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IP Connectivity

493 questions

All types493Multiple choice250Multiple select113Matching25Ordering60Fill in the blank22Short answer23
Question 41Question →

A network engineer needs to configure an IPv6 static route on router R1 to forward traffic destined for remote subnet 2001:db8:acad:50::/642001:\text{db8}:\text{acad}:50::/64. The next-hop router's link-local address is fe80::fe54:99ff:fe12:34ab\text{fe80::fe54:99ff:fe12:34ab} via local interface GigabitEthernet0/0/2. Complete the Cisco IOS command below by filling in the exit interface and the next-hop address.

Fill in the blanks below

R1(config)# ipv6 route 2001:db8:acad:50::/64
Show answer & explanation

Answer

The missing command parameters are GigabitEthernet0/0/2 for the local exit interface (blank 1) and fe80::fe54:99ff:fe12:34ab for the link-local next-hop IPv6 address (blank 2).
When configuring an IPv6 static route using a link-local IPv6 address as the next hop, Cisco IOS requires a fully specified static route syntax where the local exit interface is specified first, followed by the link-local address. Therefore, the first blank requires the exit interface GigabitEthernet0/0/2 and the second blank requires the next-hop link-local address fe80::fe54:99ff:fe12:34ab.

Step-by-Step Solution

1
Identify the destination network prefix and prefix length.
The target network prefix is 2001:db8:acad:50::/642001:\text{db8}:\text{acad}:50::/64.
This forms the first argument following the `ipv6 route` global configuration command keyword.
2
Determine the requirement for static routes configured with link-local next-hop addresses.
Because link-local addresses (fe80::/10\text{fe80::/10}) are only unique per link, Cisco IOS requires the local exit interface to be explicitly specified preceding the link-local address.
Without specifying the local exit interface, the router cannot determine which local interface to send packets out to reach the non-unique link-local address.
3
Assemble the complete IPv6 static route command string in Cisco IOS order.
`ipv6 route 2001:db8:acad:50::/64 GigabitEthernet0/0/2 fe80::fe54:99ff:fe12:34ab`
This populates GigabitEthernet0/0/2 as the first blank and fe80::fe54:99ff:fe12:34ab as the second blank.

Key Concept

Fully specified IPv6 static route syntax using a link-local next-hop address and local outbound interface.
Question 42Question →

Two Cisco routers, R1 and R2, are connected via their GigabitEthernet0/0 interfaces over a point-to-point Ethernet link. A network engineer notices that OSPFv2 fails to establish a neighbor relationship between them. The engineer issues verification commands on both routers and observes the following command outputs:

R1# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

R2# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.1.1.2/24, Area 10
Process ID 2, Router ID 2.2.2.2, Network Type BROADCAST
Timer intervals configured, Hello 15, Dead 60, Wait 60, Retransmit 5

Which TWO configuration changes will resolve the neighbor adjacency failure between R1 and R2? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: Reconfigure R2's interface so that its OSPF Area ID matches Area 0.; Adjust R2's Hello and Dead timers to 10 seconds and 40 seconds respectively to match R1.

Answer

The two required configuration changes are: reconfiguring R2's interface so its OSPF Area ID matches Area 0, and adjusting R2's Hello and Dead timers to 10 seconds and 40 seconds to match R1.
To form an OSPFv2 neighbor adjacency over a connected link, routers must exchange Hello packets that agree on several mandatory fields. The Area ID (Area 0 vs Area 10) and the Hello/Dead timer intervals (10/40 vs 15/60) are mismatched in the output. Reconfiguring R2 to use Area 0 and setting its timers to Hello 10s / Dead 40s removes both blocking conditions, allowing neighbor adjacency to complete.

Step-by-Step Solution

1
Analyze the CLI output for OSPF Area ID configuration on both routers.
R1 is in Area 0 while R2 is in Area 10.
OSPF interfaces on the same subnet must belong to the exact same area for an adjacency to form.
2
Compare the configured OSPF Hello and Dead timer values between R1 and R2.
R1 has Hello 10s / Dead 40s, whereas R2 has Hello 15s / Dead 60s.
Mismatching Hello/Dead timers prevent OSPF routers from accepting Hello packets and forming a neighbor relationship.
3
Evaluate the non-matching parameters (Process ID and Router ID) to determine if they impact adjacency.
Process IDs do not need to match (local significance only), and Router IDs must remain unique rather than identical.
Only Area ID, Hello/Dead timers, Subnet Mask, MTU, and Authentication fields must satisfy adjacency checks in Hello exchanges.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
Question 43Question →

Refer to the following excerpt from a Cisco router's IPv4 routing table:

text
Gateway of last resort is 192.168.4.1 to network 0.0.0.0

S 10.1.4.0/24 [1/0] via 192.168.1.1
O 10.1.4.64/27 [110/20] via 192.168.2.1, 00:12:45, GigabitEthernet0/0/2
D 10.1.0.0/16 [90/307200] via 192.168.3.1, 01:45:10, GigabitEthernet0/0/3
S* 0.0.0.0/0 [1/0] via 192.168.4.1

The router receives an IP packet destined for 10.1.4.6710.1.4.67. Which next-hop IP address will the router select to forward this packet?

Show answer & explanation

Answer: 192.168.2.1

Answer

The router forwards the packet to next-hop IP address 192.168.2.1.
The destination IPv4 address 10.1.4.6710.1.4.67 matches multiple routes in the routing table: 10.1.4.0/2410.1.4.0/24, 10.1.4.64/2710.1.4.64/27, 10.1.0.0/1610.1.0.0/16, and 0.0.0.0/00.0.0.0/0. Router forwarding decision logic dictates that the router will ALWAYS prefer the Longest Prefix Match (most specific subnet mask). Comparing mask lengths (/27 > /24 > /16 > /0), the route 10.1.4.64/2710.1.4.64/27 has the longest match. Therefore, the router forwards the packet to next-hop 192.168.2.1192.168.2.1.

Step-by-Step Solution

1
Evaluate all routes matching the destination IP address 10.1.4.67.
Three matching routes found: 10.1.4.0/24 (prefix length 24), 10.1.4.64/27 (prefix length 27), and 10.1.0.0/16 (prefix length 16).
The subnet 10.1.4.64/27 covers IP addresses 10.1.4.64 through 10.1.4.95, which includes 10.1.4.67.
2
Apply the Longest Prefix Match (LPM) rule.
The route 10.1.4.64/27 has a 27-bit mask, which is longer than /24, /16, and /0.
The router always prioritizes the most specific match (longest subnet mask) first before evaluating Administrative Distance.
3
Identify the next-hop IP address for the winning route 10.1.4.64/27.
The next-hop address associated with 10.1.4.64/27 is 192.168.2.1.
The routing table entry specifies 'via 192.168.2.1'.

Key Concept

Router Forwarding Decision Logic (Longest Prefix Match)
Question 44Question →

Two Cisco routers, R10 and R20, are directly connected via their GigabitEthernet0/1 interfaces. OSPFv2 is configured on both routers, but running the command 'show ip ospf neighbor' on R10 reveals no neighbor entries. IP ping between the directly connected interface addresses succeeds. Which interface configuration difference between R10 and R20 will prevent the routers from even forming an OSPF Init state?

Show answer & explanation

Answer: Mismatched OSPF Area IDs on the interconnecting interfaces

Answer

Mismatched OSPF Area IDs on the interconnecting interfaces prevent the routers from establishing an OSPF neighbor relationship.
OSPF routers require several parameters to match in incoming Hello packets before creating a neighbor entry. Because Area ID is part of the Hello header validation, a mismatch causes the router to drop the Hello packet, resulting in no neighbor entry (no Init state).

Step-by-Step Solution

1
Analyze OSPF Hello packet primary validation requirements.
For two OSPFv2 routers to establish a neighbor relationship, fields in the Hello packet must match across the link: Area ID, Subnet Mask, Hello/Dead Timers, and Authentication credentials.
If any primary parameter such as Area ID or Subnet Mask mismatches, the router ignores the incoming Hello packet entirely.
2
Evaluate the impact of an Area ID mismatch.
When R10 receives a Hello packet with a different Area ID than what is configured on its interface, it discards the packet, preventing any neighbor entry from appearing in 'show ip ospf neighbor'.
Area membership must strictly match for interfaces sharing a common network segment.
3
Contrast with MTU, Process ID, and DR Priority behavior.
Process IDs are local; DR priorities affect election roles only; MTU mismatches allow neighbor discovery up to ExStart state.
Distinguishing between parameters that break Hello discovery versus parameters that stall database synchronization is key to OSPF troubleshooting.

Key Concept

OSPFv2 Neighbor Adjacency Requirements and Hello Packet Matching
Question 45Question →

Which Cisco IOS command configures an IPv4 floating static route to destination network 172.20.0.0/16172.20.0.0/16 through next-hop IP address 10.0.0.210.0.0.2 so that it serves as a backup to a primary route learned via Internal EIGRP (Administrative Distance of 9090)?

Show answer & explanation

Answer: ip route 172.20.0.0 255.255.0.0 10.0.0.2 130

Answer

ip route 172.20.0.0 255.255.0.0 10.0.0.2 130 correctly configures the backup floating static route with an administrative distance higher than the primary EIGRP route.
The command correctly uses the standard subnet mask format for destination network 172.20.0.0/16 and sets an administrative distance of 130. Because 130 is greater than EIGRP's administrative distance of 90, the router keeps this static route inactive in reserve until the primary EIGRP route is lost.

Step-by-Step Solution

1
Identify the proper Cisco IOS static route command syntax.
Syntax is: ip route <destination-prefix> <subnet-mask> <next-hop-ip> [distance]
Cisco IOS requires a subnet mask (255.255.0.0) rather than a wildcard mask.
2
Determine the required Administrative Distance (AD) for a floating static route.
The AD must be greater than 90 (the AD of Internal EIGRP).
Routes with lower AD values are preferred in the routing table. To act as a backup, the static route must have a higher AD so it is only installed when the primary route goes down.
3
Select the command matching both syntax and AD requirements.
ip route 172.20.0.0 255.255.0.0 10.0.0.2 130
130 is greater than 90 and the syntax uses the valid subnet mask format.

Key Concept

IPv4 Floating Static Route Configuration and Administrative Distance Selection
Estimated Time:45s
Question 46Question →

A network engineer needs to configure a floating static route on a Cisco router to serve as a backup path for the destination network 172.28.32.0/21172.28.32.0/21. The primary path to this network is currently learned via OSPF, which operates with an Administrative Distance of 110. Which Cisco IOS command correctly configures the floating static route so that it only enters the routing table if the primary OSPF route fails?

Show answer & explanation

Answer: ip route 172.28.32.0 255.255.248.0 10.200.1.2 120

Answer

ip route 172.28.32.0 255.255.248.0 10.200.1.2 120
The correct command specifies 'ip route 172.28.32.0 255.255.248.0 10.200.1.2 120'. A /21 subnet mask corresponds to 255.255.248.0. Since the primary route is learned via OSPF with an Administrative Distance (AD) of 110, a backup (floating) static route must be configured with an AD greater than 110 (such as 120). This keeps the route out of the routing table until the primary OSPF route is withdrawn.

Step-by-Step Solution

1
Determine the subnet mask representation for the prefix /21
A /21 network mask consists of 21 contiguous binary 1s: 11111111.11111111.11111000.00000000, which evaluates to 255.255.248.0 in dotted-decimal notation.
Cisco IOS IPv4 static route command syntax requires the netmask parameter in dotted-decimal format.
2
Identify the administrative distance (AD) of the primary dynamic routing protocol
OSPF has a default Administrative Distance of 110.
To create a floating static route, the static route must have an AD strictly greater than the primary route protocol's AD.
3
Select an Administrative Distance value for the floating static route
An AD of 120 is higher than 110, ensuring the route stays inactive in the routing table until OSPF fails.
Lower AD values are preferred by Cisco IOS. Setting the AD to 120 keeps the static route inactive (floating) while OSPF is active.
4
Construct the full Cisco IOS static route command
'ip route 172.28.32.0 255.255.248.0 10.200.1.2 120'
Matches the syntax: 'ip route <prefix> <mask> <next-hop-ip> [distance]'.

Key Concept

IPv4 Floating Static Route Configuration and Administrative Distance Selection
Question 47Question →

A Cisco router is configured to run OSPFv2 without an explicitly configured manual router ID. The router has the following active interface IP addresses:

- Loopback0: 10.10.1.1/3210.10.1.1/32
- Loopback1: 10.20.1.1/3210.20.1.1/32
- GigabitEthernet0/0: 192.168.1.254/24192.168.1.254/24
- GigabitEthernet0/1: 172.16.50.1/24172.16.50.1/24

Which IP address will OSPFv2 select as the router ID?

Show answer & explanation

Answer: 10.20.1.110.20.1.1

Answer

The OSPFv2 router ID selected by the router is 10.20.1.110.20.1.1.
In OSPFv2, when a router ID is not manually specified with the 'router-id' command, the router selects the highest IPv4 address among all active loopback interfaces. Comparing Loopback0 (10.10.1.110.10.1.1) and Loopback1 (10.20.1.110.20.1.1), 10.20.1.110.20.1.1 is the highest loopback IP address and is selected as the OSPF Router ID.

Step-by-Step Solution

1
Check for an explicitly configured manual OSPF Router ID
No manual 'router-id' command is present.
Manual configuration takes highest precedence in OSPF Router ID election.
2
Evaluate active loopback interfaces
Two active loopbacks exist: Loopback0 (10.10.1.110.10.1.1) and Loopback1 (10.20.1.110.20.1.1).
If no manual router ID is specified, OSPF evaluates active loopback interfaces next.
3
Select the highest IP address among active loopback interfaces
Compare 10.20.1.110.20.1.1 and 10.10.1.110.10.1.1; 10.20.1.110.20.1.1 is numerically higher.
OSPF selects the highest active loopback IP address over lower loopbacks and over any physical interfaces.

Key Concept

OSPFv2 Router ID Election Precedence
Question 48Question →

An administrator executes the command `show ip ospf neighbor` on a router and observes that a connected neighbor remains continuously in the INIT state. Which condition is the most likely cause of this issue?

Show answer & explanation

Answer: One router is rejecting incoming Hello packets because of an OSPF Area ID mismatch.

Answer

One router is rejecting incoming Hello packets because of an OSPF Area ID mismatch.
The correct answer identifies that an Area ID mismatch causes incoming Hello packets to be discarded. In OSPF, a router enters the INIT state when it receives a valid Hello packet from a neighbor. However, if the neighbor drops incoming Hellos from the local router due to an Area ID mismatch, the neighbor will never add the local router's Router ID to its Hello packet neighbor field. Consequently, the local router remains stuck in the INIT state.

Step-by-Step Solution

1
Analyze OSPF neighbor state definitions.
The INIT state indicates that a router has received a Hello packet from a neighbor, but the receiving router's own Router ID was not present in that Hello packet.
For OSPF neighbors to transition from INIT to 2-WAY, bidirectional communication must be established by seeing one's own Router ID in the received Hello packet's neighbor list.
2
Evaluate the impact of an Area ID mismatch on Hello packet processing.
When Area IDs mismatch, incoming Hello packets are dropped on receipt before the neighbor list is processed.
Because Hellos are discarded due to parameter mismatch (such as Area ID, Hello/Dead timers, or subnet mask), bidirectional communication fails to establish, keeping the relationship stuck in INIT.

Key Concept

OSPFv2 Neighbor Adjacencies and Router ID
Estimated Time:1m 0s
Question 49Question →

An enterprise router learns the remote network 10.50.0.0/1610.50.0.0/16 via OSPF, which has a default administrative distance of 110. A network administrator must configure a floating static backup route to reach destination 10.50.0.0/1610.50.0.0/16 using next-hop IP address 192.168.12.2192.168.12.2. The backup route must only enter the routing table if the primary OSPF path fails. Which Cisco IOS command correctly configures this floating static route?

Show answer & explanation

Answer: ip route 10.50.0.0 255.255.0.0 192.168.12.2 120

Answer

The command 'ip route 10.50.0.0 255.255.0.0 192.168.12.2 120' correctly configures the floating static route.
A floating static route acts as a backup path and must remain dormant in the routing table until the primary path fails. Since the primary path is installed by OSPF with an Administrative Distance of 110, the backup static route must be assigned an Administrative Distance higher than 110 (e.g., 120). The command 'ip route 10.50.0.0 255.255.0.0 192.168.12.2 120' fulfills this requirement.

Step-by-Step Solution

1
Identify the primary routing protocol and its Administrative Distance (AD).
The primary route to 10.50.0.0/1610.50.0.0/16 is learned via OSPF, which has an AD of 110.
Floating static routes rely on AD comparison to remain inactive until primary routes fail.
2
Determine the required AD for the backup floating static route.
The floating static route must have an AD greater than 110 (such as 120).
Lower AD values are preferred by the router forwarding logic; a higher AD ensures the route stays out of the routing table while OSPF is active.
3
Construct the Cisco IOS static route command syntax.
'ip route 10.50.0.0 255.255.0.0 192.168.12.2 120'
Syntax requires destination prefix, subnet mask, next-hop IP, and the explicit administrative distance metric at the end.

Key Concept

Floating Static Route Administrative Distance Selection
Estimated Time:1m 30s
Question 50Question →

A core router receives a packet destined for IP address 172.16.42.95. The router's IPv4 routing table contains four candidate paths to reach this destination:

text O 172.16.40.0/21 [110/65] via 10.1.1.1, 00:14:22, GigabitEthernet0/0/0 D 172.16.42.0/24 [90/307200] via 10.2.2.2, 01:05:10, GigabitEthernet0/0/1 S 172.16.42.64/26 [1/0] via 10.3.3.3 O 172.16.42.80/28 [110/30] via 10.4.4.4, 00:02:45, GigabitEthernet0/0/2

Which next-hop IP address will the router use to forward the packet?

Show answer & explanation

Answer: 10.4.4.4

Answer

The router will forward the packet to next-hop IP address 10.4.4.4 via GigabitEthernet0/0/2 because it has the longest prefix match (/28) containing the destination address 172.16.42.95.
When a router makes a forwarding decision, it evaluates all routing table entries that match the destination IP address and selects the entry with the longest prefix mask (the most specific route). For destination 172.16.42.95, the route 172.16.42.80/28 has a 28-bit mask, which is more specific than /26, /24, or /21. Therefore, the router forwards the packet to next-hop 10.4.4.4.

Step-by-Step Solution

1
Determine which prefixes in the routing table actually match the destination IP address 172.16.42.95.
172.16.40.0/21 covers 172.16.40.0 to 172.16.47.255 (Match). 172.16.42.0/24 covers 172.16.42.0 to 172.16.42.255 (Match). 172.16.42.64/26 covers 172.16.42.64 to 172.16.42.127 (Match). 172.16.42.80/28 covers 172.16.42.80 to 172.16.42.95 (Match).
Before comparing metrics or administrative distances, the router identifies all valid matching routes.
2
Compare the prefix lengths (subnet masks) of all matching routes to apply the Longest Prefix Match rule.
Prefix lengths are /21, /24, /26, and /28. The /28 prefix length is the longest (most specific).
The Cisco IOS forwarding decision always uses the longest prefix match first, regardless of routing source or administrative distance.
3
Identify the next-hop IP address associated with the longest matching route (172.16.42.80/28).
The next-hop address associated with 172.16.42.80/28 is 10.4.4.4.
The router forwards the packet out the exit interface towards 10.4.4.4.

Key Concept

Longest Prefix Match Rule in Routing Table Lookups
Question 51Question →

An enterprise network has a shared Ethernet segment running OSPFv2 in Area 0. Router R1 (Router ID 10.1.1.1, priority 1) is currently serving as the Designated Router (DR), and Router R2 (Router ID 10.2.2.2, priority 1) is serving as the Backup Designated Router (BDR). A network engineer adds Router R5 (Router ID 10.5.5.5, priority 255) to the segment. Shortly after R5 stabilizes as a DROTHER, R1's interface experiences a temporary link failure and goes down for five minutes before recovering. Which router functions as the DR on the segment immediately after R1 recovers and reaches the FULL neighbor state?

Show answer & explanation

Answer: Router R2 functions as the DR because it was promoted from BDR when R1 failed, and OSPF DR elections are non-preemptive.

Answer

Router R2 remains the Designated Router (DR) because OSPF DR/BDR elections are non-preemptive. Upon R1's failure, R2 was promoted to DR and R5 became BDR; when R1 recovers, existing roles are maintained.
OSPF DR and BDR roles are non-preemptive. When R5 joined initially, R1 and R2 retained their roles despite R5 having a higher priority. When R1 failed, BDR R2 was promoted to DR, and R5 was elected as the new BDR. When R1 recovers, it finds an active DR (R2) and an active BDR (R5) already present on the segment, so R1 becomes a DROTHER.

Step-by-Step Solution

1
Analyze initial state and addition of R5
R1 is DR, R2 is BDR. Adding R5 with priority 255 does not trigger a re-election for DR/BDR because active roles are non-preemptive. R5 becomes a DROTHER.
OSPF avoids network instability by preventing newly connected routers from preempting active DR and BDR roles.
2
Evaluate state changes during R1 failure
When R1 fails, BDR R2 is immediately promoted to DR. An election is held to fill the vacant BDR role, which R5 wins due to having the highest priority (255).
The BDR automatically steps up to DR when the DR fails, and remaining eligible routers elect a new BDR.
3
Determine final state after R1 link recovery
R1 re-establishes adjacency, sees active DR (R2) and active BDR (R5), and assumes the DROTHER role.
Recovered or newly booted routers do not preempt existing operational DR/BDR routers on multiaccess networks.

Key Concept

Non-preemptive nature of OSPF DR/BDR elections and promotion mechanics on broadcast multiaccess networks
Question 52Question →

A network engineer is configuring a static route on router R1 to reach remote network 2001:db8:c0ff:20::/642001:\text{db8}:\text{c0ff}:20::/64. The engineer intends to forward traffic to the neighboring router using its link-local address fe80::215:c5ff:feab:3102\text{fe80}::215:c5ff:\text{feab}:3102 through R1's local interface GigabitEthernet0/0/1. Which Cisco IOS command successfully accomplishes this configuration?

Show answer & explanation

Answer: ipv6 route 2001:db8:c0ff:20::/64 GigabitEthernet0/0/1 fe80::215:c5ff:feab:3102

Answer

The command specifying 'ipv6 route 2001:db8:c0ff:20::/64 GigabitEthernet0/0/1 fe80::215:c5ff:feab:3102' is the correct configuration.
The correct command follows exact Cisco IOS syntax for a fully specified static route: 'ipv6 route <prefix/mask> <exit-interface> <link-local-address>'. Because link-local IPv6 addresses are only significant on the local link, Cisco IOS requires the exit interface to be explicitly named.

Step-by-Step Solution

1
Identify the global configuration command syntax for IPv6 static routing.
The correct base command is 'ipv6 route <destination-prefix/prefix-length>'.
IPv6 routing commands use the 'ipv6' prefix rather than 'ip'.
2
Determine the mandatory parameters for link-local next-hop addresses.
A local exit interface must be included alongside the link-local address (fully specified static route).
Link-local addresses (fe80::/10) are link-scoped; without an exit interface, the router cannot determine which link to send packets out on.
3
Assemble the complete Cisco IOS command.
'ipv6 route 2001:db8:c0ff:20::/64 GigabitEthernet0/0/1 fe80::215:c5ff:feab:3102'
This places the destination network, local exit interface, and link-local next-hop in exact positional order.

Key Concept

Fully specified IPv6 static route requiring local exit interface when using link-local next-hop
Question 53Question →

A network engineer initializes OSPFv2 process 10 on router R-HQ. The router interfaces are configured as follows:

- Loopback 0: IP address 10.1.1.1/32 (Status: administratively down, Protocol: down)
- Loopback 1: IP address 192.168.50.1/32 (Status: up, Protocol: up)
- GigabitEthernet 0/0: IP address 172.16.100.254/24 (Status: up, Protocol: up)
- GigabitEthernet 0/1: IP address 10.254.254.254/30 (Status: up, Protocol: up)

No manual router ID is configured under `router ospf 10`. When the OSPF process starts, which IP address will be selected as the OSPF Router ID for R-HQ?

Show answer & explanation

Answer: 192.168.50.1

Answer

192.168.50.1 will be selected as the OSPF Router ID.
OSPF Router ID selection follows a strict order of precedence: 1) Manually configured 'router-id' under the OSPF process, 2) Highest IPv4 address on an active (up/up) loopback interface, 3) Highest IPv4 address on an active (up/up) physical interface. Because no manual Router ID is specified and Loopback 0 is down, the router selects 192.168.50.1 (Loopback 1), which is the only operational loopback interface.

Step-by-Step Solution

1
Check for a manually configured OSPF Router ID
No explicit router-id command is configured.
Explicit configuration is the highest priority in OSPF Router ID selection.
2
Evaluate active (up/up) loopback interfaces
Loopback 0 is down, leaving Loopback 1 (192.168.50.1) as the only active loopback interface.
Active loopback interfaces take precedence over physical interfaces regardless of numerical value.
3
Select the highest IP address among active loopbacks
192.168.50.1 is selected as the OSPF Router ID.
192.168.50.1 is the highest (and only active) loopback interface IP.

Key Concept

OSPF Router ID Election Logic
Estimated Time:1m 15s
Question 54Question →

An enterprise network administrator deploys four Cisco routers (R1, R2, R3, and R4) connected to a common Layer 2 GigabitEthernet switch on the 192.168.1.0/24192.168.1.0/24 broadcast multiaccess segment. All routers are powered on simultaneously, and OSPFv2 Area 0 is initialized across all connected interfaces. The interface parameters and operational configurations are as follows:

* R1: Configured with `ip ospf priority 255` and `ip ospf network point-to-point`; Router ID is 1.1.1.11.1.1.1.
* R2: Configured with default interface priority (11); Router ID is 10.10.10.1010.10.10.10 (derived from Loopback0).
* R3: Configured with `ip ospf priority 0`; Router ID is 30.30.30.3030.30.30.30 (derived from Loopback0).
* R4: Configured with interface priority set to 100100 (`ip ospf priority 100`); Router ID is 20.20.20.2020.20.20.20 (derived from Loopback0).

Assuming all routers run default OSPF timers and IP settings match, which two outcomes correctly describe the resulting OSPFv2 operational states and DR/BDR election results on this segment? (Choose two.)

Select all that apply

Show answer & explanation

Answer: R4 is elected as the Designated Router (DR) for the broadcast multiaccess segment.; R2 is elected as the Backup Designated Router (BDR) for the broadcast multiaccess segment.

Answer

The statement identifying R4 as the DR and the statement identifying R2 as the BDR are the correct choices.
In OSPFv2, DR/BDR elections only take place on multiaccess network types (such as broadcast). Because R1's interface is set to point-to-point, it does not participate in DR/BDR elections despite its high priority of 255. Furthermore, R3's priority of 0 explicitly disqualifies it from becoming DR or BDR. Comparing the remaining eligible routers on the broadcast network (R2 with priority 1 and R4 with priority 100), R4 wins the DR election due to its higher priority (100), and R2 wins the BDR election as the remaining eligible candidate.

Step-by-Step Solution

1
Evaluate eligibility for DR/BDR election based on OSPF network type
R1 is configured with network type point-to-point. Point-to-point networks do not perform DR/BDR elections. Therefore, R1 is completely eliminated from the election regardless of its priority of 255.
DR/BDR selection only occurs on multiaccess network types (Broadcast and Non-Broadcast Multi-Access).
2
Evaluate eligibility for DR/BDR election based on interface priority values
R3 has an interface priority of 0 (`ip ospf priority 0`), which renders it ineligible to become either DR or BDR. Thus, R3 is eliminated from election consideration.
An OSPF interface priority of 0 explicitly configured on an interface prevents the router from participating in DR/BDR elections.
3
Determine DR and BDR election winners among remaining eligible candidates
The remaining eligible candidates on the broadcast segment are R4 (priority 100) and R2 (priority 1). R4 has the highest priority and is elected DR. R2 has the next highest priority among eligible routers and is elected BDR.
OSPF DR election prioritizes highest interface priority first, followed by highest Router ID as a tie-breaker.

Key Concept

OSPFv2 DR/BDR Election Mechanics and Interface Network Type Rules
Estimated Time:3m 0s
Question 55Question →

A network engineer is configuring a backup path on router HQ-R1 to reach the corporate subnet 172.20.40.0/22172.20.40.0/22. The primary route to this prefix is currently learned via Internal EIGRP with a default administrative distance of 90. The backup connection uses a point-to-point Ethernet link where HQ-R1 local interface GigabitEthernet0/1 is assigned IP address 198.51.100.1/30198.51.100.1/30 and the remote peer router interface is assigned 198.51.100.254/30198.51.100.254/30.

Which Cisco IOS command must be configured on HQ-R1 to ensure the static route acts as a floating backup route that only enters the routing table if the primary EIGRP route fails?

Show answer & explanation

Answer: ip route 172.20.40.0 255.255.252.0 198.51.100.254 95

Answer

The correct command is `ip route 172.20.40.0 255.255.252.0 198.51.100.254 95`, which specifies the destination network, valid subnet mask, remote next-hop IP, and an administrative distance higher than Internal EIGRP (90).
The correct command properly formats the /22 mask as 255.255.252.0, uses the remote neighbor's interface IP (198.51.100.254) as the next hop, and sets an administrative distance of 95. Because 95 is higher than Internal EIGRP's administrative distance of 90, the router keeps this route out of the active routing table until the EIGRP route fails.

Step-by-Step Solution

1
Convert the CIDR prefix length /22 to dotted-decimal subnet mask format.
A /22 prefix corresponds to 22 binary ones (11111111.11111111.11111100.00000000), which evaluates to 255.255.252.0.
Cisco IOS static routing syntax requires a valid IPv4 subnet mask following the destination network prefix.
2
Identify the correct next-hop IP address for static route forwarding.
The next-hop IP address must be the neighbor router's IP address on the point-to-point link, which is 198.51.100.254.
Configuring a local interface IP address as a next hop is invalid because a router cannot forward packets to its own local interface.
3
Determine the required Administrative Distance (AD) for a floating static route.
Select an administrative distance value strictly greater than 90 (e.g., 95).
Internal EIGRP has a default AD of 90. To act as a floating backup, the static route's AD must be higher than the active primary routing protocol so that it remains untrusted until the primary route drops from the routing table.

Key Concept

Floating Static Route Administrative Distance Configuration
Estimated Time:1m 30s
Question 56Question →

A network administrator configures four routers (R1, R2, R3, and R4) on a single multiaccess Ethernet segment running OSPFv2. The OSPF interface parameters are configured as follows:

- R1: OSPF Priority = 100, Router ID = 1.1.1.1
- R2: OSPF Priority = 0, Router ID = 4.4.4.4
- R3: OSPF Priority = 100, Router ID = 3.3.3.3
- R4: OSPF Priority = 50, Router ID = 2.2.2.2

All four routers boot simultaneously and form neighbor relationships. Which role will router R2 assume on this network segment?

Show answer & explanation

Answer: DROTHER, because an interface priority of 0 explicitly excludes the router from becoming a DR or BDR.

Answer

Router R2 will assume the role of DROTHER because an interface priority of 0 prevents it from participating in the DR/BDR election.
On an OSPF broadcast network, routers elect a DR and BDR based on the highest interface priority. If there is a tie in priority, the highest Router ID breaks the tie. However, any interface configured with an OSPF priority of 0 is strictly excluded from participating in the election and will immediately assume the role of DROTHER, regardless of how high its Router ID is.

Step-by-Step Solution

1
Evaluate the OSPF interface priority settings for all routers on the broadcast network segment.
R1 has priority 100, R3 has priority 100, R4 has priority 50, and R2 has priority 0.
OSPF DR/BDR elections first compare interface priority values (range 0 to 255, default 1).
2
Identify the impact of an interface priority setting of 0.
Router R2 with priority 0 is disqualified from DR and BDR election.
An OSPF interface priority of 0 explicitly instructs the OSPF process that the router must never become a DR or BDR.
3
Determine the elected roles for all participating routers.
R3 becomes DR (priority 100, highest Router ID 3.3.3.3 among eligible routers), R1 becomes BDR (priority 100, Router ID 1.1.1.1), and R2 becomes DROTHER.
Only routers with a priority greater than 0 participate in DR/BDR selection.

Key Concept

OSPFv2 DR/BDR Election Exclusion via Priority 0
Question 57Question →

A network engineer is configuring OSPFv2 between two directly connected Cisco routers over a GigabitEthernet link. Upon inspecting the neighbor status, the engineer notes that the routers fail to form a neighbor relationship. Which TWO configuration mismatches between the interconnected interfaces will prevent these routers from establishing an OSPFv2 neighbor adjacency?

Select all that apply

Show answer & explanation

Answer: Mismatched OSPF Hello and Dead timer interval settings; Mismatched OSPF Area ID assignments on the interconnecting interfaces

Answer

The two conditions preventing OSPFv2 neighbor adjacency formation are mismatched Hello/Dead timer intervals and mismatched OSPF Area IDs on the connected interfaces.
For OSPFv2 routers to establish a neighbor adjacency, specific fields exchanged inside the OSPF Hello packet must match. These include matching OSPF Area IDs and identical Hello and Dead timer intervals. If either of these parameters differs between the connected interfaces, Hello packets are dropped and the routers remain in a Down state.

Step-by-Step Solution

1
Analyze OSPF Hello packet parameter matching requirements for neighbor adjacency.
Identify parameters exchanged in Hello packets that must strictly match: Area ID, Subnet Mask, Hello/Dead Timers, Authentication, and Stub Area Flag.
OSPF Hello packets validate neighbor compatibility before establishing an adjacency.
2
Evaluate local vs network-wide OSPF configuration parameters.
Confirm that Process IDs and interface metric costs are locally significant or used for path metric calculations rather than adjacency gating.
Process IDs distinguish local routing process instances, while interface costs affect metric accumulation downstream.

Key Concept

OSPFv2 Hello Packet Parameter Verification for Neighbor Adjacencies
Question 58Question →

Place the criteria evaluated during an OSPFv2 Designated Router (DR) election in order of precedence, starting with the highest priority evaluation criteria first.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct order of precedence for OSPFv2 DR/BDR election tie-breaking is: 1. Highest interface OSPF priority value (non-zero), 2. Highest manually configured OSPF Router ID, 3. Highest IP address among active loopback interfaces, 4. Highest IP address among active physical interfaces.
In OSPFv2 DR/BDR elections on broadcast or non-broadcast multiaccess networks, the selection process follows a strict precedence: interface priority is checked first (highest wins, while priority 0 is ineligible). If priorities tie, the OSPF Router ID determines the winner. The Router ID selection itself follows a deterministic hierarchy: explicit manual configuration via 'router-id' first, followed by the highest active loopback IP address, and finally the highest active physical interface IP address.

Step-by-Step Solution

1
Evaluate OSPF Interface Priority
Highest priority (1-255) wins. A priority of 0 excludes the router from becoming DR/BDR.
OSPF prioritizes administrator design intent via interface priority over IP address tie-breakers.
2
Evaluate Explicit Router ID
If priority values are equal, the router with the highest manually configured router-id value is selected.
Manual Router ID configuration overrides dynamically determined interface IP addresses.
3
Evaluate Active Loopback Interfaces
If no manual router ID exists, the highest IP address configured on an active loopback interface is chosen.
Loopback interfaces are logically stable and preferred over physical interfaces for RID determination.
4
Evaluate Active Physical Interfaces
If no loopback interfaces exist, the highest IP address configured on an active physical interface is used as the final tie-breaker.
Physical interface IPs serve as the final fallback for Router ID determination in OSPFv2.

Key Concept

OSPFv2 DR/BDR Election Precedence and Router ID Selection Criteria
Question 59Question →

A Cisco router displays the following routing table output:

text
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
O 10.75.0.0/16 [110/20] via 192.168.12.2, 00:14:22, GigabitEthernet0/0
D 10.75.96.0/19 [90/307200] via 192.168.13.2, 01:42:05, GigabitEthernet0/1
S 10.75.100.0/24 [1/0] via 192.168.14.2, 03:02:10, GigabitEthernet0/2
O IA 10.75.100.32/27 [110/65] via 192.168.15.2, 00:05:10, GigabitEthernet0/3

The router receives an IP packet destined for 10.75.100.5010.75.100.50. Which next-hop IP address and outgoing interface will the router select to forward this packet?

Show answer & explanation

Answer: 192.168.15.2 via GigabitEthernet0/3

Answer

Next-hop address 192.168.15.2 via interface GigabitEthernet0/3
When a router receives a packet, it compares the destination IP address against all entries in its routing table and selects the route with the longest prefix match (most specific subnet mask). The destination address 10.75.100.50 matches four routes in the routing table: /16, /19, /24, and /27. The /27 route (10.75.100.32/27) has the most matching network bits (27 bits), so the router forwards the packet to next-hop 192.168.15.2 out interface GigabitEthernet0/3.

Step-by-Step Solution

1
Identify all candidate routes that encompass destination IP 10.75.100.50
The address 10.75.100.50 falls within 10.75.0.0/16, 10.75.96.0/19 (10.75.96.0 - 10.75.127.255), 10.75.100.0/24 (10.75.100.0 - 10.75.100.255), and 10.75.100.32/27 (10.75.100.32 - 10.75.100.63).
A packet destination must match the subnet range of a candidate route entry.
2
Apply the Longest Prefix Match (LPM) rule across all matching routes
The prefix lengths are /16, /19, /24, and /27. The longest prefix match is /27 (10.75.100.32/27).
Cisco routers always prefer the route with the most specific prefix (longest subnet mask) regardless of administrative distance or metric.
3
Extract the next-hop IP and outgoing interface from the longest prefix match route entry
The entry 10.75.100.32/27 specifies next-hop 192.168.15.2 via interface GigabitEthernet0/3.
The router uses the forward destination components listed in the winning routing table entry.

Key Concept

Longest Prefix Match Rule
Estimated Time:1m 15s
Question 60Question →

An administrator reviews the routing table of a Cisco router and observes the following specific route entry:

text
O 172.31.50.0/26 [110/65] via 10.1.1.2, 00:04:12, GigabitEthernet0/1

Which two statements correctly interpret the components of this routing entry? (Select two.)

Select all that apply

Show answer & explanation

Answer: The administrative distance of this route is 110, identifying OSPF as the routing protocol source.; The metric value for reaching the destination network via this path is 65.

Answer

The statement identifying the administrative distance as 110 (indicating OSPF) and the statement identifying the metric value as 65 are correct.
In Cisco IOS routing tables, a route entry displays the route source code ('O' for OSPF), destination subnet prefix, and bracketed values [Administrative Distance / Metric]. For the route entry provided, 110 is the Administrative Distance and 65 is the OSPF route metric. The egress interface is GigabitEthernet0/1, and 10.1.1.2 is the next-hop router IP address.

Step-by-Step Solution

1
Analyze the prefix code prefixing the routing entry.
The code 'O' indicates the route was dynamically learned through Open Shortest Path First (OSPF).
Routing codes at the beginning of an entry indicate the source protocol.
2
Examine the bracketed numbers [110/65].
The structure is formatted as [Administrative Distance / Metric]. Therefore, AD = 110 and Metric = 65.
Cisco IOS routing entries consistently format believable path trustworthiness (AD) first and protocol metric second.
3
Identify destination subnet and forwarding descriptors.
172.31.50.0/26 is the target subnet, 10.1.1.2 is the next-hop IP address, and GigabitEthernet0/1 is the egress interface.
The 'via' keyword introduces the next-hop IP address for remote networks.

Key Concept

Routing Table Components
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