacency between R1 and R3?

A network engineer converts an EIGRP classic configuration to EIGRP named mode on a distribution switch. After the migration, the switch no longer forms an adjacency with a neighbor on GigabitEthernet1/0/1, though the interface is up/up and IP connectivity to the neighbor's interface is confirmed via ping. The relevant configuration is: router eigrp CORP address-family ipv4 unicast autonomous-system 100 af-interface default passive-interface exit-af-interface af-interface GigabitEthernet1/0/2 no passive-interface exit-af-interface network 10.0.0.0 exit-address-family What is the most likely cause of the missing adjacency on Gi1/0/1?

You are troubleshooting an EIGRP named-mode deployment. R1 runs a legacy classic-mode EIGRP configuration for autonomous system 100, while its neighbor R2 has recently been migrated to EIGRP named mode using the same AS. The two routers form an adjacency and exchange routes, but you notice that the metrics for the same destinations differ significantly between the two routers, and traffic engineering based on delay is not behaving as expected. The link between them is a 10 Gbps interface. Which explanation best accounts for the metric discrepancy?

Two routers, R1 and R2, both redistribute routes bidirectionally between OSPF and EIGRP at different points in the network. After the change, users report intermittent connectivity to the 172.16.50.0/24 network, and traceroutes show traffic occasionally looping between R1 and R2. You confirm the route originated in OSPF. Which configuration approach at the redistribution points will BEST prevent routes from being re-injected back into their protocol of origin?

A network engineer manages a DMVPN Phase 1 hub-and-spoke topology running EIGRP over the multipoint tunnel interface Tunnel0 on the hub. Each spoke forms an EIGRP adjacency with the hub and advertises its LAN prefix successfully. However, the engineer notices that no spoke learns the LAN prefixes of any other spoke, even though the hub shows all spoke routes in its topology and routing table. The hub's Tunnel0 has no distribute-lists or route-maps applied. What is the most likely cause?

R1 has a primary EIGRP-learned route to 10.20.0.0/16 (internal EIGRP AD 90) via its WAN link. An engineer configures a static backup route, 'ip route 10.20.0.0 255.255.0.0 192.168.100.2', intending it to be used ONLY if EIGRP fails. After committing the change, users report that traffic to 10.20.0.0/16 is now flowing over the backup path even though the EIGRP neighbor is fully adjacent and the route is present in the EIGRP topology table. What is the cause and the correct fix?

R1 is an EIGRP border router advertising a manual summary route of 10.10.0.0/16 out its interface toward R2. The summary covers subnets 10.10.1.0/24 through 10.10.8.0/24, all of which are reachable via R1. A network engineer notices that when R2 sends traffic destined to 10.10.20.5 (an address inside the summary range but with no matching specific subnet on R1), the packets are silently dropped at R1 instead of being forwarded elsewhere. Why does R1 drop this traffic?

An engineer wants R1 to load-balance traffic to the 10.50.0.0/16 network across two EIGRP paths. The primary path via R2 has a feasible distance of 3072000, and the alternate path via R3 has a computed metric of 5120000 with a reported distance of 2560000. The engineer configures 'variance 2' under the EIGRP process, but the path through R3 is still not installed in the routing table. What is the reason R3's path is excluded?

A network engineer configured EIGRP named mode for VRF CUSTOMER_A on two PE routers connected back-to-back over a physical link. The EIGRP adjacency in the VRF forms successfully, and both routers show each other as neighbors under 'show eigrp address-family ipv4 vrf CUSTOMER_A neighbors'. However, routes learned from the remote PE do not appear in 'show ip route vrf CUSTOMER_A'. The interfaces belong to the correct VRF, and the autonomous-system number matches. What is the MOST likely cause?

A network engineer applies 'distribute-list 10 in' under the OSPF process on router R3 to prevent the 10.50.5.0/24 route (originated in area 0) from being installed. Access-list 10 correctly denies 10.50.5.0/24 and permits everything else. After the change, R3's routing table no longer shows 10.50.5.0/24, but R3's neighbors still learn and use 10.50.5.0/24 normally. However, a downstream router R4 (which relies on R3 to reach 10.50.5.0/24) now has a routing black hole. Why does R4 still install the route while R3 does not, and what causes the black hole?

Two ASBRs (R1 and R2) both perform mutual redistribution between OSPF and RIP. After the change, a RIP-learned prefix 10.50.0.0/24 originated behind R1 is being reinjected back into RIP by R2, creating a routing loop that surfaces as intermittent reachability. You want to use OSPF route tagging to break the loop without static filters per-prefix. Which configuration correctly prevents routes that originated in RIP from being redistributed back into RIP by the other ASBR?