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IP Connectivity

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Question 21Question →

A network engineer is configuring OSPFv2 across a broadcast Ethernet network segment and verifying router behaviors. Which two statements regarding OSPFv2 Router ID determination and neighbor adjacency formation are correct?

Select all that apply

Show answer & explanation

Answer: A router ID configured explicitly with the router-id command takes precedence over IP addresses assigned to loopback or physical interfaces.; On a broadcast network segment, two neighboring routers with an OSPF interface priority of 0 will remain in the 2-WAY neighbor state with each other.

Answer

The two correct statements are that an explicitly configured router ID takes precedence over interface IP addresses, and that two routers with an OSPF priority of 0 on a broadcast network will remain in the 2-WAY state with each other.
An explicitly configured router ID using the 'router-id' command always takes precedence over loopback and physical interface IP addresses. Furthermore, on broadcast multiaccess networks, routers configured with a priority of 0 cannot become DR or BDR; two such DROTHER routers establish bi-directional communication but remain in the 2-WAY state with each other, forming FULL adjacencies only with the elected DR and BDR.

Step-by-Step Solution

1
Evaluate OSPF Router ID election hierarchy
The election order is: 1) explicit 'router-id' CLI command, 2) highest IP address on an active loopback interface, 3) highest IP address on an active non-loopback/physical interface.
Explicit configuration always overrides automatic interface IP selection.
2
Evaluate Router ID preemption rules
Router ID selection is non-preemptive once the OSPF process initializes.
Changing interface IPs or adding loopbacks will not alter the active Router ID until 'clear ip ospf process' is executed or the router reboots.
3
Evaluate OSPF neighbor states on multiaccess broadcast networks
Setting 'ip ospf priority 0' disables DR/BDR eligibility for a router, making it a DROTHER.
DROTHER routers form FULL adjacencies only with the DR and BDR. Communication between two DROTHERs remains bidirectional in the 2-WAY state.
4
Verify Hello packet parameter matching requirements
Area ID, Subnet Mask, Hello/Dead timers, and Authentication must match between neighbors.
An Area ID mismatch causes Hello packet rejection, preventing adjacency formation entirely.

Key Concept

OSPFv2 Router ID election precedence and DROTHER 2-WAY neighbor relationship dynamics
Question 22Question →

A network administrator is configuring OSPFv2 between two routers on a point-to-point link. After configuring the router interfaces, the administrator notices that the routers fail to establish an OSPF neighbor relationship. Troubleshooting reveals that the two connecting interfaces were assigned different OSPF Area IDs. What is the outcome of this configuration mismatch?

Show answer & explanation

Answer: The routers drop the incoming Hello packets and fail to form any neighbor relationship.

Answer

The routers drop the incoming Hello packets and fail to form any neighbor relationship.
For two routers to establish an OSPFv2 neighbor adjacency, several parameters in their Hello packets must match exactly, including the Area ID, Hello/Dead timers, Subnet Mask (on broadcast networks), Authentication, and Stub area flag. If the Area ID does not match, the router discards the received Hello packet immediately, preventing any neighbor state transition.

Step-by-Step Solution

1
Identify the mandatory parameters that must match in OSPFv2 Hello packets.
OSPF Hello packets contain field parameters including Area ID, Hello and Dead intervals, Subnet Mask, Authentication key, and Stub area flag.
OSPFv2 strictly validates these fields upon receipt of a Hello packet from a potential neighbor.
2
Evaluate the effect of an Area ID mismatch.
When a router receives a Hello packet containing an Area ID that does not match the receiving interface's configured Area ID, the router discards the packet.
Receiving interfaces must belong to the exact same logical area to exchange topology information.
3
Determine the resulting neighbor state.
Because Hello packets are dropped, neither router sees itself listed in the neighbor's Hello packet, preventing any neighbor state progression.
Without valid Hello processing, the neighbor relationship remains down.

Key Concept

OSPFv2 Neighbor Adjacency Mandatory Matching Parameters
Estimated Time:45s
Question 23Question →

A network router receives its primary path to a corporate office via external BGP with an administrative distance of 20. Which administrative distance value should be assigned to a static backup route to ensure it functions as a floating static route?

Show answer & explanation

Answer: 50

Answer

An administrative distance of 50 should be assigned.
A floating static route must have an administrative distance higher than the primary route so it is ignored by the routing table during normal operation and only installed if the primary route fails. Since the primary eBGP route has an administrative distance of 20, setting the static route's administrative distance to 50 makes it a valid backup route.

Step-by-Step Solution

1
Identify the administrative distance of the primary route.
The primary route is learned via external BGP, which has an administrative distance of 20.
Routing protocols use administrative distance to rank trustworthiness, preferring lower values.
2
Determine the required administrative distance for a floating static route.
The static route must be configured with an administrative distance greater than 20.
Floating static routes act as backups and must remain inactive in the routing table until the primary route with a lower administrative distance becomes unavailable.
3
Select the valid administrative distance option.
50 is greater than 20, making it a valid floating static route distance.
A value of 50 ensures the static route is only placed into the routing table if the eBGP route fails.

Key Concept

Floating static routes require a higher administrative distance than the primary routing protocol to remain inactive as a backup until failover occurs.
Question 24Question →

A network engineer is auditing OSPFv2 interface configurations across a corporate network topology. On a multi-access Ethernet segment connecting five routers (R1 through R5), R1 must be configured so that it never becomes a Designated Router (DR) or Backup Designated Router (BDR). Additionally, on a separate point-to-point serial/Ethernet link between R1 and R2, the engineer converts the default OSPF network type to `point-to-point`. Which TWO actions or operational characteristics correctly apply to this OSPFv2 deployment? (Choose two.)

Select all that apply

Show answer & explanation

Answer: Configuring `ip ospf priority 0` on R1's multi-access interface prevents it from participating in the DR/BDR election, keeping it permanently in the DROTHER state.; Changing the interface network type to `point-to-point` on the link between R1 and R2 eliminates DR/BDR election overhead while maintaining a default Hello interval of 10 seconds on Ethernet media.

Answer

Configuring an OSPF priority of 0 prevents a router from participating in DR/BDR elections (keeping it DROTHER), and configuring a point-to-point network type suppresses DR/BDR elections while preserving the default 10-second Hello timer on Ethernet.
Configuring an interface priority of 0 ensures a router will never assume the role of DR or BDR on a multi-access network segment. Furthermore, configuring the point-to-point network type eliminates DR/BDR elections entirely on that interface while retaining the standard 10-second Hello interval on Ethernet.

Step-by-Step Solution

1
Evaluate the requirement for R1 on the multi-access Ethernet segment.
Setting `ip ospf priority 0` on the interface completely excludes R1 from becoming either DR or BDR.
In OSPFv2, a router priority of 0 signifies non-participation in elections.
2
Analyze the operational properties of the OSPF point-to-point network type.
Point-to-point interfaces form adjacencies without electing a DR or BDR, and default Hello/Dead timers on Ethernet media remain 10s/40s.
DR/BDR elections are unnecessary on links limited to two endpoints.
3
Evaluate incorrect assertions regarding DR preemption, election override, and timer changes.
Reject claims that priority 0 allows BDR election, that point-to-point on Ethernet uses 30s timers, or that higher priority preempts an active DR.
Priority 0 excludes both DR and BDR roles; timer default is 10s; elections are non-preemptive.

Key Concept

OSPFv2 DR/BDR Election Rules and Network Type Timer Defaults
Estimated Time:1m 30s
Question 25Question →

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

text
Gateway of last resort is not set

172.24.0.0/16 is variably subnetted, 4 subnets, 3 masks
O 172.24.32.0/21 [110/65] via 10.1.1.2, 00:12:45, GigabitEthernet0/0/1
D 172.24.32.0/22 [90/307200] via 10.1.1.6, 02:41:10, GigabitEthernet0/0/2
O IA 172.24.35.0/24 [110/120] via 10.1.1.10, 00:05:22, GigabitEthernet0/0/3
S 172.24.0.0/16 [1/0] via 10.1.1.1

Which next-hop IP address and exit interface will the router select to forward a packet destined for 172.24.35.10172.24.35.10?

Show answer & explanation

Answer: 10.1.1.10 via GigabitEthernet0/0/3

Answer

10.1.1.10 via GigabitEthernet0/0/3
When a router makes a forwarding decision, it evaluates all matching table entries and selects the route with the most specific match—the longest prefix length. The route entry 172.24.35.0/24 covers 172.24.35.10 with a /24 prefix length, which is longer (more specific) than /22, /21, or /16. Therefore, traffic is forwarded to next-hop 10.1.1.10 via interface GigabitEthernet0/0/3.

Step-by-Step Solution

1
Identify all candidate routes that match the destination IP address 172.24.35.10.
The destination address falls into four matching subnet ranges: 172.24.32.0/21, 172.24.32.0/22, 172.24.35.0/24, and 172.24.0.0/16.
Before comparing routing metrics or administrative distance, a router determines which entry ranges include the destination address.
2
Determine the prefix length (subnet mask bits) for each candidate route.
Prefix lengths are /16, /21, /22, and /24.
Cisco IOS routing logic enforces the Longest Prefix Match rule above administrative distance or metric considerations.
3
Select the route with the most specific match (highest number of network prefix bits).
172.24.35.0/24 has the longest prefix match (/24). The associated next-hop is 10.1.1.10 out interface GigabitEthernet0/0/3.
A /24 mask provides a more specific route match than /22, /21, or /16.

Key Concept

Longest Prefix Match Rule in Routing Table Lookup
Question 26Question →

A network administrator is establishing a backup default route on a Cisco IOS router. The primary default route is currently learned dynamically via OSPFv3 with an administrative distance of 110. The administrator wants to ensure that the backup path towards the next-hop global unicast address 2001:db8:200::2 is used only if the primary OSPFv3 route fails. Which command correctly achieves this objective?

Show answer & explanation

Answer: ipv6 route ::/0 2001:db8:200::2 130

Answer

ipv6 route ::/0 2001:db8:200::2 130
The correct command uses the IPv6 default route prefix ::/0, targets the global unicast next-hop address, and assigns an administrative distance of 130 (which is greater than OSPFv3's AD of 110), successfully floating the route until the primary path fails.

Step-by-Step Solution

1
Identify the IPv6 default route prefix notation.
The IPv6 default route representation is ::/0.
IPv6 default routes represent all possible destination IPv6 networks using the all-zeros prefix with a length of zero.
2
Determine the required administrative distance for a floating static route.
Set an administrative distance higher than 110 (e.g., 130).
A floating static route must have an administrative distance higher than the primary routing protocol (OSPFv3 AD = 110) so it remains inactive until the primary route fails.
3
Validate next-hop addressing syntax in Cisco IOS.
Using global unicast address 2001:db8:200::2 directly is valid, whereas link-local next hops mandatory require an exit interface parameter.
Cisco IOS requires explicit interface designation only when using link-local IPv6 addresses as next hops.

Key Concept

Floating IPv6 Static Default Route Configuration
Question 27Question →

A network administrator needs to configure a floating static route on a Cisco router to act as a backup link for a primary path learned via OSPF (Administrative Distance of 110). Which Administrative Distance value must be assigned to the floating static route to ensure it is used ONLY when the primary OSPF route becomes unavailable?

Show answer & explanation

Answer: 120

Answer

The floating static route must be configured with an Administrative Distance greater than 110 (such as 120).
For a static route to function as a floating static route (backup), its Administrative Distance must be configured to a value higher than that of the primary route protocol. Since OSPF has an AD of 110, an AD of 120 ensures the static route remains out of the routing table until the OSPF route fails.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary dynamic route.
The primary route is learned via OSPF, which has a default AD of 110.
Routers prefer routes with lower AD values when installing paths into the routing table.
2
Determine the AD requirement for a backup (floating) static route.
The floating static route must have an AD higher than 110 (for example, 120).
Assigning a higher AD keeps the backup route inactive ('floating') until the lower AD primary route disappears from the routing table.

Key Concept

Floating Static Route Administrative Distance Selection
Estimated Time:45s
Question 28Question →

Two directly connected Cisco routers are configured with OSPFv2 on their connecting interfaces. Router R1 has its interface assigned to OSPF Area 0, while Router R2 has its connecting interface assigned to OSPF Area 1. Assuming all other OSPF parameters are left at default settings, what is the result of this configuration?

Show answer & explanation

Answer: The routers fail to form an OSPF neighbor adjacency because the Area IDs do not match.

Answer

The routers fail to form an OSPF neighbor adjacency because the Area IDs do not match.
For two OSPFv2 routers to establish a neighbor adjacency, several interface parameters exchanged in Hello packets must match exactly. One mandatory parameter is the Area ID. Because one router's interface is configured in Area 0 and the other in Area 1, the routers reject each other's Hello packets and fail to form an adjacency.

Step-by-Step Solution

1
Identify the mandatory matching parameters required in OSPF Hello packets for neighbor formation.
Area ID, Subnet Mask, Hello/Dead Intervals, Authentication, and Option flags (such as Stub area flags) must match between neighbors on a link.
OSPF routers validate these fields upon receiving a Hello packet before accepting the sending router as a valid neighbor.
2
Evaluate the effect of placing connecting interfaces into different areas (Area 0 vs. Area 1).
Incoming OSPF Hello packets are dropped due to the Area ID mismatch.
Because the Area ID field in the Hello header does not match the local interface's configured area, neighbor discovery fails immediately.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
Question 29Question →

An engineer needs to configure a static route on a Cisco router to forward traffic destined for 2001:db8:a:10::/642001:\text{db8}:\text{a}:10::/64. The route must use the next-hop router's link-local address fe80::211:25ff:feab:1234\text{fe80}::211:25\text{ff}:\text{feab}:1234 via the local interface GigabitEthernet0/1. Which command correctly configures this IPv6 static route?

Show answer & explanation

Answer: ipv6 route 2001:db8:a:10::/64 GigabitEthernet0/1 fe80::211:25ff:feab:1234

Answer

ipv6 route 2001:db8:a:10::/64 GigabitEthernet0/1 fe80::211:25ff:feab:1234
The command 'ipv6 route 2001:db8:a:10::/64 GigabitEthernet0/1 fe80::211:25ff:feab:1234' is correct because configuring an IPv6 static route with a link-local next-hop address requires a fully specified route configuration that explicitly specifies the local exit interface prior to the link-local next-hop address.

Step-by-Step Solution

1
Identify the proper command syntax for IPv6 static routing.
The configuration must use the global configuration command starting with 'ipv6 route' followed by the target destination prefix and prefix length (2001:db8:a:10::/64).
Cisco IOS differentiates IPv4 and IPv6 static routes by requiring the 'ipv6 route' keyword for IPv6.
2
Determine the required parameters for a link-local next-hop address.
Because the next-hop is a link-local address (fe80::/10), the command must explicitly include both the exit interface (GigabitEthernet0/1) and the link-local address (fe80::211:25ff:feab:1234).
Link-local addresses are only valid on a specific physical link. Without the local exit interface context, the router cannot determine which outgoing interface to use to reach the next-hop.

Key Concept

Fully Specified IPv6 Static Route
Estimated Time:1m 0s
Question 30Question →

Branch router R2 needs to reach host devices in the remote subnet 172.20.15.0/24172.20.15.0/24. The next-hop router interface connected to R2 is assigned the IPv4 address 10.1.1.210.1.1.2. Which Cisco IOS command correctly defines this IPv4 static route?

Show answer & explanation

Answer: ip route 172.20.15.0 255.255.255.0 10.1.1.2

Answer

The correct command is 'ip route 172.20.15.0 255.255.255.0 10.1.1.2'.
The correct command adheres precisely to Cisco IOS syntax: 'ip route <destination-prefix> <subnet-mask> <next-hop-ip>'. For a /24/24 destination network of 172.20.15.0172.20.15.0 reached via 10.1.1.210.1.1.2, the proper entry is 'ip route 172.20.15.0 255.255.255.0 10.1.1.2'.

Step-by-Step Solution

1
Identify the target destination prefix and length.
Destination prefix is 172.20.15.0172.20.15.0 with prefix length /24/24.
Static routing requires specifying the destination network ID.
2
Convert prefix length /24/24 to dotted-decimal subnet mask format.
Mask is 255.255.255.0255.255.255.0.
Cisco IOS global configuration syntax requires dotted-decimal subnet masks for IPv4 static routes.
3
Assemble parameters into Cisco IOS command structure 'ip route <destination> <mask> <next-hop>'.
'ip route 172.20.15.0 255.255.255.0 10.1.1.2'
This matches standard Cisco CLI global configuration syntax.

Key Concept

IPv4 Static Route Syntax
Question 31Question →

A network administrator needs to configure a floating static default route on a Cisco IOS router to serve as a backup path to a primary default route learned via OSPF. Which two statements accurately describe the requirements for this configuration? (Select TWO)

Select all that apply

Show answer & explanation

Answer: The static route command must specify a destination network of 0.0.0.00.0.0.0 and a subnet mask of 0.0.0.00.0.0.0.; The administrative distance configured for the backup static route must be greater than 110110.

Answer

To configure a floating static default route as a backup to an OSPF route, the destination address and mask must be configured as 0.0.0.0 0.0.0.0, and the administrative distance must be configured to a value greater than 110.
A static default route requires the destination network 0.0.0.00.0.0.0 and mask 0.0.0.00.0.0.0. To make this static route function as a backup (floating route) to an OSPF-learned route, its administrative distance must be higher than OSPF's default AD of 110110.

Step-by-Step Solution

1
Identify the destination prefix and mask for a default static route.
A default route matches all traffic using the IPv4 prefix 0.0.0.0 and subnet mask 0.0.0.0.
The default static route syntax requires 'ip route 0.0.0.0 0.0.0.0 {ip-address | exit-interface}'.
2
Determine the required Administrative Distance (AD) for a floating backup route.
Since OSPF has a default AD of 110, the static route AD must be set to 111 or higher.
Cisco routers prefer routes with lower AD values. To ensure the static route is used only when the OSPF route fails, its AD must be higher than the OSPF AD.

Key Concept

IPv4 Floating Static Default Route Requirements
Question 32Question →

A network administrator configures an OSPFv2 interface on a Multiaccess Ethernet network with the command `ip ospf priority 0`. What is the impact of this configuration on the DR/BDR election process on that segment?

Show answer & explanation

Answer: The router interface is completely excluded from becoming either a Designated Router (DR) or Backup Designated Router (BDR).

Answer

The router interface is completely excluded from becoming either a Designated Router (DR) or Backup Designated Router (BDR).
In OSPFv2 multiaccess networks, setting an interface's priority to 0 explicitly revokes its eligibility to participate in DR and BDR elections. The interface remains in the DROTHER state and forms adjacencies only with the elected DR and BDR.

Step-by-Step Solution

1
Evaluate the function of OSPF interface priority values.
OSPF interface priority ranges from 0 to 255, where default priority is 1.
The priority value determines eligibility and preference during DR/BDR elections on broadcast multiaccess networks.
2
Analyze the specific behavior of priority 0.
Configuring `ip ospf priority 0` marks the interface as ineligible for DR/BDR roles.
A router with priority 0 will always transition directly to the DROTHER state upon adjacency establishment.

Key Concept

OSPF Interface Priority 0 and DR/BDR Ineligibility
Question 33Question →

A network administrator needs to configure an IPv4 static default route on a Cisco router pointing to the next-hop IP address 203.0.113.1. Which Cisco IOS command correctly accomplishes this configuration?

Show answer & explanation

Answer: ip route 0.0.0.0 0.0.0.0 203.0.113.1

Answer

ip route 0.0.0.0 0.0.0.0 203.0.113.1
The correct command uses 'ip route 0.0.0.0 0.0.0.0 203.0.113.1'. In Cisco IOS, a static default route is configured using the prefix 0.0.0.0 and mask 0.0.0.0 followed by either the next-hop IPv4 address or an outbound interface.

Step-by-Step Solution

1
Identify the standard Cisco IOS syntax for IPv4 static routing.
The standard syntax is 'ip route <destination-prefix> <subnet-mask> {ip-address | exit-interface}'.
Cisco IOS requires explicit declaration of the target network prefix, subnet mask, and forwarding target.
2
Determine the prefix and mask required for a static default route.
A default route matching all IPv4 destinations uses network prefix 0.0.0.0 and subnet mask 0.0.0.0.
The prefix 0.0.0.0/0 matches any IPv4 address when no more specific match exists in the routing table.
3
Append the specified next-hop IP address.
Combining the default prefix/mask with next-hop IP 203.0.113.1 yields 'ip route 0.0.0.0 0.0.0.0 203.0.113.1'.
The next-hop IP address designates the forwarding address of the adjacent router.

Key Concept

IPv4 Static Default Route Configuration
Question 34Question →

An administrator is configuring OSPFv2 on Router R1. The router has no explicit 'router-id' command under its OSPF process, but has the following active interfaces:
- Loopback0: 172.16.1.1/32
- Loopback1: 10.200.1.1/32
- GigabitEthernet0/0: 10.0.0.1/24 (configured with 'ip ospf 1 area 0')

Directly connected on GigabitEthernet0/0 is Router R2, which has its OSPF process configured with an explicit 'router-id 172.16.1.1' and GigabitEthernet0/0 (10.0.0.2/24) in Area 0. When both routers attempt to form an adjacency, R1 shows R2 stuck in the INIT state, and R2 logs report duplicate OSPF packets.

Based on OSPF election logic and neighbor requirements, why is the neighbor relationship stuck in this state?

Show answer & explanation

Answer: R1 selected 172.16.1.1 as its Router ID because it has the highest IP address among active loopback interfaces, causing a duplicate Router ID conflict with R2.

Answer

R1 selected 172.16.1.1 as its Router ID because it has the highest IP address among active loopback interfaces, causing a duplicate Router ID conflict with R2.
The correct answer identifies that R1 automatically selects 172.16.1.1 as its Router ID because 172.16.1.1 is the highest IP address among its active loopback interfaces (172.16.1.1 vs 10.200.1.1). Because R2 has an explicit Router ID of 172.16.1.1, both routers share the same Router ID on the segment. Routers with duplicate Router IDs ignore or reject each other's Hello packets and fail to list each other as valid neighbors, causing the relationship to remain stuck in the INIT state.

Step-by-Step Solution

1
Determine the OSPF Router ID selection logic on R1.
Since no explicit 'router-id' command is configured, R1 checks active loopback interfaces. It compares Loopback0 (172.16.1.1) and Loopback1 (10.200.1.1). Because 172.16.1.1 is numerically higher than 10.200.1.1, R1 selects 172.16.1.1 as its OSPF Router ID.
OSPF Router ID election order is: 1) Explicit 'router-id' command, 2) Highest IP address among active loopbacks, 3) Highest IP address among active physical interfaces.
2
Compare R1's Router ID with R2's Router ID.
R2 is explicitly configured with OSPF Router ID 172.16.1.1. Both R1 and R2 are using the identical Router ID (172.16.1.1) on the same network segment.
Duplicate OSPF Router IDs on the same segment prevent routers from establishing bi-directional communication.
3
Analyze the impact of duplicate Router IDs on OSPF neighbor state.
When R2 receives Hello packets from R1 with a source Router ID identical to its own, it discards them or fails to list R1 as a valid neighbor. R1 receives Hello packets but never sees its own Router ID listed in R2's Hello packet neighbor field, keeping the state stuck in INIT.
Transitioning from INIT to 2-WAY requires a router to see its own Router ID in the received Hello packet's neighbor list.

Key Concept

OSPFv2 Router ID Election Hierarchy and Duplicate Router ID Neighbor Behaviors
Question 35Question →

Two Cisco routers, R1 and R2, are connected via their GigabitEthernet0/0 interfaces on the 192.168.12.0/24 subnet in OSPF Area 0.

R1 operational status:
- Loopback 0: IP 10.10.10.1/32 (up/up)
- Loopback 1: IP 172.16.1.1/32 (up/down)
- GigabitEthernet0/0: IP 192.168.12.1/24 (up/up, MTU 1500)
- OSPF Process 1 was initialized while only Loopback 0 was active. Later, the command `router-id 1.1.1.1` was added under `router ospf 1`, but `clear ip ospf process` has not been executed.

R2 operational status:
- Loopback 0: IP 10.20.20.1/32 (up/up)
- GigabitEthernet0/0: IP 192.168.12.2/24 (up/up, MTU 1400)
- Hello and Dead timers match R1 on GigabitEthernet0/0.

Which TWO statements accurately describe the OSPF operational state and Router ID behavior between R1 and R2?

Select all that apply

Show answer & explanation

Answer: R1 currently uses 10.10.10.1 as its active OSPF Router ID because the `router-id` command requires clearing the OSPF process to take effect.; R1 and R2 will establish a neighbor relationship up to the 2-Way state but will remain stuck in the ExStart/Exchange state.

Answer

R1 uses 10.10.10.1 as its active OSPF Router ID because the manual `router-id` configuration requires resetting the OSPF process, and the routers will successfully achieve 2-Way neighbor state but become stuck in ExStart/Exchange state due to the MTU mismatch on GigabitEthernet0/0.
The statement regarding R1 using 10.10.10.1 is correct because manual `router-id` commands under `router ospf` are non-preemptive and require an explicit process reset (`clear ip ospf process`) to replace an already active router ID. The statement regarding the routers reaching 2-Way state but stalling in ExStart/Exchange is correct because MTU is not checked in Hello packets (allowing 2-Way neighbor discovery), but MTU mismatches fail verification during Database Description (DBD) packet exchanges in ExStart state.

Step-by-Step Solution

1
Determine active OSPF Router ID for R1.
R1 selected 10.10.10.1 (Loopback 0) during process initialization. Adding `router-id 1.1.1.1` later is non-preemptive and does not take effect until `clear ip ospf process` is executed. Loopback 1 (172.16.1.1) is down and ignored.
OSPF Router ID election logic evaluates active (up/up) interfaces at process startup, and manual CLI configuration changes are non-preemptive.
2
Evaluate Hello packet negotiation parameters between R1 and R2.
Hello and Dead timers match, Area IDs match (Area 0), subnet masks match, and authentication matches. Both routers receive each other's Hello packets and transition through INIT to 2-Way state.
MTU parameters are not included in OSPF Hello packets and do not prevent 2-Way neighbor state formation.
3
Evaluate Database Description (DBD) exchange between R1 and R2.
During ExStart/Exchange state, routers exchange DBD packets containing interface MTU values. Because R1 (MTU 1500) and R2 (MTU 1400) mismatch, neither router accepts the slave/master DBD renegotiation, trapping adjacency progression in ExStart/Exchange.
Cisco IOS enforces strict MTU matching in DBD packets during OSPF ExStart/Exchange states.

Key Concept

OSPF Router ID Non-Preemptive Election and MTU Mismatch Adjacency Impact
Estimated Time:3m 0s
Question 36Question →

Two Cisco routers, R1 and R2, are directly connected via their GigabitEthernet0/0 interfaces and configured to run OSPFv2. During verification, a network engineer notices that the routers fail to form an OSPF neighbor relationship. Which TWO parameters configured on the interfaces or within the OSPF process must match identically in the OSPF Hello packets for the neighbor adjacency to successfully form? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: Area ID; Hello and Dead timer intervals

Answer

The parameters that must match identically for OSPFv2 neighbor adjacency formation are Area ID and Hello and Dead timer intervals.
To successfully establish an OSPFv2 neighbor adjacency, directly connected routers must agree on several Hello packet fields, including identical Area IDs, matching Hello and Dead timers, matching IP subnet/mask, matching authentication settings, and matching stub area flags. Area ID and Hello/Dead timer intervals are mandatory matching parameters.

Step-by-Step Solution

1
Analyze OSPF Hello packet contents required for neighbor validation.
Identify parameters that are verified upon receipt of a Hello packet on an OSPF-enabled interface.
For two OSPF routers to establish a neighbor relationship, key fields in the Hello packet header must match.
2
Evaluate the Area ID requirement.
Confirm that both connected interfaces must share the exact same Area ID.
If the Area ID field in the incoming Hello packet does not match the receiving interface's configured area, the packet is dropped.
3
Evaluate the Hello and Dead timer requirements.
Confirm that Hello intervals and Dead intervals must match exactly.
Mismatched timers prevent the routers from agreeing on keepalive expectations, causing adjacency initialization to fail.
4
Evaluate local vs global parameters (Process ID and Router ID).
Determine that Process ID is local only and Router ID must be unique.
Process ID is assigned locally per device, while Router IDs must never match to avoid duplicate router ID conflicts in the OSPF topology.

Key Concept

OSPFv2 Neighbor Adjacency Requirements
Question 37Question →

Match each OSPFv2 network type on the left to its corresponding DR/BDR election behavior, neighbor adjacency capability, and default Hello/Dead timer characteristics on the right.

Click a left item, then click its matching right item

Items

Broadcast Network Type
Point-to-Point Network Type
Point-to-Multipoint Network Type
Loopback Interface Network Type

Matches

Show answer & explanation

Answer

Broadcast Network Type matches with DR/BDR election and Hello 10s/Dead 40s. Point-to-Point Network Type matches with no DR/BDR election and Hello 10s/Dead 40s. Point-to-Multipoint Network Type matches with no DR/BDR election and Hello 30s/Dead 120s. Loopback Interface Network Type matches with no DR/BDR election, no neighbors, and /32 host route advertisement.
Each OSPF network type is designed for specific underlying link technologies. Broadcast multiaccess networks perform DR/BDR election with 10s/40s timers. Point-to-point bypasses DR election with 10s/40s timers. Point-to-multipoint bypasses DR election with 30s/120s timers. Loopback interfaces advertise /32 host routes without forming adjacencies.

Step-by-Step Solution

1
Analyze Broadcast multiaccess network characteristics.
Broadcast networks (e.g., Ethernet interfaces running OSPF) require DR and BDR elections to avoid N(N−1)/2N(N-1)/2 full mesh adjacencies. The standard Hello timer is 10 seconds, and the Dead timer is 40 seconds.
DR/BDR selection is mandatory on multiaccess media.
2
Analyze Point-to-Point network characteristics.
Point-to-Point networks connect two endpoints directly. DR/BDR election is bypassed because only two routers exist on the segment. Timers default to Hello 10s and Dead 40s.
Electing a DR/BDR on a two-node point-to-point connection adds unnecessary protocol overhead.
3
Analyze Point-to-Multipoint network characteristics.
Point-to-Multipoint treats the physical or logical multiaccess network as multiple point-to-point connections. It disables DR/BDR elections and defaults to longer timers (Hello 30s, Dead 120s).
Used primarily in WAN/hub-and-spoke topologies where direct spoke-to-spoke broadcast reachability is absent.
4
Analyze Loopback network characteristics.
In Cisco IOS OSPFv2, loopback interfaces automatically default to the Loopback network type, advertising the interface IP as a /32 host route regardless of the configured netmask, without participating in neighbor discovery.
Loopback interfaces represent logical local endpoints, not actual transit links.

Key Concept

OSPFv2 Network Types, Adjacencies, Timers, and DR/BDR Requirements
Question 38Question →

A network engineer is configuring router R1 to reach destination network 172.16.10.0/24172.16.10.0/24. The primary path must use next-hop IP address 192.168.12.2192.168.12.2 with standard default static priority. A backup path must use next-hop IP address 10.0.0.210.0.0.2 and function as a floating static route that only enters the routing table if the primary path is unavailable. Which TWO commands must be configured on R1 to meet these requirements? (Select two.)

Select all that apply

Show answer & explanation

Answer: ip route 172.16.10.0 255.255.255.0 192.168.12.2; ip route 172.16.10.0 255.255.255.0 10.0.0.2 10

Answer

The correct commands are 'ip route 172.16.10.0 255.255.255.0 192.168.12.2' for the primary path and 'ip route 172.16.10.0 255.255.255.0 10.0.0.2 10' for the backup floating path.
The command specifying next-hop 192.168.12.2 without an explicit administrative distance correctly creates the primary static route with the default Administrative Distance of 1. The command specifying next-hop 10.0.0.2 with an explicit administrative distance of 10 correctly creates a floating static route, because its higher administrative distance keeps it out of the routing table until the primary route disappears.

Step-by-Step Solution

1
Identify the proper Cisco IOS IPv4 static route command structure.
The correct command format is 'ip route <destination-prefix> <subnet-mask> <next-hop-ip> [administrative-distance]'.
Cisco IOS requires a subnet mask (not a wildcard mask) and optional administrative distance parameter.
2
Configure the primary static route.
'ip route 172.16.10.0 255.255.255.0 192.168.12.2' is generated.
When no administrative distance is specified at the end of the command, Cisco IOS assigns a default Administrative Distance of 1.
3
Configure the backup floating static route.
'ip route 172.16.10.0 255.255.255.0 10.0.0.2 10' is generated.
A floating static route must have an Administrative Distance higher than the primary route (in this case, 10 > 1) so it remains inactive in the RIB until the primary route fails.

Key Concept

IPv4 Static Route Syntax and Administrative Distance for Floating Static Routes
Question 39Question →

A network engineer starts an OSPFv2 routing process on router Branch-GW. Prior to launching the OSPF process, no explicit router-id command is configured. The router has the following interfaces configured:

- Interface Loopback0: IP address 10.200.1.1/3210.200.1.1/32, Operational Status: Administratively Down
- Interface Loopback10: IP address 172.16.50.1/24172.16.50.1/24, Operational Status: Up/Up
- Interface Loopback20: IP address 172.16.20.1/24172.16.20.1/24, Operational Status: Up/Up
- Interface GigabitEthernet0/0: IP address 192.168.10.1/24192.168.10.1/24, Operational Status: Up/Up

What IPv4 address will OSPFv2 select as the Router ID for Branch-GW?

Show answer & explanation

Answer: 172.16.50.1

Answer

172.16.50.1
In Cisco OSPFv2, the Router ID selection order follows three rules: 1) Explicitly configured `router-id` command, 2) Highest IPv4 address among active (Up/Up) loopback interfaces, 3) Highest IPv4 address among active (Up/Up) physical interfaces. Since no manual Router ID is configured, OSPF evaluates loopback interfaces. Loopback0 is administratively down, so it is ineligible. Between the active loopbacks (Loopback10 at 172.16.50.1 and Loopback20 at 172.16.20.1), 172.16.50.1 is the highest IP address, making it the selected Router ID.

Step-by-Step Solution

1
Check for a manually configured OSPF Router ID
No explicit OSPF router-id command was entered under the routing process.
Explicit configuration always takes top precedence in OSPF Router ID election.
2
Evaluate active (Up/Up) Loopback interfaces
Loopback0 (10.200.1.110.200.1.1) is administratively down and ignored. Active loopbacks are Loopback10 (172.16.50.1172.16.50.1) and Loopback20 (172.16.20.1172.16.20.1).
Only active interfaces with operational status Up/Up are eligible for Router ID selection.
3
Select the highest IPv4 address among eligible active loopback interfaces
Comparing 172.16.50.1172.16.50.1 and 172.16.20.1172.16.20.1, the address 172.16.50.1172.16.50.1 is numerically higher.
OSPF tie-breaking logic favors the highest IPv4 address among active loopback interfaces over physical interface IP addresses.

Key Concept

OSPFv2 Router ID Election Order and Interface State Requirements
Question 40Question →

A network engineer is configuring OSPFv2 on router R1, which has the following interface states and IPv4 configurations:

- Loopback0: 192.168.254.1/32 (administratively down, line protocol down)
- Loopback1: 172.16.50.1/24 (up, line protocol up)
- Loopback2: 10.255.255.1/24 (up, line protocol up)
- GigabitEthernet0/0: 10.1.1.1/30 (up, line protocol up)
- GigabitEthernet0/1: 192.168.1.1/30 (up, line protocol up)

The engineer starts the OSPF process using `router ospf 1`. Immediately after, the engineer configures `router-id 1.1.1.1` under the OSPF process prompt but does NOT issue a `clear ip ospf process` command. Router R1 then attempts to establish an OSPFv2 adjacency over GigabitEthernet0/0 (Area 0) with directly connected router R2. Router R2 already has a manually configured static OSPF Router ID of 172.16.50.1172.16.50.1.

Which outcome correctly describes the OSPF neighbor state and behavior between R1 and R2?

Show answer & explanation

Answer: The routers fail to establish an OSPF adjacency because both routers are using the duplicate Router ID 172.16.50.1.

Answer

The routers fail to establish an OSPF adjacency because both routers are using the duplicate Router ID 172.16.50.1.
When the OSPF process initializes on R1, it evaluates active interfaces. Loopback0 is down, so it is ignored. Loopback1 (172.16.50.1172.16.50.1) is chosen over Loopback2 (10.255.255.110.255.255.1) because it has the higher IPv4 address. Although the administrator configures `router-id 1.1.1.1`, OSPF Router ID changes are non-preemptive and require the process to be cleared (`clear ip ospf process`) before taking effect. Consequently, R1 maintains its elected Router ID of 172.16.50.1172.16.50.1, which matches R2's Router ID. OSPF neighbors cannot form an adjacency with duplicate Router IDs on the same network segment.

Step-by-Step Solution

1
Evaluate initial dynamic OSPF Router ID election on R1
Loopback0 is down and disqualified. Among active loopbacks (Loopback1: 172.16.50.1 and Loopback2: 10.255.255.1), 172.16.50.1 is numerically highest and becomes R1's active Router ID.
OSPF Router ID selection order is: manual 'router-id' command > highest IP on active loopbacks > highest IP on active physical interfaces.
2
Evaluate the effect of the manual 'router-id 1.1.1.1' command
R1 retains 172.16.50.1 as its active Router ID because the process was not cleared.
In Cisco IOS OSPFv2, manually modifying the Router ID on an existing process is non-preemptive until 'clear ip ospf process' is executed.
3
Analyze neighbor adjacency formation with R2
Both R1 and R2 present a Router ID of 172.16.50.1 in Hello packets, leading to duplicate Router ID conflict and adjacency failure.
OSPF routers on the same segment or area require unique Router IDs to establish neighbor relationships.

Key Concept

OSPFv2 Router ID Selection Precedence and Non-Preemptive CLI Behavior
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