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43 questions

All types1,987 Multiple choice1,020 Multiple select452 Numeric18 Matching229 Ordering203 Fill in the blank22 Short answer43

Question 1Question →

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 $10.200.1.1/32$ , Operational Status: Administratively Down
- Interface Loopback10: IP address $172.16.50.1/24$ , Operational Status: Up/Up
- Interface Loopback20: IP address $172.16.20.1/24$ , Operational Status: Up/Up
- Interface GigabitEthernet0/0: IP address $192.168.10.1/24$ , 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

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.

Evaluate active (Up/Up) Loopback interfaces

Loopback0 (

10.200.1.1

) is administratively down and ignored. Active loopbacks are Loopback10 (

172.16.50.1

) and Loopback20 (

172.16.20.1

Only active interfaces with operational status Up/Up are eligible for Router ID selection.

Select the highest IPv4 address among eligible active loopback interfaces

Comparing

172.16.50.1

and

172.16.20.1

, the address

172.16.50.1

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 2Question →

A Cisco router interface GigabitEthernet0/1 has a burned-in MAC address of `0050.56a1.b2c3`. The interface is enabled and configured with the Cisco IOS command `ipv6 address fe80::/64 eui-64`. What is the complete compressed IPv6 link-local address assigned to this interface?

Show answer & explanation

Answer: fe80::250:56ff:fea1:b2c3; fe80::0250:56ff:fea1:b2c3; FE80::250:56FF:FEA1:B2C3; FE80::0250:56FF:FEA1:B2C3

Answer

fe80::250:56ff:fea1:b2c3

The standard EUI-64 process converts the 48-bit MAC address `0050.56a1.b2c3` by inserting `FFFE` into the midpoint (`0050.56FF.FEa1.b2c3`) and inverting the 7th bit of the first byte (`00` hex = `00000000` binary -> `00000010` binary = `02` hex). Appending this 64-bit interface identifier `0250:56ff:fea1:b2c3` to the link-local prefix `fe80::/64` yields `fe80::250:56ff:fea1:b2c3` after omitting leading zeros.

Step-by-Step Solution

Split the 48-bit MAC address into two 24-bit halves and insert the 16-bit hex value `FFFE` in the middle.

The MAC `0050.56a1.b2c3` becomes `0050:56FF:FEA1:B2C3`.

EUI-64 standard requires expanding a 48-bit MAC address into a 64-bit interface identifier by inserting `FFFE` between the OUI and the vendor-assigned NIC bytes.

Invert the 7th bit (Universal/Local bit) of the first byte.

First byte `00` (

0000\ 0000_2

) with the 7th bit inverted becomes `02` (

0000\ 0010_2

). The 64-bit Interface Identifier is `0250:56ff:fea1:b2c3`.

The 7th bit of the first byte indicates universal (

0

) vs local (

1

) scope; inverting

0

yields

1

(

02

hex).

Combine the link-local prefix `fe80::/64` with the generated EUI-64 interface identifier and apply standard IPv6 leading-zero compression.

`fe80::250:56ff:fea1:b2c3`

Link-local addresses always begin with `fe80::/64`, followed by the 64-bit interface identifier, suppressing leading zeros in hex quartets.

Key Concept

EUI-64 Link-Local IPv6 Address Calculation

Estimated Time:1m 30s

Question 3Question →

A Cisco router initializes OSPFv2 process 100 with no explicitly configured router ID and no active loopback interfaces. The router has three physical interfaces configured with the following IPv4 addresses and operational states:

- GigabitEthernet0/0: 10.200.15.1/24 (Status: up, Line Protocol: up)
- GigabitEthernet0/1: 172.25.1.254/16 (Status: up, Line Protocol: up)
- GigabitEthernet0/2: 192.168.50.5/30 (Status: up, Line Protocol: down)

Which IPv4 address will OSPFv2 select as the Router ID when the routing process starts?

Show answer & explanation

Answer: 172.25.1.254; 172.25.1.254/16

Answer

172.25.1.254

When OSPF process initializes without an explicit 'router-id' command or any active loopback interfaces, it chooses the highest IPv4 address among operationally active (up/up) physical interfaces. Since GigabitEthernet0/2 is inactive (up/down), only GigabitEthernet0/0 (10.200.15.1) and GigabitEthernet0/1 (172.25.1.254) are eligible. 172.25.1.254 is the highest active address.

Step-by-Step Solution

Evaluate the OSPF Router ID election precedence rules

OSPF selects the router ID using the following priority: 1) Manually configured 'router-id' command, 2) Highest IPv4 address of any active loopback interface, 3) Highest IPv4 address of any active non-loopback physical interface.

Since no manual router ID or loopback interface exists, the selection defaults to active physical interfaces.

Filter interfaces by operational status

GigabitEthernet0/0 (10.200.15.1) and GigabitEthernet0/1 (172.25.1.254) are both up/up (active). GigabitEthernet0/2 (192.168.50.5) is up/down (inactive) and is disqualified from router ID selection.

OSPF only considers interfaces whose line protocol is operationally active (up/up).

Determine the highest active IPv4 address

Compare active addresses 10.200.15.1 and 172.25.1.254. 172.25.1.254 is numerically higher.

Numeric comparison of active physical interface IPv4 addresses yields 172.25.1.254 as the highest value.

Key Concept

OSPFv2 Router ID election logic prioritizes active loopback interfaces over physical interfaces, selecting the highest IPv4 address among operationally active (up/up) interfaces when no manual router ID is specified.

Question 4Question →

A network administrator modifies the OSPFv2 router ID on an operational Cisco router by issuing the command `router-id 10.50.1.1` under `router ospf 1`. A system message informs the administrator that the change will not take effect until the OSPF process is reset. Which privileged EXEC mode command must be executed to immediately apply the new router ID?

Show answer & explanation

Answer: clear ip ospf process; clear ip ospf 1 process; clear ip ospf process 1

Answer

clear ip ospf process

When the `router-id` command is configured under an active OSPF process, Cisco IOS does not immediately change the operational Router ID in order to prevent unexpected disruption of established neighbor adjacencies. Executing the privileged EXEC command `clear ip ospf process` clears all OSPF structures and restarts the process, allowing the new router ID to take effect immediately.

Step-by-Step Solution

Analyze how OSPF handles changes to the Router ID on an active process.

Configuring the `router-id` command dynamically alters the intended configuration, but Cisco IOS holds the current active Router ID to prevent disrupting current neighbor adjacencies.

To apply the new Router ID without rebooting the entire device, the OSPF software process must be manually restarted.

Determine the exact command required in privileged EXEC mode to restart the OSPF process.

The command `clear ip ospf process` resets the OSPF database and neighbor relationships, forcing OSPF to re-initialize with the newly configured Router ID.

Running `clear ip ospf process` forces the OSPF process to tear down adjacencies and rebuild them using the new router ID.

Key Concept

OSPFv2 Router ID Modification and Process Reset

Question 5Question →

A network administrator initializes OSPFv2 process 1 on a Cisco router named Branch-GW. The router does not have an explicit `router-id` command configured under `router ospf 1`. The operational status and IPv4 addresses of the router interfaces are listed below:

- Loopback1: 10.10.10.10/32 (Up / Up)
- Loopback2: 10.200.1.1/32 (Up / Up)
- Loopback3: 172.16.50.1/32 (Up / Up)
- GigabitEthernet0/0: 192.168.100.1/24 (Up / Up)
- GigabitEthernet0/1: 192.168.200.1/24 (Up / Up)

What IPv4 address will OSPFv2 elect as the router ID for Branch-GW?

Show answer & explanation

Answer: 172.16.50.1

Answer

172.16.50.1

OSPFv2 elects the router ID using a strict three-tier precedence hierarchy: 1) Manually configured `router-id` command, 2) Highest IPv4 address among active (Up/Up) loopback interfaces, 3) Highest IPv4 address among active physical interfaces. Because no manual router ID is specified, OSPF evaluates the active loopback interfaces. Among Loopback1 (10.10.10.10), Loopback2 (10.200.1.1), and Loopback3 (172.16.50.1), Loopback3 has the highest IPv4 address (172.16.50.1). Active loopbacks take precedence over physical interfaces, so the higher physical interface addresses (192.168.100.1 and 192.168.200.1) are ignored.

Step-by-Step Solution

Check for an explicitly configured router ID under the OSPF process.

No `router-id` command is configured.

A manually configured router ID has the highest priority in OSPF election logic.

Identify active (Up/Up) loopback interfaces on the router.

Three active loopback interfaces exist: Loopback1 (10.10.10.10), Loopback2 (10.200.1.1), and Loopback3 (172.16.50.1).

Active loopback interfaces take precedence over all physical interfaces regardless of IP address values.

Determine the highest IPv4 address among the active loopback interfaces.

172.16.50.1 is numerically greater than 10.200.1.1 and 10.10.10.10.

When multiple active loopbacks are present, OSPF selects the highest numeric IPv4 address as the router ID.

Key Concept

OSPFv2 Router ID Election Priority Logic

Estimated Time:1m 0s

Question 6Question →

Router R3 has OSPFv2 configured and active. When the OSPF process initialized, the operational statuses of its interfaces were as follows:

- GigabitEthernet0/0 (IP: 192.168.10.1/24): Up / Line Protocol: Up
- GigabitEthernet0/1 (IP: 10.254.254.1/24): Up / Line Protocol: Up
- Loopback10 (IP: 172.16.1.1/24): Up / Line Protocol: Down
- Loopback20 (IP: 172.16.2.1/24): Up / Line Protocol: Up
- Loopback30 (IP: 172.16.3.1/24): Administratively Down / Line Protocol: Down

Shortly after initialization, a network administrator configured the command `router-id 1.1.1.1` under `router ospf 1`. No additional commands or resets were executed on the router.

What is the active OSPFv2 Router ID currently used by Router R3?

Show answer & explanation

Answer: 172.16.2.1

Answer

172.16.2.1

The active OSPF Router ID is 172.16.2.1. OSPF selects a Router ID during process initialization using the following tie-breaker order: (1) an explicitly configured `router-id` command, (2) the highest IP address of any operationally active (Up/Up) loopback interface, and (3) the highest IP address of any operationally active physical interface. At initialization, Loopback20 (172.16.2.1) was the only up/up loopback interface. Although `router-id 1.1.1.1` was subsequently configured, OSPF Router ID updates are non-preemptive and require executing `clear ip ospf process` to take effect.

Step-by-Step Solution

Evaluate interface operational states for OSPF Router ID candidacy.

Only interfaces in an 'Up / Line Protocol: Up' state are eligible. Loopback10 (line protocol down) and Loopback20 (up/up) are evaluated. Loopback20 is the only active loopback interface.

OSPF requires an interface to be operationally active to use its IP address as a Router ID.

Determine the initial active OSPF Router ID during process startup.

Because no explicit `router-id` command was configured at startup, OSPF selected the highest IP address among active loopback interfaces, which is 172.16.2.1 (Loopback20).

Active loopbacks take precedence over physical interfaces regardless of IP address values.

Evaluate the effect of configuring `router-id 1.1.1.1` after process initialization.

The active OSPF Router ID remains 172.16.2.1.

OSPF Router ID election is non-preemptive. Changing the `router-id` CLI setting takes effect only after reloading the device or resetting the OSPF process using the `clear ip ospf process` command.

Key Concept

OSPFv2 Router ID Election Order and Non-Preemptive Behavior

Question 7Question →

A network engineer is configuring Port Address Translation (PAT) on a Cisco IOS router using an IP address pool. Access Control List 10 matches internal private IPv4 addresses, and the NAT pool is named `OUTSIDE_POOL`. To allow multiple internal hosts to share the addresses in `OUTSIDE_POOL` by multiplexing Layer 4 port numbers, which single keyword must be appended to the end of the `ip nat inside source list 10 pool OUTSIDE_POOL` command?

Show answer & explanation

Answer: overload; the overload keyword

Answer

overload

Appending the keyword `overload` to the `ip nat inside source list <acl> pool <pool_name>` command enables Port Address Translation (PAT). This allows Cisco IOS to translate both IP addresses and transport layer port numbers, enabling many internal private IP addresses to share public IP addresses in the pool.

Step-by-Step Solution

Identify the requirement for Port Address Translation (PAT) using an IP address pool.

Standard dynamic NAT assigns public IP addresses to private IP addresses on a one-to-one basis.

Without port tracking, dynamic pool NAT runs out of available public IPv4 addresses when internal hosts exceed the pool size.

Determine the necessary Cisco IOS command modifier to enable port multiplexing.

Appending the keyword `overload` instructs the router to track transport-layer port numbers (PAT).

The `overload` keyword enables multiple internal sockets to share pool addresses simultaneously.

Key Concept

Inside Source Port Address Translation (PAT) Pool-based Configuration

Estimated Time:1m 0s

Question 8Question →

During the standard EUI-64 process used to derive a 64-bit IPv6 interface identifier from a 48-bit MAC address, what 16-bit hexadecimal value is inserted into the middle of the MAC address?

Show answer & explanation

Answer: FFFE; fffe; 0xFFFE; 0xfffe

Answer

FFFE

To create a 64-bit interface identifier using the IEEE EUI-64 format, a host takes its 48-bit MAC address, splits it into two 24-bit halves (OUI and NIC identifier), and inserts the 16-bit hexadecimal value FFFE (0xFFFE) in the middle. Additionally, the 7th bit (Universal/Local bit) of the first byte is inverted.

Step-by-Step Solution

Split the 48-bit MAC address into two equal 24-bit halves

The first 24 bits represent the Organizationally Unique Identifier (OUI) and the last 24 bits represent the Extension Identifier.

EUI-64 modification requires inserting a specific 16-bit sequence between these two halves.

Insert the 16-bit value FFFE into the midpoint between the two 24-bit halves

The 48-bit address expands to a 64-bit value containing FFFE in the center.

Inserting 0xFFFE creates the mandatory 64-bit structure required for an EUI-64 interface identifier.

Key Concept

EUI-64 Interface Identifier Generation

Question 9Question →

A network engineer is deploying IPv6 on a Cisco router interface with the burned-in MAC address `b496.910a.bcde`. The interface is enabled to automatically formulate its link-local address using standard EUI-64 formatting. What complete, compressed link-local IPv6 address will be generated on this interface?

Show answer & explanation

Answer: fe80::b696:91ff:fe0a:bcde; FE80::B696:91FF:FE0A:BCDE; fe80:0000:0000:0000:b696:91ff:fe0a:bcde; FE80:0000:0000:0000:B696:91FF:FE0A:BCDE

Answer

fe80::b696:91ff:fe0a:bcde

The MAC address `b496.910a.bcde` consists of octets `b4`, `96`, `91`, `0a`, `bc`, `de`. Inverting the 7th bit of `b4` (binary `1011 0100`) gives binary `1011 0110` (hex `b6`). Inserting `fffe` into the midpoint between `91` and `0a` creates the 64-bit interface ID `b696:91ff:fe0a:bcde`. Combining this with the IPv6 link-local prefix `fe80::/64` yields `fe80::b696:91ff:fe0a:bcde`.

Step-by-Step Solution

Split the 48-bit MAC address into two 24-bit halves.

Left half: `b4-96-91`, Right half: `0a-bc-de`.

The 64-bit interface identifier is constructed by inserting a 16-bit hex delimiter into the middle of the 48-bit MAC address.

Invert the Universal/Local (U/L) bit (the 7th bit of the first byte).

The first byte `b4` (hex `1011 0100` in binary) becomes `1011 0110` (hex `b6`).

In EUI-64 conversion, the 7th bit (U/L bit) of the first byte must be inverted (0 becomes 1 for globally unique MAC addresses).

Insert the 16-bit hexadecimal string `FFFE` between the two halves.

The 64-bit interface identifier becomes `b696:91ff:fe0a:bcde`.

Inserting `FFFE` expands the 48-bit MAC address into a valid 64-bit EUI-64 interface identifier.

Prepend the IPv6 link-local prefix (`fe80::/64`).

The full link-local address is `fe80::b696:91ff:fe0a:bcde`.

IPv6 link-local addresses use the reserved prefix `fe80::/10` with standard interface configuration defaulting to `fe80::/64` followed by the 64-bit EUI-64 interface ID.

Key Concept

IPv6 EUI-64 Link-Local Address Derivation

Estimated Time:2m 0s

Question 10Question →

When deriving an IPv6 64-bit interface identifier using the EUI-64 process from a 48-bit MAC address, which bit position (counted from left to right within the first octet) is inverted?

Show answer & explanation

Answer: 7; 7th; 7th bit; bit 7; seventh; seventh bit

Answer

The 7th bit position (the Universal/Local bit) in the first octet of the MAC address is inverted.

During the EUI-64 calculation process, the 7th bit from the left of the MAC address's first octet (the Universal/Local bit) is inverted. For example, a universal MAC address starting with binary 00000000 (0x00) becomes 00000010 (0x02) after the 7th bit is flipped from 0 to 1.

Step-by-Step Solution

Identify the modification rules for converting a 48-bit MAC address to an EUI-64 64-bit interface identifier.

The two required modifications are inserting the 16-bit value

FFFE

into the middle of the MAC address and inverting the Universal/Local (U/L) bit.

EUI-64 formatting requires extending the 48-bit MAC address to 64 bits and adjusting the scope indicator.

Determine the exact location of the Universal/Local (U/L) bit.

The U/L bit is located at the 7th bit position from the left in the first octet of the MAC address.

In standard IEEE MAC address formatting, bit 7 specifies whether the address is universally administered (0) or locally administered (1).

Key Concept

EUI-64 Universal/Local Bit Inversion

Question 11Question →

A Cisco router interface with a burned-in MAC address of `d46d.5012.abcd` is configured with the command `ipv6 address 2001:db8:100:1::/64 eui-64`. What is the complete 128-bit IPv6 global unicast address assigned to this interface?

Show answer & explanation

Answer: 2001:db8:100:1:d66d:50ff:fe12:abcd; 2001:DB8:100:1:D66D:50FF:FE12:ABCD; 2001:db8:100:1:d66d:50ff:fe12:abcd/64; 2001:0db8:0100:0001:d66d:50ff:fe12:abcd

Answer

2001:db8:100:1:d66d:50ff:fe12:abcd

To derive the IPv6 global unicast address using EUI-64, the router takes the assigned `/64` prefix `2001:db8:100:1::/64` and appends a 64-bit interface identifier generated from the interface's MAC address (`d46d.5012.abcd`). The MAC address is split into `d46d:50` and `12:abcd`, `FFFE` is inserted into the middle (`d46d:50ff:fe12:abcd`), and the 7th bit of the first byte is inverted (`d4` = `1101 0100` becomes `d6` = `1101 0110`). The final address is `2001:db8:100:1:d66d:50ff:fe12:abcd`.

Step-by-Step Solution

Split the 48-bit MAC address into two 24-bit halves.

Left half: `d4:6d:50`, Right half: `12:ab:cd`.

EUI-64 process requires dividing the MAC address to insert the 16-bit midpoint padding.

Insert the 16-bit hexadecimal value `FFFE` between the two MAC halves.

Resulting 64-bit sequence: `d46d:50ff:fe12:abcd`.

EUI-64 expands a 48-bit MAC address into a 64-bit interface identifier using `FFFE`.

Invert the 7th bit (Universal/Local bit) of the first byte (`d4`).

Byte `d4` in binary is `1101 0100`. Inverting the 7th bit yields `1101 0110`, which is `d6` in hexadecimal. Modified interface identifier: `d66d:50ff:fe12:abcd`.

IEEE EUI-64 standard requires flipping the U/L bit (7th bit from left) to indicate global uniqueness.

Combine the configured `/64` IPv6 prefix with the modified 64-bit EUI-64 interface identifier.

Complete IPv6 address: `2001:db8:100:1:d66d:50ff:fe12:abcd`.

Appending the derived 64-bit Interface ID to the `2001:db8:100:1::/64` network prefix completes the 128-bit global unicast address.

Key Concept

Derivation of IPv6 EUI-64 Interface Identifier from a 48-bit MAC address

Estimated Time:2m 0s

Question 12Question →

A network administrator is inspecting a host connected to a Cisco switch. The host network interface has a hardware MAC address of `5254.0012.3456`. Assuming IPv6 stateless autoconfiguration uses the standard IEEE EUI-64 format, what is the resulting 64-bit interface identifier?

Show answer & explanation

Answer: 5054:00ff:fe12:3456; 5054:00FF:FE12:3456; 5054:ff:fe12:3456; 5054:FF:FE12:3456

Answer

5054:00ff:fe12:3456

To derive the EUI-64 interface identifier from MAC address `5254.0012.3456`:
1. Split the MAC address: `52:54:00` and `12:34:56`.
2. Insert `FFFE` in the middle: `5254:00FF:FE12:3456`.
3. Invert the 7th bit of the first byte: `0x52` (`0101 0010` in binary) becomes `0x50` (`0101 0000` in binary).

Combining these operations yields the final 64-bit interface identifier `5054:00ff:fe12:3456`.

Step-by-Step Solution

Split the 48-bit MAC address into two 24-bit halves.

Left half: `52:54:00`, Right half: `12:34:56`

EUI-64 insertion occurs directly between the OUI and the vendor-assigned NIC portion.

Insert the 16-bit hex value `FFFE` into the middle.

`5254:00FF:FE12:3456`

Inserting `FFFE` expands the 48-bit MAC address into a 64-bit structure.

Invert the 7th bit (Universal/Local bit) of the first byte.

First byte `0x52` in binary is `0101 0010`. Inverting the 7th bit yields `0101 0000`, which is `0x50` in hex.

IEEE EUI-64 standards dictate that the 7th bit is flipped (0 becomes 1 for local uniqueness, 1 becomes 0 for universal).

Assemble the final 64-bit interface identifier in standard IPv6 hexadecimal notation.

`5054:00ff:fe12:3456`

Combines the modified first byte, remaining MAC components, and inserted `FFFE` into 16-bit colon-separated fields.

Key Concept

EUI-64 Interface Identifier Derivation

Question 13Question →

A Cisco router has the following interface operational statuses and IPv4 configurations:

- GigabitEthernet0/0 ( $192.168.50.1/24$ ): line protocol up, operational state UP
- GigabitEthernet0/1 ( $192.168.100.1/24$ ): line protocol up, operational state UP
- Loopback 0 ( $10.254.1.1/32$ ): administratively down, line protocol down
- Loopback 1 ( $10.200.1.1/32$ ): line protocol down, operational state DOWN
- Loopback 2 ( $172.16.20.1/32$ ): line protocol up, operational state UP

The OSPFv2 routing process is initialized using `router ospf 1` without an explicitly configured static Router ID via the `router-id` command. What exact IPv4 address will OSPFv2 elect as the active Router ID for this process?

Show answer & explanation

Answer: 172.16.20.1

Answer

172.16.20.1

OSPFv2 uses a strict three-tier precedence logic to select the Router ID: 1) An explicitly configured 'router-id' command, 2) The highest IPv4 address among active (up/up) loopback interfaces, 3) The highest IPv4 address among active (up/up) physical non-loopback interfaces. Here, no explicit Router ID exists. Loopback 0 and Loopback 1 are both down, making Loopback 2 (172.16.20.1) the sole active loopback interface. Because active loopback interfaces take precedence over physical interfaces (such as GigabitEthernet0/1 with 192.168.100.1), 172.16.20.1 is selected as the Router ID.

Step-by-Step Solution

Check for an explicitly configured router ID using the 'router-id' command.

No explicit router ID is configured under 'router ospf 1'.

Explicit configuration takes highest priority in OSPF Router ID selection.

Evaluate all active (up/up) loopback interfaces to find the highest IPv4 address.

Loopback 0 and Loopback 1 are inactive (down). Loopback 2 (172.16.20.1) is active (up/up).

OSPF prefers active loopback interfaces over physical interfaces, but inactive loopbacks cannot participate in election.

Compare candidate addresses among active loopbacks.

172.16.20.1 is the only active loopback address.

Since Loopback 2 is the only active loopback interface, its IP address (172.16.20.1) is chosen over the physical interface IP addresses.

Key Concept

OSPFv2 Router ID Selection Priority Logic

Estimated Time:1m 30s

Question 14Question →

A Cisco router is configured to run OSPFv2 without an explicitly set `router-id` under the OSPF process. The current interface IP address configurations and states are as follows:

- Loopback0: $10.10.10.10/32$ (Administratively Down)
- Loopback1: $172.16.1.1/24$ (Up / Up)
- Loopback2: $192.168.100.1/24$ (Up / Up)
- GigabitEthernet0/0: $10.200.200.1/30$ (Up / Up)

What IP address will OSPF select as the router ID when the OSPF process initializes?

Show answer & explanation

Answer: 192.168.100.1

Answer

192.168.100.1

OSPFv2 uses a strict order of preference to establish its Router ID: 1) Explicitly configured `router-id` command, 2) Highest IP address among active (Up/Up) loopback interfaces, 3) Highest IP address among active physical interfaces. In this scenario, no manual router ID exists. Loopback0 is down so it is excluded. Loopback1 (

172.16.1.1

) and Loopback2 (

192.168.100.1

) are both active loopback interfaces. OSPF selects

192.168.100.1

because it is the highest active loopback IP address.

Step-by-Step Solution

Check for an explicitly configured OSPF router ID

No manual OSPF router ID command is configured.

Explicit configuration takes precedence over all automatic election rules.

Evaluate all active (Up/Up) loopback interfaces

Loopback1 (

172.16.1.1

) and Loopback2 (

192.168.100.1

) are active. Loopback0 (

10.10.10.10

) is administratively down and ignored.

OSPF election logic prioritizes active loopback interfaces over physical interfaces.

Compare IP addresses of active loopbacks to select the highest value

Between

172.16.1.1

and

192.168.100.1

192.168.100.1

is numerically higher.

OSPF selects the highest IP address among operational loopback interfaces.

Key Concept

OSPFv2 Router ID Selection Precedence

Estimated Time:1m 0s

Question 15Question →

A Cisco router running OSPFv2 has no manually configured router ID (`router-id` command). The router has the following active and operational interfaces configured with IP addresses:

- Loopback 0: 10.10.10.1/32
- Loopback 1: 172.16.50.1/32
- GigabitEthernet0/0: 192.168.1.1/24
- GigabitEthernet0/1: 192.168.2.1/24

What IP address will OSPF automatically select as the OSPF Router ID?

Show answer & explanation

Answer: 172.16.50.1

Answer

172.16.50.1

In OSPFv2, the router ID election order is: 1) Manually configured router ID (`router-id` command), 2) Highest IP address among active loopback interfaces, and 3) Highest IP address among active non-loopback (physical) interfaces. Because active loopbacks exist and no manual router ID is specified, OSPF chooses the highest loopback IP address, which is 172.16.50.1.

Step-by-Step Solution

Check for a manually configured OSPF router ID

No manually configured OSPF router ID exists

Manual configuration using the `router-id` command takes highest priority.

Evaluate active loopback interfaces

Two active loopback interfaces exist: 10.10.10.1 and 172.16.50.1

If no explicit router ID is set, OSPF evaluates the IP addresses assigned to up/up loopback interfaces.

Select the highest IP address among active loopback interfaces

172.16.50.1 is selected as the Router ID

Comparing numerically, 172.16.50.1 is greater than 10.10.10.1.

Key Concept

OSPFv2 Router ID Election Order

Question 16Question →

A network switch interface has a burned-in MAC address of `3464.a912.3456`. When calculating an IPv6 address using the standard IEEE EUI-64 process, what is the generated 64-bit interface identifier expressed in standard colon-separated IPv6 hexadecimal notation?

Show answer & explanation

Answer: 3664:a9ff:fe12:3456; 3664:A9FF:FE12:3456

Answer

3664:a9ff:fe12:3456

Converting the MAC address 34-64-A9-12-34-56 to EUI-64 format requires inserting FFFE into the middle (34-64-A9-FF-FE-12-34-56) and inverting the 7th bit of the first byte (0x34 becomes 0x36). Grouping into 16-bit hexadecimal blocks separated by colons yields 3664:a9ff:fe12:3456.

Step-by-Step Solution

Split the 48-bit MAC address into two 24-bit halves.

Left half: 34:64:A9, Right half: 12:34:56.

EUI-64 requires inserting a specific 16-bit hexadecimal value into the midpoint of the MAC address.

Insert the hexadecimal value FFFE between the two halves.

34-64-A9-FF-FE-12-34-56

IEEE EUI-64 standard specifies the insertion of FFFE to expand a 48-bit MAC address to 64 bits.

Invert the 7th bit (Universal/Local bit) of the first byte (0x34).

Binary 0x34 = 0011 0100; inverted 7th bit = 0011 0110 = 0x36. The updated byte sequence is 36-64-A9-FF-FE-12-34-56.

The Universal/Local (U/L) bit is flipped to indicate a globally unique, modified EUI-64 address.

Format the resulting 64-bit value into IPv6 colon-separated hexadecimal quads.

3664:a9ff:fe12:3456

IPv6 interface identifiers are written as four 16-bit hexadecimal blocks separated by colons.

Key Concept

EUI-64 Interface Identifier Derivation

Question 17Question →

Two Cisco routers, R1 and R2, are directly connected via their GigabitEthernet0/0 interfaces in OSPFv2 Area 0. Interface GigabitEthernet0/0 on R1 is configured with a maximum transmission unit (MTU) of 1500 bytes, whereas GigabitEthernet0/0 on R2 is configured with an MTU of 1400 bytes. Neither router has the ip ospf mtu-ignore command configured.

In which specific OSPF neighbor state will the adjacency between R1 and R2 become stuck?

Show answer & explanation

Answer: ExStart; EXSTART; ExStart state; EXSTART state

Answer

ExStart

When OSPF neighbor parameters in Hello packets match, routers establish two-way communication and progress to the ExStart state to begin Database Description (DBD) packet negotiation. Because interface MTU values are advertised inside DBD headers, an MTU mismatch (1500 bytes on R1 vs. 1400 bytes on R2) causes the router with the lower MTU to ignore incoming DBD packets. As a result, Master/Slave election and DBD sequence number synchronization fail, leaving the neighbor relationship stuck in the ExStart state.

Step-by-Step Solution

Analyze OSPF Hello packet processing and initial state transitions

Hello parameters (Area ID, Subnet Mask, Hello/Dead timers, Authentication) match between R1 and R2, allowing the routers to pass the Init state and transition into the 2-Way state.

MTU values are not contained or evaluated within OSPF Hello packets.

Evaluate Database Description (DBD) packet negotiation during ExStart

Upon transitioning to the ExStart state, routers exchange Database Description (DBD) packets to elect a Master/Slave relationship and negotiate sequence numbers. The configured interface MTU is explicitly included in the Interface MTU field of the DBD header.

OSPF requires matching interface MTUs to ensure Database Description packets carrying Link-State Advertisements can be exchanged without fragmentation issues.

Determine the impact of the MTU mismatch

R2 receives a DBD packet from R1 advertising an MTU of 1500 bytes, which exceeds R2's interface MTU of 1400 bytes. Consequently, R2 drops the packet, causing Master/Slave negotiation to stall endlessly in the ExStart state.

Without ip ospf mtu-ignore enabled, any mismatch in advertised MTU prevents completion of DBD packet exchange.

Key Concept

OSPF Neighbor States and MTU Mismatch Behavior

Question 18Question →

A Cisco router initializes an OSPFv2 process without a manually configured router ID. The router currently has the following operational interfaces:

- Loopback0: $10.10.10.1/32$
- Loopback10: $10.20.20.1/32$
- GigabitEthernet0/1: $192.168.100.1/24$

What IP address will OSPFv2 automatically select as the router ID?

Show answer & explanation

Answer: 10.20.20.1

Answer

10.20.20.1

When an explicit OSPF router ID is not configured using the 'router-id' command, OSPFv2 selects the highest IPv4 address among all active loopback interfaces. Because loopback interfaces take priority over physical interfaces, the router compares 10.10.10.1 and 10.20.20.1, choosing 10.20.20.1 as the router ID.

Step-by-Step Solution

Check if a manual OSPF router ID is configured.

No manual router ID is specified under the OSPF process.

Explicit configuration taking highest precedence is absent.

Evaluate active loopback interfaces.

Two active loopback interfaces exist: Loopback0 (

10.10.10.1

) and Loopback10 (

10.20.20.1

OSPF criteria prioritize active loopback interface IP addresses over active physical interface IP addresses.

Determine the highest IP address among active loopback interfaces.

10.20.20.1

is numerically higher than

10.10.10.1

10.20.20.1

is chosen as the router ID.

Key Concept

OSPFv2 Router ID Election Order

Estimated Time:45s

Question 19Question →

A Cisco router initializes an OSPFv2 routing process without an explicitly configured router ID. The router currently has the following operational interfaces and statuses:

- Loopback 0: 10.254.254.1/24 (Administratively Down / Down)
- Loopback 1: 172.16.40.1/24 (Up / Up)
- Loopback 2: 172.16.100.1/24 (Up / Up)
- GigabitEthernet0/0: 192.168.10.1/24 (Up / Up)

What IP address will be selected as the OSPFv2 router ID for this router?

Show answer & explanation

Answer: 172.16.100.1

Answer

172.16.100.1

When no manual router ID is configured, OSPFv2 selects the highest IPv4 address among active (up/up) loopback interfaces. Loopback 0 is administratively down, so it is ignored. Between the operational loopbacks (172.16.40.1 and 172.16.100.1), 172.16.100.1 is the highest IP address.

Step-by-Step Solution

Check for an explicitly configured router ID under the OSPF process.

No manual 'router-id' command is configured.

Explicit configuration takes highest priority in OSPF router ID election.

Evaluate active (up/up) loopback interfaces.

Loopback 1 (172.16.40.1) and Loopback 2 (172.16.100.1) are active. Loopback 0 (10.254.254.1) is administratively down and ignored.

Inactive interfaces cannot be used for router ID election, and active loopbacks take priority over physical interfaces.

Select the highest IP address among the active loopback interfaces.

172.16.100.1 is selected because 172.16.100.1 is greater than 172.16.40.1.

OSPF selects the numerically highest IPv4 address among operational loopback interfaces.

Key Concept

OSPFv2 Router ID Election Order

Question 20Question →

A Cisco router initializes OSPFv2 Process 1 with the following operational interfaces:
- Loopback0: 10.1.1.1/32 (status up, line protocol up)
- GigabitEthernet0/0: 172.16.10.1/24 (status up, line protocol up)

After OSPF Process 1 is fully active, a network engineer performs the following configuration actions in exact chronological sequence:
1. Configures Loopback1 with IP address 10.254.254.254/32 (status up, line protocol up).
2. Enters OSPF router configuration mode and executes the command `router-id 192.168.99.1`.
3. Configures GigabitEthernet0/1 with IP address 192.168.200.1/24 (status up, line protocol up).

Assuming no router reboot or `clear ip ospf process` command has been executed, what IP address is currently used as the operational OSPFv2 Router ID for Process 1?

Show answer & explanation

Answer: 10.1.1.1

Answer

10.1.1.1

When OSPF Process 1 initially started, 10.1.1.1 was elected as the Router ID because Loopback0 was the sole operational loopback interface. OSPF Router ID selection is non-preemptive once an active process has selected its ID. Adding a loopback with a higher IP address (10.254.254.254), configuring an explicit `router-id 192.168.99.1` command, or activating new physical interfaces will not modify the operational Router ID until `clear ip ospf process` is executed or the router reboots. Thus, the active Router ID remains 10.1.1.1.

Step-by-Step Solution

Determine the initial OSPFv2 Router ID selected when Process 1 started.

Loopback0 (10.1.1.1) was selected because operational loopback interface IP addresses take priority over physical interface IP addresses during initial Router ID election.

In Cisco IOS OSPFv2, if no manual router ID is configured at startup, the router chooses the highest IP address among operational loopback interfaces.

Evaluate the impact of configuring Loopback1 (10.254.254.254/32).

The active Router ID remains 10.1.1.1.

OSPF Router ID election is non-preemptive. Dynamically elected Router IDs do not change automatically when new interfaces with higher IP addresses come online.

Evaluate the impact of executing `router-id 192.168.99.1`.

The active Router ID remains 10.1.1.1.

Configuring a manual OSPF router ID on an already running OSPF process does not take effect until the process is reset or the router is rebooted.

Evaluate the impact of adding GigabitEthernet0/1 and determine the final operational Router ID.

The active Router ID stays 10.1.1.1.

Without a process reset via `clear ip ospf process` or a device reload, none of the subsequent configuration changes displace the active Router ID.

Key Concept

OSPFv2 Router ID Non-Preemption and Reset Requirement

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