Route Summarization

A network engineer is troubleshooting a connectivity issue between two branches connected via a WAN link. Router R1 (10.1.0.0/16) is summarizing its directly connected subnets (10.1.1.0/24, 10.1.2.0/24, 10.1.3.0/24) as a single 10.1.0.0/16 route to Router R2 via EIGRP. Users at R2 report that they cannot reach the 10.1.4.0/24 subnet, which was recently added to R1. What is the most likely cause of the problem?

A network engineer is troubleshooting an OSPF network where routers R1, R2, and R3 are in area 0. R1 has a summary route 192.168.0.0/22 configured on its interface to R2, summarizing four /24 subnets (192.168.0.0/24 through 192.168.3.0/24). After the configuration, R3 loses connectivity to the 192.168.0.0/24 subnet, although other subnets are reachable. What is the most likely cause?

A network engineer is troubleshooting BGP route summarization on a border router that advertises a summary route 172.16.0.0/16 to an ISP neighbor. The engineer notices that the ISP is receiving the summary route but also receiving the more specific routes (172.16.1.0/24, 172.16.2.0/24), causing suboptimal routing. What should the engineer do to ensure only the summary route is advertised?

A network engineer is troubleshooting a redistribution issue between EIGRP and OSPF. Router R1 redistributes EIGRP routes into OSPF. The engineer configured a summary route 10.0.0.0/8 using the 'summary-address' command under the OSPF process. After the configuration, OSPF neighbors lose connectivity to the 10.1.0.0/16 subnet, which is one of the component routes. What is the most likely cause?

A network engineer is troubleshooting a route summarization issue in a network using RIP. Router R1 is configured with the 'ip summary-address rip 192.168.0.0 255.255.252.0' command on its serial interface. After the configuration, R2, which is connected via that interface, can no longer reach the 192.168.1.0/24 subnet, although other subnets within the summary are reachable. What is the most likely cause?

A network engineer is troubleshooting an OSPF network where an ABR (R1) is configured with the 'area 1 range 10.0.0.0 255.255.0.0' command to summarize routes from area 1 into area 0. After the configuration, routers in area 0 lose connectivity to the 10.0.1.0/24 subnet, although the summary route 10.0.0.0/16 is present in their routing tables. What is the most likely cause?

A network engineer is troubleshooting a BGP route summarization issue. Router R1 is configured with the 'aggregate-address 192.168.0.0 255.255.252.0' command without any keywords. The engineer notices that the ISP neighbor is receiving both the aggregate route and the more specific routes (192.168.0.0/24, 192.168.1.0/24, etc.), causing the ISP to prefer the specific routes. What should the engineer do to ensure the aggregate route is preferred?

A network engineer is troubleshooting an EIGRP network where route summarization is configured. Router R1 has the 'ip summary-address eigrp 100 10.0.0.0 255.0.0.0' command on its interface facing R2. After the configuration, R2 loses connectivity to the 10.1.0.0/16 subnet, which is one of the component routes. The engineer checks the routing table on R2 and sees the summary route 10.0.0.0/8 but not the specific route. What is the most likely cause?

A network engineer is troubleshooting a route summarization issue in a network using OSPFv3. Router R1 is configured with the 'ipv6 summary-address 2001:db8:0:1::/64' command under the OSPFv3 process. After the configuration, routers in the same area lose connectivity to the 2001:db8:0:1:1::/80 subnet, which is one of the component routes. What is the most likely cause?

A network engineer runs the following command on Router R1: R1# show ip eigrp topology 10.0.0.0 255.255.252.0 IP-EIGRP (AS 100): Topology entry for 10.0.0.0/22 State: Passive, Origin: Internal, Metric [90/2172416], Tag 0 Number of successors: 1 FD is 2172416, Serno: 5 Route is Summary Advertised by R2 (via Serial0/0/0) Reply status: 0 Based on this output, what is true about the route 10.0.0.0/22?

A network engineer runs the following command on Router R1: R1# show ip route 172.16.0.0 255.255.240.0 Routing entry for 172.16.0.0/20 Known via "ospf 1", distance 110, metric 20, type intra area Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:15 ago Routing Descriptor Blocks: * 192.168.1.2, from 192.168.1.2, 00:00:15 ago, via GigabitEthernet0/0 Route metric is 20, traffic share count is 1 Based on this output, what can be concluded about the route 172.16.0.0/20?

A network engineer runs the following command on Router R1: R1# show ip bgp 192.168.0.0 255.255.252.0 BGP routing table entry for 192.168.0.0/22, version 5 Paths: (1 available, best #1, table default) Advertised to update-groups: 1 Refresh Epoch 1 65001 10.1.1.1 from 10.1.1.1 (10.1.1.1) Origin IGP, metric 0, localpref 100, valid, external, best Community: 65001:100 rx pathid: 0, tx pathid: 0x0 Aggregator: 65001, 10.1.1.1 Based on this output, what is true about this route?

A network engineer runs the following command on Router R1: R1# show ip eigrp interfaces detail GigabitEthernet0/0 IP-EIGRP interfaces for process 100 Interface Peers Xmit Queue Mean Pacing Time Multicast Pending Un/Reliable SRTT Un/Reliable Flow Timer Routes Gi0/0 1 0/0 10 0/10 50 0 Hello interval: 5 sec, Hold time: 15 sec Split horizon is enabled Summary address: 10.0.0.0/8 Next xmit serial <none> Un/reliable mcasts: 0/0 Un/reliable ucasts: 0/0 Mcast exceptions: 0 CR packets: 0 ACKs suppressed: 0 Retransmissions: 0 Retry timer: 15 Hello packets sent: 100, received: 99 Based on this output, what is the purpose of the summary address configured on this interface?

A network engineer runs the following command on Router R1: R1# show ip ospf database summary 172.16.0.0 OSPF Router with ID (1.1.1.1) (Process ID 1) Summary Net Link States (Area 0) LS age: 100 Options: (No TOS-capability, DC) LS Type: Summary Links(Network) Link State ID: 172.16.0.0 (Summary Network Number) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0x1234 Length: 28 Network Mask: /20 TOS: 0 Metric: 10 Based on this output, what does this LSA represent?

A network engineer runs the following command on Router R1: R1# show ip route summary IP routing table maximum-paths: 32 IP routing table has 15 routes, using 900 bytes of memory Number of prefixes: /8: 1, /16: 2, /20: 3, /24: 9 Route types: Connected: 4, Static: 1, OSPF: 10 Route sources: OSPF: 10, Connected: 4, Static: 1 Based on this output, what is a potential issue regarding route summarization?

A network engineer runs the following command on Router R1: R1# show ip bgp neighbors 10.1.1.1 advertised-routes BGP table version is 10, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete Network Next Hop Metric LocPrf Weight Path *> 10.0.0.0/8 0.0.0.0 0 32768 i *> 10.1.0.0/16 0.0.0.0 0 32768 i *> 10.1.1.0/24 0.0.0.0 0 32768 i *> 10.1.2.0/24 0.0.0.0 0 32768 i Based on this output, what is a problem with the BGP advertisements?

A network engineer runs the following command on Router R1: R1# show ip route 10.0.0.0 255.255.252.0 longer-prefixes Routing entry for 10.0.0.0/22 Known via "eigrp 100", distance 90, metric 2172416, type internal Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:10 ago Routing Descriptor Blocks: * 192.168.1.2, from 192.168.1.2, 00:00:10 ago, via GigabitEthernet0/0 Route metric is 2172416, traffic share count is 1 Routing entry for 10.0.1.0/24 Known via "eigrp 100", distance 90, metric 2812416, type internal Last update from 192.168.1.2 on GigabitEthernet0/0, 00:00:10 ago Routing Descriptor Blocks: * 192.168.1.2, from 192.168.1.2, 00:00:10 ago, via GigabitEthernet0/0 Route metric is 2812416, traffic share count is 1 Based on this output, what is the effect of the summary route 10.0.0.0/22?

A network engineer runs the following command on Router R1: R1# show ip eigrp topology summary IP-EIGRP Topology Table for AS(100)/ID(1.1.1.1) Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply, r - reply Status, s - sia Status P 10.0.0.0/8, 1 successors, FD is 2812416, serno 10 via Summary (2812416/0), Null0 P 10.0.0.0/24, 1 successors, FD is 2172416, serno 5 via 192.168.1.2 (2172416/2812416), GigabitEthernet0/0 P 10.0.1.0/24, 1 successors, FD is 2172416, serno 6 via 192.168.1.2 (2172416/2812416), GigabitEthernet0/0 Based on this output, what is the purpose of the route 10.0.0.0/8 via Null0?

Consider the following partial EIGRP configuration on router R1: router eigrp 100 network 10.0.0.0 network 192.168.1.0 summary-address 10.0.0.0 255.255.255.0 5 What is the effect of the 'summary-address 10.0.0.0 255.255.255.0 5' command?

Given the following OSPF configuration on router R2: router ospf 1 network 10.0.0.0 0.255.255.255 area 0 area 0 range 10.0.0.0 255.255.252.0 Which statement is true about this configuration?

Examine this BGP configuration on router R3: router bgp 65001 neighbor 10.1.1.1 remote-as 65002 address-family ipv4 network 192.168.0.0 mask 255.255.252.0 aggregate-address 192.168.0.0 255.255.252.0 summary-only What is the effect of the 'aggregate-address' command with the 'summary-only' keyword?

Review the following OSPF configuration on router R4: router ospf 1 network 10.0.0.0 0.255.255.255 area 0 area 1 range 10.0.0.0 255.255.255.0 What is missing or incorrect in this configuration for proper route summarization?

Consider this EIGRP configuration on router R5: interface GigabitEthernet0/0 ip summary-address eigrp 100 10.10.0.0 255.255.0.0 5 router eigrp 100 network 10.0.0.0 What is the effect of the 'ip summary-address eigrp' command on interface GigabitEthernet0/0?

Given this partial configuration on router R6: router bgp 65000 neighbor 192.168.1.1 remote-as 65001 address-family ipv4 network 172.16.0.0 mask 255.255.0.0 aggregate-address 172.16.0.0 255.255.0.0 What is missing if the administrator wants to ensure that only the aggregate route is advertised to neighbor 192.168.1.1?

In OSPF, what is the default metric for a Type 3 summary LSA generated by an ABR using the 'area range' command?

In EIGRP, what is the default administrative distance of a summary route created with the 'ip summary-address eigrp' command?

Which of the following best describes the behavior of BGP when an 'aggregate-address' command is used without the 'summary-only' keyword?

Which TWO commands can be used to verify route summarization on a Cisco router running OSPF? (Choose TWO.)

Which TWO statements about route summarization in EIGRP are true? (Choose TWO.)

Which TWO configuration steps are required to implement manual route summarization in OSPF on an ABR? (Choose TWO.)

Which THREE symptoms indicate that route summarization may be causing routing issues in a network? (Choose THREE.)

Which TWO statements about route summarization in BGP are true? (Choose TWO.)

A large enterprise network is experiencing intermittent reachability to a specific /24 subnet (192.168.10.0/24) from remote sites. Router R1 has the following relevant configuration: interface GigabitEthernet0/0 ip address 10.0.1.1 255.255.255.0 ! router eigrp 100 network 10.0.0.0 network 192.168.0.0 ! interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip summary-address eigrp 100 192.168.0.0 255.255.252.0 ! Router R2 shows: R2# show ip route 192.168.10.0 Routing entry for 192.168.0.0/22, supernet Known via "eigrp 100", distance 90, metric 30720, type internal Last update from 10.0.1.1 on GigabitEthernet0/0, 00:00:15 ago * 10.0.1.1, via GigabitEthernet0/0, 00:00:15 ago What is the root cause?

A redistribution setup between OSPF and EIGRP is causing a routing loop for subnet 10.1.1.0/24. Router R1 runs OSPF and EIGRP with redistribution. R1's configuration: router ospf 1 redistribute eigrp 100 subnets ! router eigrp 100 redistribute ospf 1 metric 10000 100 255 1 1500 ! interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.0 ip summary-address eigrp 100 10.1.0.0 255.255.255.0 ! Router R2 (EIGRP neighbor) shows: R2# show ip route 10.1.1.0 Routing entry for 10.1.0.0/24, supernet Known via "eigrp 100", distance 90, metric 30720, type internal Last update from 10.1.1.1 on GigabitEthernet0/0, 00:00:05 ago What is the root cause?

BGP route summarization is causing unexpected path selection for prefix 172.16.0.0/16. Router R1 (AS 65001) has: router bgp 65001 neighbor 10.0.0.2 remote-as 65002 network 172.16.0.0 mask 255.255.0.0 aggregate-address 172.16.0.0 255.255.0.0 summary-only ! Router R2 (AS 65002) receives the aggregate and shows: R2# show ip bgp 172.16.0.0/16 BGP routing table entry for 172.16.0.0/16, version 2 Paths: (1 available, best #1) 65001, (aggregated by 65001 10.0.0.1) 10.0.0.1 from 10.0.0.1 (10.0.0.1) Origin IGP, localpref 100, valid, external, best However, R2 has a more specific route for 172.16.1.0/24 via another path with higher local preference. What is the root cause?

OSPF network type mismatch on a multi-access link is causing route summarization issues. Router R1 and R2 are connected via Ethernet, but R1 has: interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.0 ip ospf network point-to-point ip ospf 1 area 0 ! Router R2 has default OSPF network type (broadcast). R1 is configured with: router ospf 1 area 0 range 10.0.0.0 255.255.255.0 ! R2 shows: R2# show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.0.0.1 0 FULL/ - 00:00:30 10.0.0.1 GigabitEthernet0/0 But R2 does not have the summary route in its routing table. What is the root cause?

EIGRP stuck-in-active (SIA) is occurring due to route summarization. Router R1 is the hub in a hub-and-spoke topology with R2 and R3 as spokes. R1's configuration: interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0 ! router eigrp 100 network 10.0.0.0 ! R2 and R3 are connected via Frame Relay. R2 shows: R2# show ip eigrp topology 10.0.1.0/24 P 10.0.1.0/24, 1 successors, FD is 30720 via 10.0.0.1 (30720/28160), GigabitEthernet0/0 R3 has a similar entry. However, when R1's summary is active, R2 and R3 go into active state for the summary. What is the root cause?

DMVPN spoke-to-spoke tunnel failures are occurring due to route summarization. Hub router R1 has: interface Tunnel0 ip address 172.16.0.1 255.255.255.0 ip nhrp network-id 1 ip nhrp map multicast dynamic tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ! router eigrp 100 network 172.16.0.0 ! Spoke R2 has: interface Tunnel0 ip address 172.16.0.2 255.255.255.0 ip nhrp network-id 1 ip nhrp nhs 172.16.0.1 tunnel source GigabitEthernet0/0 tunnel mode gre multipoint ! R1 also has: interface GigabitEthernet0/0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0 ! Spokes cannot establish direct tunnels to each other for subnets within 10.0.0.0/22. What is the root cause?

MPLS LDP neighbor mismatch is causing label distribution failures for summary routes. Router R1 and R2 are LDP peers. R1 has: mpls ip ! interface GigabitEthernet0/0 ip address 10.0.0.1 255.255.255.0 mpls ip ! router ospf 1 network 10.0.0.0 0.0.0.255 area 0 area 0 range 10.0.0.0 255.255.252.0 ! R2 shows: R2# show mpls ldp neighbor Peer LDP Ident: 10.0.0.2:0, Transport address: 10.0.0.2 TCP connection: 10.0.0.2.646 - 10.0.0.1.646 State: Oper, Msgs sent/rcvd: 10/10 R2# show mpls forwarding-table 10.0.0.0/22 Local tag outgoing tag prefix tag(s) next-hop 16 Untagged 10.0.0.0/22 0 10.0.0.1 But R2 cannot forward traffic for 10.0.1.0/24. What is the root cause?

An ACL implicit deny is blocking management traffic due to route summarization. Router R1 has: access-list 100 permit ip 10.0.0.0 0.0.3.255 any ! interface GigabitEthernet0/0 ip access-group 100 in ! router eigrp 100 network 10.0.0.0 ! R1 also has: interface GigabitEthernet0/1 ip address 192.168.1.1 255.255.255.0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0 ! A management station at 10.0.1.100 cannot SSH to R1's loopback 0 (10.0.0.1). What is the root cause?

CoPP rate-limit is impacting legitimate traffic due to route summarization. Router R1 has: access-list 100 permit ip 10.0.0.0 0.0.3.255 any ! class-map match-all COPP-CLASS match access-group 100 ! policy-map COPP-POLICY class COPP-CLASS police 10000 conform-action transmit exceed-action drop ! control-plane service-policy input COPP-POLICY ! router eigrp 100 network 10.0.0.0 ! interface GigabitEthernet0/0 ip summary-address eigrp 100 10.0.0.0 255.255.252.0 ! R2 (10.0.1.1) sends EIGRP packets to R1, but they are being dropped. What is the root cause?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip eigrp topology 10.0.0.0/16 IP-EIGRP topology entry for 10.0.0.0/16 State is Passive, Query origin flag is 1, 1 Successor(s), FD is 128576 Routing Descriptor Blocks: 10.1.1.2 (Serial0/0/0), from 10.1.1.2, Send flag is 0x0 Composite metric is (128576/156160), Route is Internal Vector metric: Minimum bandwidth is 10000 Kbit Total delay is 2000 microseconds Reliability is 255/255 Load is 1/255 Minimum MTU is 1500 Hop count is 2 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# debug eigrp packets EIGRP: Sending HELLO on Serial0/0/0 src 10.1.1.1, dst 224.0.0.10 EIGRP: Received UPDATE on Serial0/0/0 from 10.1.1.2 src 10.1.1.2, dst 224.0.0.10 update type: route prefix 10.0.0.0/16 metric 128576 EIGRP: Sending UPDATE on Serial0/0/0 to 10.1.1.2 update type: summary prefix 10.0.0.0/16 metric 128576 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip ospf database summary 10.0.0.0 OSPF Router with ID (1.1.1.1) (Process ID 1) Summary Net Link States (Area 0) LS age: 100 Options: (No TOS-capability, DC) LS Type: Summary Links(Network) Link State ID: 10.0.0.0 (summary Network Number) Advertising Router: 2.2.2.2 LS Seq Number: 80000001 Checksum: 0x1234 Length: 28 Network Mask: /16 TOS: 0 Metric: 20 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip bgp 10.0.0.0/16 BGP routing table entry for 10.0.0.0/16, version 2 Paths: (1 available, best #1, table default) Advertised to update-groups: 1 Refresh Epoch 1 Local, (aggregated by 65000 1.1.1.1) 10.1.1.2 from 10.1.1.2 (2.2.2.2) Origin IGP, metric 0, localpref 100, valid, external, best Atomic-aggregate What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip route summary IP routing table maximum-paths limit: 32 IP routing table entry count: 15 IP routing table active entry count: 15 Number of prefixes: 15 Number of /0: 0 Number of /8: 1 Number of /16: 2 Number of /24: 12 Number of /32: 0 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# debug ip routing IP: route table change: 10.0.0.0/16 via 10.1.1.2, Serial0/0/0, distance 90, metric 128576 IP: route table change: 10.0.1.0/24 via 10.1.1.2, Serial0/0/0, distance 90, metric 128576 IP: route table change: 10.0.2.0/24 via 10.1.1.2, Serial0/0/0, distance 90, metric 128576 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show mpls ldp bindings 10.0.0.0/16 tib entry: 10.0.0.0/16, rev 1 local binding: label: 16 remote binding: lsr: 2.2.2.2:0, label: 17 What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip nhrp detail 10.0.0.0/16 via 10.1.1.2, Tunnel0 created 00:01:00, expire 01:59:00 Type: summary, Flags: used NBMA address: 192.168.1.2 Registration: never What does this output indicate?

A network engineer runs the following command to troubleshoot a Route Summarization issue: R1# show ip access-lists CoPP-ACL extended IP access list CoPP-ACL 10 permit eigrp any any (100 matches) 20 permit ospf any any (50 matches) 30 permit bgp any any (200 matches) 40 deny ip any any (0 matches) What does this output indicate?

In EIGRP, what is the default behavior of auto-summary in IOS-XE versions 15.0 and later?

What is the default OSPF reference bandwidth used in the cost calculation formula on Cisco IOS?

Which BGP attribute is used as the first tie-breaker when selecting the best path for route summarization?

In OSPF, what is the default hello interval on a point-to-point network type?

Which EIGRP packet type is used to confirm receipt of a route update during reliable transport?

What is the maximum hop count for a route in RIPv2 by default?

Which OSPF LSA type is used to advertise a summary route for a network outside the area but within the same OSPF domain?

In BGP, what is the default administrative distance for routes learned from an eBGP peer?

Which OSPF network type defaults to a 30-second hello interval and requires a DR/BDR election?

Drag and drop the steps to configure OSPF inter-area summarization on an ABR into the correct order, from first to last.

Drag and drop the steps to troubleshoot route summarization adjacency or connectivity failures into the correct order, from first to last.

Drag and drop the steps to verify and validate route summarization operational state into the correct order, from first to last.

Which TWO statements correctly describe the behavior of EIGRP route summarization when using the 'summary-address' command under an interface? (Choose TWO.)

An engineer is troubleshooting an OSPF network where route summarization is configured on an ABR. Which TWO statements correctly describe the behavior of OSPF inter-area route summarization using the 'area range' command? (Choose TWO.)

Which TWO statements correctly describe the behavior of BGP route summarization using the 'aggregate-address' command? (Choose TWO.)

Which TWO statements correctly describe the verification of route summarization using Cisco IOS commands? (Choose TWO.)

Which TWO statements correctly describe the behavior of route summarization in RIP? (Choose TWO.)

An engineer configures OSPF on two routers connected via a point-to-point link. The routers are stuck in EXSTART state. 'show ip ospf neighbor' shows neighbor state EXSTART/EXCHANGE. Which is the most likely explanation?

An engineer configures EIGRP named mode on a router. After configuration, a directly connected EIGRP neighbor is not forming an adjacency. 'show ip eigrp neighbors' shows nothing. Which is the most likely explanation?

An engineer configures a BGP route reflector in a network. After configuration, some iBGP routes are being dropped, causing routing loops. Which is the most likely explanation?

An engineer configures mutual redistribution between OSPF and EIGRP on a router. After configuration, routing loops occur. Which is the most likely explanation?

An engineer configures a DMVPN Phase 2 network. Spoke-to-spoke tunnels are not forming. Which is the most likely explanation?

An engineer configures IPsec between two routers. The tunnel does not come up. 'show crypto isakmp sa' shows MM_NO_STATE. Which is the most likely explanation?

An engineer configures Control Plane Policing (CoPP) on a router. After configuration, OSPF neighbors are flapping. Which is the most likely explanation?

An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on an interface. After configuration, legitimate traffic from a directly connected network is being dropped. Which is the most likely explanation?

An engineer configures route summarization in OSPF using the 'area range' command. After configuration, some routes are missing from the routing table of other routers. Which is the most likely explanation?