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IP Connectivity

493 questions

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Question 361Question →

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

text
Codes: C - connected, S - static, D - EIGRP, O - OSPF
Gateway of last resort is 172.16.1.1 to network 0.0.0.0

D 10.140.0.0/16 [90/2172416] via 10.1.1.1, 00:04:12, GigabitEthernet0/0
O 10.140.32.0/19 [110/20] via 10.2.2.2, 00:12:35, GigabitEthernet0/1
S 10.140.32.0/24 [1/0] via 10.3.3.3
C 10.140.32.128/25 is directly connected, GigabitEthernet0/2

The router receives an IP packet addressed to 10.140.32.13510.140.32.135. Which path or next hop will the router select to forward this traffic?

Show answer & explanation

Answer: Interface GigabitEthernet0/2

Answer

The router will forward the packet out interface GigabitEthernet0/2 because the route 10.140.32.128/25 has the longest matching prefix for destination host 10.140.32.135.
When an IPv4 router receives a packet, it compares the destination IPv4 address against all routes in its routing table. When multiple routes match the destination host, the router always selects the route with the longest prefix length (the most specific subnet mask). Destination 10.140.32.13510.140.32.135 falls within 10.140.32.128/2510.140.32.128/25 (usable range 10.140.32.12910.140.32.129 to 10.140.32.25410.140.32.254). Since /25/25 is more specific than /24/24, /19/19, and /16/16, the router forwards traffic out GigabitEthernet0/2.

Step-by-Step Solution

1
Identify all matching routes for the destination address 10.140.32.135.
Matches found: 10.140.0.0/16, 10.140.32.0/19, 10.140.32.0/24, and 10.140.32.128/25.
The destination address falls within the address range of all four listed subnets.
2
Compare prefix lengths among the candidate routes.
Prefix lengths are /16, /19, /24, and /25.
Router forwarding logic relies on the Longest Prefix Match (LPM) rule prior to evaluating Administrative Distance or Metric.
3
Select the most specific route (/25 prefix).
The route 10.140.32.128/25 connected to GigabitEthernet0/2 is chosen.
A /25 subnet mask matches 25 continuous network bits, which is more specific than /24, /19, or /16.

Key Concept

Longest Prefix Match (LPM) Precedence
Question 362Question →

A network administrator views the OSPFv2 status of interface GigabitEthernet0/0 on router R1:

text
R1# show ip ospf interface gigabitEthernet 0/0
GigabitEthernet0/0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 1
State DROTHER, Priority 0
Designated Router (ID) 3.3.3.3, Interface address 10.10.10.3
Backup Designated router (ID) 2.2.2.2, Interface address 10.10.10.2

If the current Designated Router (Router ID 3.3.3.3) unexpectedly reboots and fails on this multiaccess segment, which behavior will R1 exhibit during the subsequent election process?

Show answer & explanation

Answer: R1 will remain in the DROTHER state and will not participate in the election for either the DR or BDR role.

Answer

R1 will remain in the DROTHER state and will not participate in the election for either the DR or BDR role.
In OSPFv2 multiaccess networks, setting `ip ospf priority 0` on an interface makes that interface completely ineligible to participate in Designated Router (DR) or Backup Designated Router (BDR) elections. When the primary DR fails, the existing BDR (Router ID 2.2.2.2) promotes to DR, and a new BDR is elected only among other eligible routers with priority values of 1 to 255. Therefore, the router displaying priority 0 will remain in the DROTHER state throughout the election process.

Step-by-Step Solution

1
Analyze the CLI output for interface priority.
Interface GigabitEthernet0/0 has 'Priority 0'.
In OSPFv2, the interface priority parameter dictates DR/BDR election eligibility.
2
Apply OSPF DR/BDR election rules for priority 0.
A router interface with an OSPF priority value of 0 is ineligible to become either DR or BDR.
Priority 0 is a explicitly defined mechanism to prevent a router interface from ever becoming a Designated Router or Backup Designated Router on broadcast or non-broadcast multiaccess networks.
3
Determine the state transition when DR fails.
The current BDR (Router ID 2.2.2.2) promotes to DR, and a new BDR election occurs among eligible routers (priority > 0). R1 remains DROTHER.
Because R1 is ineligible, it cannot be elected as BDR or DR during the promotion or re-election phase.

Key Concept

OSPF DR/BDR Election Ineligibility with Priority 0
Question 363Question →

A Cisco router receives an IPv4 packet destined for host 192.168.50.45192.168.50.45. The router's routing table contains a static route `ip route 192.168.50.0 255.255.255.0 10.1.1.2` and a directly connected network 10.1.1.0/3010.1.1.0/30 out interface `GigabitEthernet0/1`. Arrange the sequential processing steps executed by the router to forward this packet from initial ingress lookup to egress transmission.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct sequence is: Extract the destination IP address -> Perform primary routing lookup -> Perform recursive lookup for the next-hop IP -> Query ARP table for next-hop MAC -> Encapsulate packet and transmit out egress interface.
When a Cisco router receives an IP packet, it first extracts the destination IP address. Using longest prefix match, it matches the static route entry pointing to next-hop IP 10.1.1.2. Because this route specifies an IP next-hop rather than an outbound interface directly, the router performs a recursive lookup to find which connected interface reaches 10.1.1.2 (GigabitEthernet0/1). Knowing the egress Ethernet interface, it resolves the next-hop MAC address using ARP, encapsulates the IPv4 packet in an Ethernet frame, and transmits it out GigabitEthernet0/1.

Step-by-Step Solution

1
Inspect destination header
Destination IPv4 address 192.168.50.45192.168.50.45 is extracted from the incoming frame.
Routing decisions are based on the packet's destination IPv4 address.
2
Execute primary route table lookup
Matches route `192.168.50.0 255.255.255.0` with next-hop IP 10.1.1.210.1.1.2.
The router uses longest prefix matching to select the route entry.
3
Execute recursive lookup
Resolves 10.1.1.210.1.1.2 to directly connected route 10.1.1.0/3010.1.1.0/30 on interface `GigabitEthernet0/1`.
Static routes configured solely with a next-hop IP address require a recursive lookup to identify the exit interface.
4
Resolve Layer 2 next-hop address
Obtains the target MAC address for 10.1.1.210.1.1.2 from the ARP table.
Ethernet encapsulation requires a destination Layer 2 MAC address for the next hop.
5
Encapsulate and egress
Frame is sent out `GigabitEthernet0/1`.
Final physical delivery step out the egress interface.

Key Concept

IPv4 Static Route Recursive Lookup & Packet Forwarding Pipeline
Estimated Time:2m 0s
Question 364Question →

A network administrator is troubleshooting an OSPFv2 adjacency issue between two routers, R1 and R2, connected via their GigabitEthernet0/0/0 interfaces. The administrator notices that the OSPF neighbor state on R1 remains persistently stuck in the EXSTART/EXCHANGE state. Interface GigabitEthernet0/0/0 on R1 has an IP MTU of 1500 bytes, while GigabitEthernet0/0/0 on R2 has been configured with an IP MTU of 1450 bytes. If modifying the interface MTU on R2 is prohibited by organizational policy, which command configured on R1 will allow the OSPF neighbor adjacency to proceed successfully to the FULL state?

Show answer & explanation

Answer: Configure ip ospf mtu-ignore under interface GigabitEthernet0/0/0 on R1.

Answer

Configure ip ospf mtu-ignore under interface GigabitEthernet0/0/0 on R1.
During the EXSTART state, OSPF routers exchange Database Descriptor (DBD) packets to agree on initial sequence numbers and master/slave roles. The DBD packet includes the interface MTU. If neighboring routers have mismatched MTU values, DBD packet processing fails and the neighbor relationship remains stuck in EXSTART/EXCHANGE. Configuring 'ip ospf mtu-ignore' in interface configuration mode forces the router to ignore MTU checks in received DBD packets, allowing the adjacency to transition to FULL.

Step-by-Step Solution

1
Identify the cause of the EXSTART/EXCHANGE neighbor state stagnation.
During the EXSTART and EXCHANGE states, OSPF neighbors elect a Master/Slave relationship and exchange Database Descriptor (DBD) packets containing interface MTU values.
If the receiving interface detects an MTU larger than its own or mismatched MTU parameters, the DBD packet processing fails and the neighbor relationship halts at EXSTART/EXCHANGE.
2
Determine the appropriate configuration workaround when interface MTU modification is restricted.
Cisco IOS provides the 'ip ospf mtu-ignore' interface configuration command.
This command disables the MTU consistency check on incoming OSPF DBD packets for that specific interface, permitting adjacency completion.

Key Concept

OSPFv2 MTU Mismatch Behavior and ip ospf mtu-ignore
Estimated Time:2m 0s
Question 365Question →

Match each OSPFv2 network type configuration on the left with its corresponding DR/BDR election requirement and default Hello/Dead timer characteristics on the right.

Click a left item, then click its matching right item

Items

Broadcast (`ip ospf network broadcast`)
Point-to-Point (`ip ospf network point-to-point`)
Point-to-Multipoint (`ip ospf network point-to-multipoint`)
Non-Broadcast (`ip ospf network non-broadcast`)

Matches

Show answer & explanation

Answer

Broadcast pairs with DR/BDR elected automatically with 10s/40s timers. Point-to-Point pairs with no DR/BDR election and 10s/40s timers. Point-to-Multipoint pairs with no DR/BDR election and 30s/120s timers. Non-Broadcast pairs with DR/BDR elected using manual neighbor statements and 30s/120s timers.
Each OSPF network type has distinct rules regarding whether DR/BDR elections occur, whether multicast is used for neighbor discovery, and which default timers apply. LAN-oriented types (Broadcast and Point-to-Point) use 10-second Hello and 40-second Dead timers, while WAN-oriented types (Point-to-Multipoint and Non-Broadcast) use 30-second Hello and 120-second Dead timers. Multiaccess types (Broadcast and Non-Broadcast) perform DR/BDR elections, whereas point-to-point styles (Point-to-Point and Point-to-Multipoint) bypass DR/BDR elections entirely.

Step-by-Step Solution

1
Analyze election requirements for multiaccess vs. point-to-point network types.
Broadcast and Non-Broadcast networks require DR/BDR elections because multiple routers share the medium. Point-to-Point and Point-to-Multipoint networks do not elect DR/BDRs.
DR/BDR selection reduces adjacency formation overhead from n(n−1)/2n(n-1)/2 to 2n−32n-3 on multiaccess segments.
2
Analyze neighbor discovery mechanisms (multicast vs. unicast).
Broadcast, Point-to-Point, and Point-to-Multipoint use multicast (224.0.0.5224.0.0.5). Non-Broadcast multiaccess requires manual unicast neighbor definition via the `neighbor` command.
NBMA media do not natively support broadcast or multicast traffic.
3
Evaluate default OSPFv2 Hello and Dead timers for each network type.
LAN types (Broadcast, Point-to-Point) default to 10s Hello / 40s Dead. WAN types (Point-to-Multipoint, Non-Broadcast) default to 30s Hello / 120s Dead.
Slower WAN links require higher timer tolerances to avoid unnecessary neighbor flap state transitions.

Key Concept

OSPFv2 Network Types, DR/BDR Election Requirements, and Default Timers
Estimated Time:2m 0s
Question 366Question →

Match each IPv4 routing table entry from a Cisco IOS 'show ip route' display to its correct protocol source classification and administrative properties.

Click a left item, then click its matching right item

Items

D 192.168.10.0/24 [90/2170112] via 10.1.1.2
O 172.16.1.0/24 [110/20] via 10.1.1.6
S 10.2.2.0/24 [1/0] via 10.1.1.2
C 10.1.1.0/30 is directly connected, GigabitEthernet0/0

Matches

Show answer & explanation

Answer

Each routing table entry matches its corresponding protocol code, administrative distance, and metric: 'D' maps to Internal EIGRP (AD 90), 'O' maps to OSPF (AD 110), 'S' maps to Static (AD 1), and 'C' maps to Directly Connected (AD 0).
Routing table entries display their learning source code followed by the prefix, bracketed administrative distance and metric values [AD/Metric], next-hop IP, and outgoing interface. Code D identifies EIGRP (default AD 90), code O identifies OSPF (default AD 110), code S identifies Static routes (default AD 1), and code C identifies Connected networks (AD 0).

Step-by-Step Solution

1
Identify the protocol code prefix at the start of each routing entry.
D corresponds to EIGRP, O corresponds to OSPF, S corresponds to Static, and C corresponds to Connected.
Cisco IOS routing tables use single-letter prefix codes to identify how each route was learned.
2
Parse the bracketed values [Administrative Distance / Metric] for dynamically learned and static routes.
The first number inside brackets represents Administrative Distance (AD), and the second number represents the protocol metric.
AD measures route source trustworthiness while metric determines path preference within the protocol.
3
Match each entry to its exact functional description based on AD and metric values.
Entry with code D matches EIGRP (AD 90), entry with code O matches OSPF (AD 110), entry with code S matches Static (AD 1), and entry with code C matches Directly Connected (AD 0).
Each routing protocol has standard default administrative distance values recognized by Cisco IOS.

Key Concept

Routing Table Components and Source Codes
Question 367Question →

An enterprise router learns the primary route to network 172.16.50.0/24172.16.50.0/24 via internal BGP (iBGP), which carries a default Administrative Distance of 200200. An administrator wants to configure a floating static backup route pointing to next-hop IP 10.1.1.210.1.1.2. Complete the Cisco IOS command below by entering the minimum administrative distance value required to ensure this route functions strictly as a backup path.

Fill in the blanks below

Router(config)# ip route 172.16.50.0 255.255.255.0 10.1.1.2
Show answer & explanation

Answer

201
A floating static route must be configured with an Administrative Distance (AD) strictly greater than that of the primary route. Since iBGP has a default AD of 200, the static route requires an AD of at least 201 to remain inactive until the primary iBGP route fails.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary routing protocol.
The primary path is learned via iBGP, which has a default Administrative Distance of 200200.
Cisco IOS prefers routing information sources with lower Administrative Distance values.
2
Determine the required AD relationship for a floating static backup route.
The static route's administrative distance must be higher than the primary route's AD (>200> 200).
If the static route has an AD lower than or equal to 200200, Cisco IOS will install the static route into the routing table instead of holding it in reserve as a backup.
3
Calculate the minimum integer AD value for the command.
The smallest integer greater than 200200 is 201201.
Specifying 201201 at the end of the `ip route` command sets the static route's AD to 201, keeping it inactive until the iBGP route is removed.

Key Concept

Floating Static Routes and Administrative Distance
Estimated Time:1m 0s
Question 368Question →

Complete the Cisco IOS command required to configure a floating static backup route.

Fill in the blanks below

An edge router receives its primary path to destination subnet 172.20.10.0/24172.20.10.0/24 via internal EIGRP, which carries a default Administrative Distance of 9090. A network engineer needs to configure a floating static backup route toward the same destination using next-hop IP 10.1.1.210.1.1.2 with an Administrative Distance of 100100.

What value should be entered to complete the Cisco IOS command below?

`ip route 172.20.10.0 255.255.255.0 10.1.1.2`
Show answer & explanation

Answer

The missing parameter to complete the command is 100.
Appending the integer 100 to the end of the `ip route` syntax sets the static route's Administrative Distance to 100. Because 100 is higher than internal EIGRP's default AD of 90, the router will keep the static route inactive (floating) in the routing table until the primary EIGRP path becomes unavailable.

Step-by-Step Solution

1
Analyze the Cisco IOS static route command structure.
The command syntax is `ip route <prefix> <mask> <next-hop> [distance]`.
The optional trailing integer at the end of the command specifies the Administrative Distance (AD) for the route.
2
Determine the required Administrative Distance value for the floating static route.
The problem specifies an Administrative Distance of 100 for the backup route.
Setting the AD to 100 ensures the static route remains higher (less preferred) than the primary internal EIGRP route (AD 90), causing it to float outside the routing table until the primary path fails.

Key Concept

Floating Static Route Configuration and Administrative Distance
Estimated Time:1m 0s
Question 369Question →

A network administrator executes the `show ip ospf interface GigabitEthernet0/1` command on router R1 connected to an enterprise Ethernet segment, yielding the following operational output:

text
GigabitEthernet0/1 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 10.10.10.1, Network Type BROADCAST, Cost: 1
State DR, Priority 1
Designated Router (ID) 10.10.10.1, Interface address 10.10.10.1
Backup Designated Router (ID) 10.10.10.2, Interface address 10.10.10.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

If the administrator modifies the OSPF interface network type on all routers attached to this segment by executing the `ip ospf network point-to-point` command, which two operational changes will take effect on this interface? (Choose two.)

Select all that apply

Show answer & explanation

Answer: OSPF will completely bypass the election of a Designated Router (DR) and Backup Designated Router (BDR) on the interface.; Neighbors will continue to dynamically form adjacencies using 224.0.0.5 while maintaining the default 10-second Hello and 40-second Dead timers.

Answer

Reconfiguring the Ethernet interface to an OSPF point-to-point network type eliminates DR/BDR elections entirely while retaining dynamic multicast neighbor discovery (224.0.0.5) with 10-second Hello and 40-second Dead timers.
Changing an OSPF interface from broadcast to point-to-point disables DR and BDR elections because the link is assumed to connect exactly two routers. Additionally, point-to-point network types retain dynamic neighbor discovery via multicast (224.0.0.5) using standard 10-second Hello and 40-second Dead timers.

Step-by-Step Solution

1
Analyze the default OSPF operational traits for the BROADCAST network type.
The BROADCAST network type elects a DR and BDR and uses default timers of 10s Hello and 40s Dead with multicast address 224.0.0.5.
Ethernet interfaces default to BROADCAST network type in Cisco IOS.
2
Evaluate the impact of changing the interface network type to POINT_TO_POINT.
POINT_TO_POINT network types do not elect DR or BDR roles because only two nodes reside on the segment.
DR/BDR elections exist solely to reduce adjacency counts on multiaccess media.
3
Determine timer and neighbor discovery behaviors for POINT_TO_POINT mode.
POINT_TO_POINT retains dynamic neighbor discovery using multicast (224.0.0.5) and keeps the 10s Hello / 40s Dead timer values.
Timers only change to 30s/120s on non-broadcast or point-to-multipoint OSPF network types.

Key Concept

OSPFv2 Network Types (Broadcast vs. Point-to-Point) and DR/BDR Election Rules
Question 370Question →

An network engineer changes an Ethernet interface configuration to use the OSPFv2 Point-to-Point network type (`ip ospf network point-to-point`). Which two statements correctly describe the default operational characteristics of this OSPFv2 network type? (Select two.)

Select all that apply

Show answer & explanation

Answer: OSPF suppresses Designated Router (DR) and Backup Designated Router (BDR) elections on the interface.; OSPF neighbors are dynamically discovered using multicast Hello packets sent to 224.0.0.5.

Answer

The OSPFv2 Point-to-Point network type automatically suppresses DR and BDR elections on the interface, and dynamically discovers adjacent neighbors using OSPF multicast messages sent to the AllSPFRouters address (224.0.0.5).
The OSPF Point-to-Point network type is designed specifically for two-router links. As a result, OSPF disables DR and BDR elections entirely, and neighbors dynamically discover each other via multicast sent to the AllSPFRouters address (224.0.0.5).

Step-by-Step Solution

1
Analyze DR/BDR requirement for Point-to-Point network type
DR and BDR elections are unnecessary because only two endpoints exist on a point-to-point link.
Suppressing DR/BDR election reduces protocol overhead and simplifies adjacency establishment.
2
Determine neighbor discovery mechanism and timers
Neighbors are discovered automatically using 224.0.0.5 multicast with standard 10-second Hello and 40-second Dead timers.
Multicast capabilities are supported natively on Point-to-Point logical and physical interfaces.

Key Concept

OSPFv2 Point-to-Point Network Type Behavior
Estimated Time:1m 0s
Question 371Question →

Four Cisco routers (Alpha, Beta, Gamma, and Delta) are connected to the same Ethernet broadcast domain and simultaneously boot and establish OSPFv2 adjacencies. The routers are configured with the following OSPF interface priorities and Router IDs:

- Router Alpha: Priority = 2, Router ID = 1.1.1.1
- Router Beta: Priority = 2, Router ID = 2.2.2.2
- Router Gamma: Priority = 1, Router ID = 10.10.10.10
- Router Delta: Priority = 0, Router ID = 192.168.1.1

Arrange the routers in order of their resulting OSPF roles on the shared segment, starting from the Designated Router (DR) at the top, followed by the Backup Designated Router (BDR), the eligible DROTHER router, and ending with the router completely excluded from election eligibility.

Drag items to arrange them in the correct order

Show answer & explanation

Answer

The correct order of routers from DR to election-ineligible is: Router Beta (DR), Router Alpha (BDR), Router Gamma (DROTHER), and Router Delta (Ineligible).
In OSPFv2 multiaccess networks, DR and BDR election precedence is determined first by highest interface priority. Router Alpha and Router Beta tie with the highest priority (2), so their Router IDs break the tie: Router Beta (2.2.2.2) becomes DR and Router Alpha (1.1.1.1) becomes BDR. Router Gamma has a lower priority (1) and becomes an eligible DROTHER. Router Delta has a priority of 0, which makes it ineligible to participate in DR/BDR selection regardless of its high Router ID.

Step-by-Step Solution

1
Evaluate interface priority values for all routers.
Routers Alpha and Beta have priority 2, Router Gamma has priority 1, and Router Delta has priority 0.
OSPFv2 uses interface priority as the primary criterion for DR/BDR election.
2
Identify election eligibility.
Router Delta (priority 0) is marked ineligible for DR/BDR roles and placed at the bottom of election hierarchy.
An interface priority of 0 explicitly prevents a router from becoming a DR or BDR.
3
Determine DR and BDR roles among highest priority routers using Router ID as a tie-breaker.
Router Beta (2.2.2.2>1.1.1.12.2.2.2 > 1.1.1.1) is elected DR, and Router Alpha becomes BDR.
When priority values are equal, the router with the highest Router ID wins the DR election, and the next highest becomes BDR.
4
Place remaining eligible routers into DROTHER status.
Router Gamma (priority 1) assumes the DROTHER role.
Eligible routers with non-zero priority that do not win DR or BDR election remain DROTHERs.

Key Concept

OSPFv2 DR/BDR election precedence prioritizes non-zero interface priority first (highest wins; 0 is ineligible), followed by highest Router ID as a tie-breaker.
Question 372Question →

An engineer inspects the following OSPFv2 neighbor table on router R1, which is connected to a shared Layer 2 Ethernet segment:

text
R1# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 FULL/DROTHER 00:00:34 192.168.1.2 GigabitEthernet0/0
3.3.3.3 2 FULL/BDR 00:00:38 192.168.1.3 GigabitEthernet0/0
4.4.4.4 255 FULL/DR 00:00:31 192.168.1.4 GigabitEthernet0/0

Router R1's GigabitEthernet0/0 interface is configured with an OSPF priority of 10 and IP address 192.168.1.1. If the current Designated Router (R4) unexpectedly reboots, which router will be selected as the new Backup Designated Router (BDR) after OSPF reconverges?

Show answer & explanation

Answer: R1

Answer

Router R1 will become the new Backup Designated Router (BDR).
When the Designated Router (R4) reboots, OSPF rules mandate that the existing Backup Designated Router (R3) is immediately promoted to Designated Router (DR). A new election is then conducted to fill the vacant BDR position. Router R2 has an OSPF interface priority of 0, which completely excludes it from DR/BDR eligibility. Consequently, R1—configured with a priority of 10—is the highest-priority eligible candidate and is elected as the new BDR. Furthermore, when R4 completes its reboot, it will not preempt R3 or R1 because OSPF DR/BDR elections are non-preemptive.

Step-by-Step Solution

1
Determine the immediate role transition following DR failure.
Router R4 (the current DR) fails. In accordance with OSPF non-preemptive election mechanics, the active BDR (R3 with Router ID 3.3.3.3) is automatically promoted to DR.
OSPF ensures network stability on multi-access segments by promoting the current BDR to DR immediately when the DR goes offline.
2
Identify candidate routers eligible for the newly opened BDR role.
Active candidate routers remaining on the segment are R1 (Priority 10, RID 1.1.1.1) and R2 (Priority 0, RID 2.2.2.2).
R4 is offline rebooting, and R3 has already transitioned to DR.
3
Evaluate candidate eligibility and OSPF priority values.
R2 has an OSPF priority of 0, making it entirely ineligible for DR/BDR roles regardless of its Router ID. R1 has a valid OSPF priority of 10.
An OSPF interface priority of 0 prevents a router from ever participating in DR or BDR elections.
4
Conclude the BDR election result.
R1 is elected as the new BDR.
R1 is the sole eligible candidate remaining on the multi-access segment with an OSPF priority greater than 0.

Key Concept

OSPF DR/BDR election precedence, priority 0 ineligibility, and non-preemptive role promotion mechanics.
Estimated Time:2m 0s
Question 373Question →

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

text
Gateway of last resort is 10.0.0.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 10.0.0.1
O 10.50.16.0/20 [110/20] via 10.1.1.2, GigabitEthernet0/0
O 10.50.16.32/27 [110/30] via 10.3.3.2, GigabitEthernet0/2
D 10.50.16.32/28 [90/307200] via 10.2.2.2, GigabitEthernet0/1

The router receives two separate IPv4 unicast packets with destination IP addresses 10.50.16.45 and 10.50.16.20. Which two statements accurately describe how the router forwards these packets?

Select all that apply

Show answer & explanation

Answer: The packet destined for 10.50.16.45 is egressed out GigabitEthernet0/1 because the 10.50.16.32/28 entry represents the longest prefix match.; The packet destined for 10.50.16.20 is egressed out GigabitEthernet0/0 because 10.50.16.0/20 is the most specific matching prefix for that destination address.

Answer

The router forwards the packet destined for 10.50.16.45 out interface GigabitEthernet0/1 using the 10.50.16.32/28 route, and forwards the packet destined for 10.50.16.20 out interface GigabitEthernet0/0 using the 10.50.16.0/20 route.
When a router receives a packet, it compares the packet's destination IP address against all routing table entries and selects the route with the most specific subnet mask (the longest prefix length). For the destination host 10.50.16.45, the route 10.50.16.32/28 has a 28-bit mask, which is more specific than /27, /20, or /0, so it is egressed out interface GigabitEthernet0/1. For the destination host 10.50.16.20, it falls within the subnet range 10.50.16.0 to 10.50.31.255 associated with the 10.50.16.0/20 prefix. Since host address .20 does not reach the starting boundary .32 of the /27 or /28 subnets, 10.50.16.0/20 is its most specific matching route, and the packet is egressed out interface GigabitEthernet0/0.

Step-by-Step Solution

1
Analyze matching prefix entries for destination IP address 10.50.16.45.
Destination 10.50.16.45 matches 0.0.0.0/0 (/0), 10.50.16.0/20 (/20), 10.50.16.32/27 (/27: 10.50.16.32 - 10.50.16.63), and 10.50.16.32/28 (/28: 10.50.16.32 - 10.50.16.47).
Routers compare the destination host IP against all entries in the IP routing table to find candidate matching routes.
2
Apply Longest Prefix Match (LPM) logic for destination IP address 10.50.16.45.
The prefix 10.50.16.32/28 has the longest prefix length (/28 > /27 > /20 > /0). Traffic is forwarded via 10.2.2.2 out interface GigabitEthernet0/1.
The router always selects the matching route with the highest number of network bits (most specific mask), regardless of administrative distance or metric.
3
Analyze matching prefix entries for destination IP address 10.50.16.20.
Destination 10.50.16.20 matches 0.0.0.0/0 (/0) and 10.50.16.0/20 (/20: 10.50.16.0 - 10.50.31.255). It does NOT match 10.50.16.32/27 or 10.50.16.32/28 because host value 20 is lower than subnet base address 32.
Subnet ranges must be accurately calculated to determine valid match candidacy.
4
Apply Longest Prefix Match logic for destination IP address 10.50.16.20.
The most specific matching prefix is 10.50.16.0/20 (/20 > /0). Traffic is forwarded via 10.1.1.2 out interface GigabitEthernet0/0.
Between /20 and /0, /20 is the longer prefix match.

Key Concept

Longest Prefix Match (LPM) Decision Logic
Question 374Question →

A network administrator configures the command `ip ospf priority 0` on a router's Ethernet interface connected to a broadcast multiaccess network. Which behavior will this router exhibit during OSPF DR and BDR elections on this segment?

Show answer & explanation

Answer: The router is completely excluded from participating in DR and BDR elections on the interface.

Answer

The router is completely excluded from participating in DR and BDR elections on the interface.
In OSPFv2 broadcast and non-broadcast multiaccess networks, setting the interface priority to 0 prevents the router from participating in the DR/BDR election. The router will always remain a DROther on that segment.

Step-by-Step Solution

1
Analyze the interface configuration command
The command `ip ospf priority 0` sets the OSPF router priority for the specific interface to zero.
In OSPFv2, interface priority determines candidate eligibility for DR/BDR election on multiaccess networks.
2
Evaluate the effect of a priority value of 0
A priority value of 0 explicitly disqualifies the interface from becoming either the Designated Router (DR) or Backup Designated Router (BDR).
Routers with priority 0 remain in the DROther state regardless of their Router ID, loopback addresses, or physical IP addresses.

Key Concept

OSPF Interface Priority 0 Election Disqualification
Estimated Time:45s
Question 375Question →

A network engineer is administering an active OSPFv2 routing process on router R1. The router currently has the following interface IP addresses and operational statuses:

- Loopback 0: 192.168.10.1/24192.168.10.1/24 (Status: Up, Line Protocol: Down)
- Loopback 1: 172.16.1.1/24172.16.1.1/24 (Status: Up, Line Protocol: Up)
- GigabitEthernet0/0: 10.0.0.1/3010.0.0.1/30 (Status: Up, Line Protocol: Up)
- GigabitEthernet0/1: 10.0.0.5/3010.0.0.5/30 (Status: Up, Line Protocol: Up)

While OSPFv2 process 1 is actively running with active neighbor adjacencies, the engineer executes the command `router-id 1.1.1.1` under the OSPF router configuration mode. Which TWO statements accurately describe the resulting OSPF Router ID behavior and operational state on R1? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: The currently active OSPF Router ID will remain unchanged until the OSPF process is explicitly reset or the router is rebooted.; Once the OSPF process is cleared, the statically configured value 1.1.1.1 will take precedence over all loopback and physical interface IP addresses.

Answer

The active OSPF Router ID will remain unchanged until the process is reset or rebooted, and once cleared, the manually configured router-id command takes precedence over all interface IP addresses.
In OSPFv2, explicit manual configuration using the `router-id` command takes top priority over all loopback and physical interfaces once applied. However, OSPF Router ID evaluation is strictly non-preemptive on an active process. Changing the Router ID while neighbors are active will store the configured value in running-config without changing the operational Router ID until the OSPF process is cleared via `clear ip ospf process` or the device reboots.

Step-by-Step Solution

1
Evaluate OSPF Router ID election precedence order
Order of precedence is: 1) Explicit `router-id` command, 2) Highest IP address among active (Up/Up) loopback interfaces, 3) Highest IP address among active (Up/Up) physical interfaces.
OSPF criteria mandate explicit manual configuration over dynamic interface evaluation.
2
Analyze non-preemptive behavior of an active OSPF process
When a new Router ID is configured on an already active OSPF process with existing adjacencies, OSPF does not dynamically tear down adjacencies to change the ID immediately.
Preventing unnecessary network instability and SPF re-computations requires manual invocation of `clear ip ospf process` to force re-election.
3
Verify interface operational readiness for default election
Loopback 0 (192.168.10.1192.168.10.1) has Line Protocol Down, making it ineligible. Loopback 1 (172.16.1.1172.16.1.1) would be selected if no manual ID existed.
Only interfaces in an operational Up/Up state participate in automatic Router ID selection.

Key Concept

OSPFv2 Router ID Selection Logic & Non-Preemptive Execution
Question 376Question →

A network administrator needs to configure a floating static backup route on a Cisco router to reach the destination subnet 192.168.50.0/24192.168.50.0/24 via the next-hop IP address 10.0.12.210.0.12.2. The primary path to this network is currently dynamically learned via OSPF. Which command correctly configures the floating static route so that it only enters the routing table when the OSPF route fails?

Show answer & explanation

Answer: ip route 192.168.50.0 255.255.255.0 10.0.12.2 120

Answer

The command 'ip route 192.168.50.0 255.255.255.0 10.0.12.2 120' correctly configures the floating static route by setting an administrative distance of 120, which is greater than OSPF's default administrative distance of 110.
The correct command specifies an administrative distance of 120 at the end of the static route definition. Because OSPF has a default administrative distance of 110, Cisco IOS will prefer the OSPF route and keep the static route out of the active routing table (floating) until the OSPF route is removed.

Step-by-Step Solution

1
Identify the Administrative Distance (AD) of the primary routing protocol.
OSPF has a standard default Administrative Distance of 110 on Cisco IOS devices.
Floating static routes rely on AD comparisons to determine route preference.
2
Determine the required Administrative Distance for the floating static route.
The backup static route must have an AD strictly greater than 110 (e.g., 120).
Cisco routers prefer routes with lower AD values; setting a higher AD keeps the route out of the routing table until the primary route fails.
3
Verify Cisco IOS static route syntax.
The syntax 'ip route <prefix> <mask> <next-hop-ip> <distance>' is satisfied by appending 120 at the end of the command.
Appending an integer after the next-hop address specifies a custom Administrative Distance.

Key Concept

Floating Static Route Administrative Distance Calibration
Estimated Time:1m 0s
Question 377Question →

A Cisco router displays the following partial routing table output:

text
Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 4 subnets, 3 masks
O 10.150.0.0/16 [110/20] via 192.168.12.2, 00:14:22, GigabitEthernet0/0
D 10.150.32.0/19 [90/2578560] via 192.168.23.2, 00:05:10, GigabitEthernet0/1
S 10.150.32.0/24 [1/0] via 192.168.34.2, 01:12:00, GigabitEthernet0/2
O IA 10.150.32.128/26 [110/30] via 192.168.45.2, 00:02:45, GigabitEthernet0/3

The router receives a packet destined for 10.150.32.13510.150.32.135. Which interface will the router select to forward this packet?

Show answer & explanation

Answer: GigabitEthernet0/3

Answer

The router forwards the packet out of interface GigabitEthernet0/3 because the route 10.150.32.128/26 provides the longest matching prefix for destination IP 10.150.32.135.
When a router makes a forwarding decision, it evaluates all matching routing table entries and selects the route with the longest prefix mask (the most specific match). The destination IP address 10.150.32.135 falls into the subnet 10.150.32.128/26 (range 10.150.32.128 through 10.150.32.191). Because /26 is the longest subnet mask among all valid candidate routes, the router forwards the packet out of GigabitEthernet0/3.

Step-by-Step Solution

1
Identify candidate routes matching the destination IP address 10.150.32.135
Candidate routes include: 10.150.0.0/16, 10.150.32.0/19, 10.150.32.0/24, and 10.150.32.128/26 (range 10.150.32.128–10.150.32.191). All four candidate routes match the destination IP address.
Before comparing routing metrics or administrative distances, a router first evaluates all installed routes to determine which prefix ranges encompass the destination IP.
2
Apply the Longest Prefix Match (LPM) rule
The prefix 10.150.32.128/26 has a 26-bit mask length, which is longer than /24, /19, and /16.
Routers always prefer the route with the most specific (longest prefix mask) match regardless of administrative distance or metric.
3
Determine the egress interface associated with the longest prefix match route
The entry 10.150.32.128/26 specifies next-hop 192.168.45.2 via interface GigabitEthernet0/3.
The forwarding decision is dictated by the interface bound to the winning longest prefix route.

Key Concept

Longest Prefix Match (LPM) Logic
Question 378Question →

An enterprise network administrator is deploying First Hop Redundancy Protocols across two Cisco Catalyst routers, R1 (primary, physical IP 192.168.10.2/24192.168.10.2/24) and R2 (secondary, physical IP 192.168.10.3/24192.168.10.3/24), servicing subnet 192.168.10.0/24192.168.10.0/24. The administrator is evaluating feature parities and operational behaviors between HSRPv2 and VRRPv3 for dual-stack IPv4/IPv6 resilience.

Which two statements accurately describe the operational differences and configuration mechanics between HSRPv2 and VRRPv3 in this environment? (Choose two.)

Select all that apply

Show answer & explanation

Answer: VRRPv3 permits the virtual IP address to be identical to the physical interface IP address of the Master router (setting its priority automatically to 255), whereas HSRPv2 requires the virtual IP to be a unique IP address distinct from any physical interface IP assigned to routers in the standby group.; VRRPv3 preemption is enabled by default when configuring a VRRP group, whereas HSRPv2 requires explicit configuration of the preemption command for a higher-priority router to claim the Active role upon boot or recovery.

Answer

The correct statements are: (1) VRRPv3 permits using the physical interface IP address as the virtual IP (which automatically forces priority to 255), while HSRPv2 mandates a unique virtual IP; and (2) VRRPv3 has preemption enabled by default, whereas HSRPv2 requires explicit configuration of the preemption command.
VRRPv3 supports IP address ownership where the virtual IP matches the physical interface IP of the master router, automatically setting priority to 255. Additionally, VRRP preemption is enabled by default. In contrast, HSRP requires a unique virtual IP address not assigned to any physical interface, and requires the explicit 'standby preemption' command to allow a higher-priority router to claim the Active role.

Step-by-Step Solution

1
Analyze Virtual IP Address Assignment Rules for HSRP and VRRP
VRRP allows the IP Address Owner to share its physical interface IP with the virtual IP address, giving it an unalterable priority of 255. HSRP requires that the virtual IP address be unassigned to any physical interface in the local group to prevent ARP and routing loop issues.
Understanding protocol specification RFCs (RFC 5798 for VRRPv3 vs Cisco proprietary HSRPv2 specs) is essential for proper IP addressing design.
2
Analyze Default Preemption Behavior
VRRP router processes preempt higher-priority backup nodes by default unless explicitly disabled using 'no vrrp preempt'. HSRP router processes do not preempt active nodes by default, requiring 'standby <group> preempt' to be configured on primary nodes.
Failing to configure preemption on an HSRP primary router will cause sub-optimal traffic flow after a router reboots.
3
Verify Multicast Addresses and Virtual MAC Allocations
HSRPv1 uses 224.0.0.2224.0.0.2 (`0000.0C07.ACxx`), HSRPv2 uses 224.0.0.102224.0.0.102 (`0000.0C9F.Fxxx`), and VRRPv2/v3 uses 224.0.0.18224.0.0.18 (`0000.5E00.01xx`).
Distinguishing between multicast control planes and MAC structures avoids misidentifying frame header captures.

Key Concept

First Hop Redundancy Protocol (FHSRP) Operational Mechanics and Feature Comparison
Question 379Question →

Router R1 and Router R2 are directly connected via their GigabitEthernet0/0 interfaces across an Ethernet link. A network engineer attempts to establish an OSPFv2 neighbor relationship between them in Area 0.

The interfaces are configured with the following parameters:
- Router R1 (Gi0/0): IP address 172.16.10.1/24 (Subnet Mask: 255.255.255.0), OSPF Area 0, Hello timer 10s, Dead timer 40s
- Router R2 (Gi0/0): IP address 172.16.10.2/25 (Subnet Mask: 255.255.255.128), OSPF Area 0, Hello timer 10s, Dead timer 40s

Executing `show ip ospf neighbor` on both routers reveals no neighbor entries. Which statement correctly explains why R1 and R2 fail to establish an OSPFv2 neighbor adjacency?

Show answer & explanation

Answer: The subnet masks configured on the interconnecting interfaces do not match.

Answer

The correct explanation is that the subnet masks configured on the interconnecting interfaces do not match.
For OSPFv2 neighbor adjacencies to form over broadcast multiaccess or point-to-point networks, specific fields within the OSPF Hello packet must match across neighboring interfaces. One critical requirement is that the subnet mask of the primary IP address configured on the interface must be identical on both ends. Since R1 is configured with a /24 subnet mask and R2 is configured with a /25 subnet mask, the routers drop received Hello packets and fail to establish an OSPF neighbor relationship.

Step-by-Step Solution

1
Analyze OSPF Hello packet fields required for neighbor adjacency formation on broadcast/multiaccess interfaces.
OSPF Hello packets require matching parameters: Area ID, Subnet Mask, Hello and Dead timers, Authentication password/type, and Stub area flag.
If any of these mandatory fields mismatch, the routers drop received Hello packets and will not transition to the INIT or 2-WAY neighbor state.
2
Compare the interface configurations between R1 and R2.
R1 uses a /24 subnet mask (255.255.255.0) while R2 uses a /25 subnet mask (255.255.255.128).
Because the subnet masks differ, OSPF rejects the Hello packet exchange.
3
Verify secondary configuration items such as process IDs.
Process IDs are local to the router instance and do not affect neighbor adjacency formation.
Process IDs do not appear in Hello packets as adjacency requirements.

Key Concept

OSPFv2 Hello Packet Parameter Matching Requirements
Question 380Question →

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

text
Codes: C - connected, S - static, D - EIGRP, O - OSPF

Gateway of last resort is 192.168.40.1 to network 0.0.0.0

S* 0.0.0.0/0 [1/0] via 192.168.40.1
S 172.20.0.0/16 [1/0] via 192.168.30.1
D 172.20.100.0/24 [90/2170112] via 192.168.20.1
O 172.20.100.64/26 [110/20] via 192.168.10.1

The router receives a packet destined for IP address 172.20.100.85. Which next-hop IP address will the router choose to forward this packet?

Show answer & explanation

Answer: 192.168.10.1

Answer

192.168.10.1
When a router makes a packet forwarding decision, it evaluates matching routes using the Longest Prefix Match (LPM) rule. The destination address 172.20.100.85 matches the route 172.20.100.64/26 because the range of valid hosts for 172.20.100.64/26 spans from 172.20.100.65 to 172.20.100.126. Because /26 is more specific than /24, /16, or /0, the router chooses the next-hop address 192.168.10.1 regardless of Administrative Distance.

Step-by-Step Solution

1
Identify all matching routes in the routing table for the destination IP address 172.20.100.85.
The IP address 172.20.100.85 matches 0.0.0.0/0, 172.20.0.0/16, 172.20.100.0/24, and 172.20.100.64/26 (usable range 172.20.100.65 - 172.20.100.126).
Before comparing routing metrics or administrative distance, the router identifies all routes whose network addresses encompass the destination host IP.
2
Determine the prefix length for each matching route.
Prefix lengths are /0 (Default), /16 (Static), /24 (EIGRP), and /26 (OSPF).
Cisco forwarding logic strictly adheres to the Longest Prefix Match rule above all other criteria.
3
Select the route with the most specific (longest) subnet mask.
172.20.100.64/26 has the longest prefix (/26). The next-hop IP associated with this route is 192.168.10.1.
Administrative Distance is only evaluated when comparing identical prefix lengths learned from different routing protocols, not when comparing routes of different prefix lengths.

Key Concept

Router Forwarding Decision Logic (Longest Prefix Match precedence over Administrative Distance)
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