Network Implementation

424 questions

Question 161Question

A workstation on a remote branch subnet initializes network communications and requires an IP address assignment from a centralized DHCP server located across a routed WAN link. Place the chronological steps of the DHCP Relay forwarding process in the correct sequence from initial client packet creation to the reception of the initial offer.

Drag items to arrange them in the correct order

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Answer

The correct sequence starts with the workstation broadcasting a DHCPDISCOVER request locally on UDP port 67. Next, the Layer 3 relay agent inserts its interface IP into the giaddr field and unicasts the request to the central DHCP server. The DHCP server processes the giaddr field to match the scope and responds with a unicast DHCPOFFER to the relay agent. Finally, the relay agent forwards the DHCPOFFER down to the workstation segment on UDP port 68.
The DHCP sequence across subnets requires specific steps to bridge broadcast domains. First, the unconfigured client broadcasts a DHCPDISCOVER message using destination UDP port 6767. Second, the local relay agent captures the broadcast, inserts its local interface IP into the gateway IP address (giaddrgiaddr) header field, and unicasts the message across the routed network to the DHCP server on UDP port 6767. Third, the server uses the giaddrgiaddr value to select the proper subnet pool and unicasts a DHCPOFFER back to the relay agent interface. Fourth, the relay agent delivers the offer back to the client host on UDP port 6868.

Step-by-Step Solution

1
Identify the initial client request behavior.
The client broadcasts a DHCPDISCOVER packet locally using source UDP port 6868 and destination UDP port 6767.
Unconfigured hosts must broadcast locally to locate network configuration services.
2
Determine the action of the DHCP relay agent (IP Helper).
The relay agent captures the broadcast, appends its local interface address into the Gateway IP Address (giaddrgiaddr) field, and forwards the packet as a unicast transmission to the central DHCP server on UDP port 6767.
Routers drop local broadcasts by default; relaying requires converting the broadcast to a routable unicast packet and tagging the originating subnet.
3
Trace the DHCP server processing and reply.
The server matches the giaddrgiaddr to the appropriate scope, generates a DHCPOFFER, and unicasts it back to the relay agent interface on UDP port 6767.
The server relies on giaddrgiaddr rather than the IP header source address to determine which pool of addresses to draw from.
4
Trace the relay agent's final delivery to the client.
The relay agent forwards the DHCPOFFER packet onto the client's local subnet to UDP port 6868.
The client listens for DHCP server replies on UDP port 6868 to parse lease parameters.

Key Concept

DHCP Relay Agent Unicast Conversion and giaddr Field Processing
Estimated Time:2m 0s
Question 162Question

A network administrator is connecting a wireless access point (AP) to switch port GigabitEthernet 1/0/5. The AP transmits management traffic untagged on VLAN 100, while wireless user traffic on the 'Guest' SSID is transmitted with an 802.1Q tag for VLAN 200. The switch port is currently operating with default 802.1Q trunk settings, where VLAN 1 is the default native VLAN. Which interface configuration command must be executed on Switch GigabitEthernet 1/0/5 so that untagged management frames are correctly processed on VLAN 100?

Show answer & explanation

Answer: switchport trunk native vlan 100

Answer

The command 'switchport trunk native vlan 100' correctly designates VLAN 100 as the native VLAN on the 802.1Q trunk interface.
On an IEEE 802.1Q trunk link, untagged frames are associated with the port's native VLAN. By default, Cisco switches assign VLAN 1 as the native VLAN. Issuing the command 'switchport trunk native vlan 100' changes this mapping so that all untagged traffic (in this case, AP management traffic) is placed into VLAN 100 while maintaining 802.1Q tagging for VLAN 200.

Step-by-Step Solution

1
Analyze the framing requirements for the connected network device
Management traffic requires untagged framing for VLAN 100, and Guest traffic requires tagged 802.1Q framing for VLAN 200.
802.1Q trunks pass tagged frames for specified VLANs and untagged frames for a single designated native VLAN.
2
Identify the switch port's current state and necessary modification
The trunk defaults to VLAN 1 as the native VLAN. Untagged traffic will inadvertently enter VLAN 1 unless reconfigured.
To route untagged management frames into VLAN 100, the switch port's native VLAN ID must be explicitly changed from VLAN 1 to VLAN 100.
3
Select the appropriate interface CLI command
Executing 'switchport trunk native vlan 100' maps untagged ingress and egress frames to VLAN 100.
This command specifically redefines the 802.1Q native VLAN for the trunk port without disrupting tagged VLAN 200 traffic.

Key Concept

802.1Q Native VLAN Configuration
Question 163Question

A network administrator is inspecting an IEEE 802.1Q trunk link between two switches, `SW-CAMPUS-01` and `SW-CAMPUS-02`. The configuration of interface GigabitEthernet0/1 on `SW-CAMPUS-01` is:

text
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk native vlan 50
switchport trunk allowed vlan 10,20,30,50

The configuration of interface GigabitEthernet0/1 on `SW-CAMPUS-02` is:

text
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk native vlan 1
switchport trunk allowed vlan 10,20,30,50

A workstation connected to an access port on `SW-CAMPUS-01` in VLAN 50 transmits an unencapsulated Ethernet broadcast frame destined across the trunk link. Which of the following best describes how `SW-CAMPUS-02` will handle this frame upon arrival at its GigabitEthernet0/1 interface?

Show answer & explanation

Answer: It will accept the untagged frame and forward it to all active ports belonging to VLAN 1.

Answer

The switch receiving the frame will accept the untagged frame and forward it to all active ports belonging to VLAN 1.
Under IEEE 802.1Q trunking, traffic belonging to the native VLAN configured on an egress trunk port is sent untagged. Because the sending switch has VLAN 50 set as native, VLAN 50 frames enter the link without an 802.1Q tag. Upon receiving an untagged frame on a trunk port, the receiving switch assigns it to its own configured native VLAN (VLAN 1). Consequently, the frame is forwarded within VLAN 1 on the receiving switch.

Step-by-Step Solution

1
Analyze frame egress processing on the transmitting switch.
SW-CAMPUS-01 has native VLAN configured as VLAN 50. Therefore, frames belonging to VLAN 50 egress GigabitEthernet0/1 untagged (without an 802.1Q header).
By definition of IEEE 802.1Q standard, traffic belonging to the native VLAN of a trunk interface is transmitted untagged.
2
Analyze frame ingress processing on the receiving switch.
SW-CAMPUS-02 receives an untagged frame on GigabitEthernet0/1. Its configured native VLAN for that port is VLAN 1.
When a switch receives an untagged frame on an 802.1Q trunk port, it associates the frame with the local native VLAN configured on that specific receiving port.
3
Determine the forwarding behavior and operational impact.
The frame is processed as belonging to VLAN 1 on SW-CAMPUS-02, causing traffic to leak from VLAN 50 on the source switch into VLAN 1 on the destination switch.
Native VLAN mismatches do not block untagged frame forwarding by default; they result in cross-VLAN traffic leakage between different VLAN IDs.

Key Concept

802.1Q Native VLAN Operation and Mismatch Behavior
Question 164Question

A security analyst is investigating a potential incident involving a server assigned the IP address 10.80.35.138/2610.80.35.138/26. What is the broadcast IPv4 address of the subnet to which this server belongs?

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Answer: 10.80.35.191

Answer

10.80.35.191
The CIDR prefix /26/26 indicates a subnet mask of 255.255.255.192255.255.255.192, creating subnet increments of 6464 addresses in the fourth octet. The subnets start at .0.0, .64.64, .128.128, and .192.192. The IP address 10.80.35.13810.80.35.138 lies within the .128.128 subnet block (10.80.35.128/2610.80.35.128/26). The broadcast address is the final IP in this block, which is 10.80.35.19110.80.35.191.

Step-by-Step Solution

1
Determine the subnet mask and block size from the CIDR prefix length.
A /26/26 prefix corresponds to a subnet mask of 255.255.255.192255.255.255.192. The block size in the fourth octet is 256192=64256 - 192 = 64.
The CIDR notation /26/26 uses 26 network bits, leaving 6 host bits (26=642^6 = 64 addresses per subnet).
2
Identify the network boundaries for the fourth octet.
Subnet boundaries increment by 6464: 00, 6464, 128128, and 192192.
Each subnet span covers 64 continuous IP addresses.
3
Locate the given IP address within its subnet range.
The IP address 10.80.35.13810.80.35.138 falls within the range 10.80.35.12810.80.35.128 through 10.80.35.19110.80.35.191.
Since 128138<192128 \le 138 < 192, the network address for this subnet is 10.80.35.12810.80.35.128.
4
Determine the broadcast address for the subnet.
The broadcast address is 10.80.35.19110.80.35.191.
The broadcast address is the last address in the subnet range, immediately preceding the next subnet network ID (10.80.35.1921=10.80.35.19110.80.35.192 - 1 = 10.80.35.191).

Key Concept

IPv4 Subnetting and Broadcast Address Calculation
Question 165Question

A network administrator is configuring a newly created VLAN dedicated to VoIP phones. The router interface handling this subnet is configured with the network block 172.16.45.128/27172.16.45.128/27. What is the maximum number of usable host IP addresses that can be assigned to end devices within this subnet?

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Answer: 30

Answer

The maximum number of usable host IP addresses that can be assigned is 30.
A prefix length of /27 leaves 5 host bits (3227=532 - 27 = 5). Calculating 252^5 yields 32 total IP addresses in the subnet block. Subtracting 2 for the unassignable network and broadcast addresses results in 30 usable host IP addresses.

Step-by-Step Solution

1
Determine the number of host bits remaining from the CIDR prefix.
3227=532 - 27 = 5 host bits
An IPv4 address consists of 32 total bits. Subtracting the prefix length gives the bits reserved for host addressing.
2
Calculate the total number of IP addresses in the subnet.
25=322^5 = 32 total IP addresses
The total number of addresses is calculated as 2 raised to the power of the host bits.
3
Subtract reserved addresses to find usable host IP addresses.
322=3032 - 2 = 30 usable host IP addresses
The first address in the block is reserved for the network ID and the last address is reserved for the network broadcast address.

Key Concept

IPv4 Subnet Host Calculation
Estimated Time:1m 15s
Question 166Question

A network technician is setting up a new network segment using the IP block 10.0.4.0/2410.0.4.0/24. Following standard configuration guidelines, the default gateway will be assigned the last usable host IP address in the subnet. What is the last usable IPv4 host address in the 10.0.4.0/2410.0.4.0/24 network?

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Answer: 10.0.4.254

Answer

The last usable IPv4 host address in the 10.0.4.0/2410.0.4.0/24 network is 10.0.4.25410.0.4.254.
For a /24/24 IPv4 subnet, the address space covers 256 total IP addresses (10.0.4.010.0.4.0 to 10.0.4.25510.0.4.255). The first address (10.0.4.010.0.4.0) serves as the network ID and the last address (10.0.4.25510.0.4.255) serves as the broadcast address. Thus, the assignable host range is 10.0.4.110.0.4.1 through 10.0.4.25410.0.4.254, making 10.0.4.25410.0.4.254 the last usable host address.

Step-by-Step Solution

1
Identify the network address and broadcast address for 10.0.4.0/2410.0.4.0/24.
Network address is 10.0.4.010.0.4.0 and broadcast address is 10.0.4.25510.0.4.255.
A /24/24 prefix provides 256 total IP addresses in the final octet (range 0–255).
2
Determine the range of usable host IP addresses.
The usable host address range spans from 10.0.4.110.0.4.1 to 10.0.4.25410.0.4.254.
The network address (10.0.4.010.0.4.0) and the broadcast address (10.0.4.25510.0.4.255) cannot be assigned to hosts.
3
Select the last assignable host address.
10.0.4.25410.0.4.254
It is the highest usable IP address immediately preceding the broadcast address.

Key Concept

IPv4 Usable Host Range and Subnet Boundary Calculation
Question 167Question

A client workstation connected to VLAN 45 (10.45.0.0/2410.45.0.0/24) issues a Layer 2 broadcast `DHCPDISCOVER` packet (255.255.255.255255.255.255.255) to obtain an IP configuration. The default gateway interface for VLAN 45 on the Layer 3 switch is configured with the IP address 10.45.0.110.45.0.1 and an IP helper address pointing to a centralized DHCP server at 10.200.10.2510.200.10.25. Which set of header and payload modifications will the Layer 3 switch perform on this packet before forwarding it across the routed core to the DHCP server?

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Answer: It sets the Gateway IP Address (`giaddr`) field in the DHCP payload to 10.45.0.110.45.0.1, sets the source IP address to 10.45.0.110.45.0.1, sets the destination IP address to 10.200.10.2510.200.10.25, and forwards the unicast packet using UDP destination port 67.

Answer

The Layer 3 switch populates the Gateway IP Address (`giaddr`) field in the DHCP payload with 10.45.0.110.45.0.1, rewrites the source IP address as 10.45.0.110.45.0.1, sets the destination IP to 10.200.10.2510.200.10.25, and forwards the unicast packet using UDP port 67.
The correct answer accurately describes the dual modification executed by a DHCP relay agent (IP helper): it writes the receiving Layer 3 interface's IP address (10.45.0.110.45.0.1) into the DHCP payload's `giaddr` (gateway IP address) field, and encapsulates the request into a standard unicast IP packet (Source: 10.45.0.110.45.0.1, Destination: 10.200.10.2510.200.10.25, UDP Port 67) so it can be routed across network subnets.

Step-by-Step Solution

1
Analyze incoming broadcast packet parameters
The client issues a `DHCPDISCOVER` frame with Source IP 0.0.0.00.0.0.0, Destination IP 255.255.255.255255.255.255.255, Source MAC = Client MAC, Destination MAC = `FF:FF:FF:FF:FF:FF`, Source Port = UDP 68, Destination Port = UDP 67.
Unconfigured clients must broadcast their initial DHCP discovery requests.
2
Identify DHCP Relay Agent (IP Helper) processing
The Layer 3 interface receiving the broadcast (10.45.0.110.45.0.1) intercepts the frame, inspects the `giaddr` field in the DHCP payload, and writes its own interface IP (10.45.0.110.45.0.1) into `giaddr`.
The centralized DHCP server uses the `giaddr` field to determine which address pool/scope (10.45.0.0/2410.45.0.0/24) to allocate an IP address from.
3
Examine IP and UDP header modifications for unicast transit
The switch constructs a unicast IP packet: Source IP = 10.45.0.110.45.0.1, Destination IP = 10.200.10.2510.200.10.25, Source Port = UDP 67, Destination Port = UDP 67.
Routers cannot route 255.255.255.255255.255.255.255 broadcasts. Converting the request to unicast allows it to traverse standard Layer 3 routed hops to reach the remote server.

Key Concept

DHCP Relay Agent Payload and Header Modification (`giaddr` Insertion & Broadcast-to-Unicast Conversion)
Question 168Question

A network administrator is designing an IP addressing scheme for a new branch office location. The department requires static IP assignments for 60 user workstations, 2 IP phone gateways, and 2 redundant default gateway router interfaces. The administrator must assign the smallest possible subnet block from `10.150.0.0/16` that will successfully accommodate all required host devices. Which CIDR prefix and subnet mask should the administrator configure?

Show answer & explanation

Answer: /25 (255.255.255.128)

Answer

/25 (255.255.255.128)
The scenario requires 64 usable IPv4 addresses (60 workstations + 2 phone gateways + 2 router interfaces). Using the formula for usable hosts 2h22^h - 2, a /26 prefix has 6 host bits (262=622^6 - 2 = 62), which falls short by 2 addresses. Therefore, the network administrator must allocate a /25 prefix with 7 host bits (272=1262^7 - 2 = 126), which is the smallest subnet block capable of supporting all 64 host IP addresses.

Step-by-Step Solution

1
Calculate the total number of host IP addresses required.
60 workstations + 2 IP phone gateways + 2 router interfaces = 64 host IP addresses.
Every active device and router gateway interface on the subnet requires a unique usable IPv4 address.
2
Apply the usable host formula 2h2642^h - 2 \ge 64 to determine the required host bits (hh).
For h=6h = 6, 262=622^6 - 2 = 62 usable hosts (insufficient). For h=7h = 7, 272=1262^7 - 2 = 126 usable hosts (sufficient).
Subnetting calculations require subtracting 2 reserved addresses (the Network ID and Broadcast ID) from total addresses (2h2^h).
3
Determine the prefix length and dotted-decimal subnet mask.
Prefix length is 327=/2532 - 7 = /25, which corresponds to the subnet mask `255.255.255.128`.
Subtracting 7 host bits from 32 total IPv4 bits leaves 25 network bits.

Key Concept

IPv4 Subnetting and Usable Host Calculation
Question 169Question

Match each switchport configuration scenario on the left with its corresponding IEEE 802.1Q frame processing behavior or operational link state on the right.

Click a left item, then click its matching right item

Items

An untagged Ethernet frame arrives on a trunk port configured with 'switchport trunk native vlan 99'.
An 802.1Q tagged Ethernet frame with VLAN ID 30 arrives on a port configured with 'switchport trunk allowed vlan 10,20'.
A switchport configured with 'switchport mode dynamic auto' connects to an adjacent switchport configured with 'switchport mode dynamic desirable'.
An 802.1Q tagged frame with VLAN ID 50 arrives on a switchport configured with 'switchport mode access' and 'switchport access vlan 10'.

Matches

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Answer

Untagged ingress on native VLAN 99 maps internally to VLAN 99 without a tag; VLAN ID 30 on a trunk allowing only 10,20 is dropped due to pruning rules; dynamic auto connecting to dynamic desirable successfully negotiates an 802.1Q trunk link via DTP; tagged VLAN 50 traffic arriving on an access port for VLAN 10 is dropped.
The correct pairings accurately reflect standard IEEE 802.1Q frame processing rules and Cisco DTP negotiation matrix rules: native VLAN frames travel untagged; non-allowed VLAN traffic is dropped by trunk interface filtering; dynamic auto and dynamic desirable negotiate a functional trunk; and unexpected tagged traffic on access ports is dropped.

Step-by-Step Solution

1
Analyze Native VLAN Ingress Tagging Rules
Untagged frames arriving on an 802.1Q trunk port are processed on the native VLAN defined by 'switchport trunk native vlan <id>'.
802.1Q trunking specifies that native VLAN traffic does not carry a 4-byte VLAN tag across the link.
2
Evaluate Trunk Allowed List Filtering
VLAN 30 traffic is dropped upon arrival.
The command 'switchport trunk allowed vlan 10,20' restricts trunk traversal exclusively to VLANs 10 and 20, pruning all unlisted VLAN IDs.
3
Evaluate Dynamic Trunking Protocol (DTP) Negotiation
The link operates as a trunk.
Dynamic desirable actively sends DTP requests to convert the link to a trunk, and dynamic auto responds positively to trunking requests.
4
Assess Access Port Tagged Ingress Behavior
The frame tagged with VLAN ID 50 is dropped on the access port assigned to VLAN 10.
Standard access ports operate within a single broadcast domain and drop ingress frames containing mismatched 802.1Q tags.

Key Concept

VLAN Configuration and 802.1Q Trunking
Estimated Time:3m 0s
Question 170Question

A network router receives two identical candidate routes to destination network 192.168.50.0/24192.168.50.0/24: one route was learned via OSPF (default administrative distance of 110110) and the other route was learned via RIP (default administrative distance of 120120). Assuming default protocol configurations are used, which route will the router install into its active routing table?

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Answer: The OSPF route, because it has a lower administrative distance value than RIP

Answer

The OSPF route will be installed into the routing table because its default administrative distance of 110 is lower and therefore more trustworthy than RIP's default administrative distance of 120.
When a router receives multiple paths to the exact same destination network prefix from different routing protocols, it compares their Administrative Distance (AD). AD represents the trustworthiness of the source. Lower AD values are preferred over higher AD values. Because OSPF has a default AD of 110 and RIP has a default AD of 120, the router selects the OSPF route for installation into the routing table.

Step-by-Step Solution

1
Identify the administrative distance (AD) values for both competing routing sources.
OSPF has a default AD of 110; RIP has a default AD of 120.
Administrative distance measures the trustworthiness of a routing information source when multiple protocols learn the same prefix.
2
Compare the AD values to determine path selection precedence.
110 is less than 120.
Lower administrative distance values are always preferred by the router selection algorithm.
3
Select the winning route for entry into the active routing table.
The router installs the OSPF route into its active routing table.
The candidate route with the lowest administrative distance is selected.

Key Concept

Administrative Distance Precedence in Route Selection
Question 171Question

A network administrator attempts to add VLAN 30 to an active 802.1Q trunk line between two distribution switches that currently carries VLAN 10 and VLAN 20. The administrator enters the following command on interface GigabitEthernet1/0/1:

`SW-Dist1(config-if)# switchport trunk allowed vlan 30`

Immediately after executing this command, users on VLAN 10 and VLAN 20 report loss of connectivity across the trunk link. Which of the following best explains why this connectivity issue occurred?

Show answer & explanation

Answer: The command replaced the existing allowed VLAN list with only VLAN 30 rather than appending it to the list.

Answer

The command executed overwrote the entire allowed VLAN list on the trunk interface with VLAN 30, thereby pruning VLAN 10 and VLAN 20 from traversing the link.
When executing `switchport trunk allowed vlan <vlan-list>` without using the `add` keyword, the switch replaces the entire set of allowed VLANs on the trunk with only the specified VLAN(s). As a result, VLAN 10 and VLAN 20 were pruned from GigabitEthernet1/0/1, causing cross-switch traffic for those VLANs to be dropped.

Step-by-Step Solution

1
Analyze the CLI command executed on the switch interface.
Identified the command as `switchport trunk allowed vlan 30` without the `add` parameter.
In Cisco IOS and standard network switch syntax, `switchport trunk allowed vlan <vlan-id>` sets an explicit allowed list, replacing any previously permitted VLANs.
2
Evaluate the impact on active VLAN traffic traversing the trunk.
VLAN 10 and VLAN 20 are immediately removed from the trunk's allowed list.
Frames belonging to VLAN 10 and VLAN 20 are dropped at egress/ingress on GigabitEthernet1/0/1 because those VLAN IDs are no longer permitted on the trunk interface.
3
Determine the proper command syntax to prevent this issue.
The administrator should have executed `switchport trunk allowed vlan add 30`.
Using the `add` parameter appends the specified VLAN ID to the existing list of permitted VLANs without clearing current entries.

Key Concept

802.1Q Trunk Allowed VLAN List Overwrite vs. Append
Estimated Time:1m 30s
Question 172Question

An enterprise network administrator configures a Layer 3 access switch as a DHCP relay agent for VLAN 55 (10.55.0.0/2410.55.0.0/24), designating a remote server at 192.168.1.100192.168.1.100 via an `ip helper-address` command on SVI 55. Downstream Layer 2 edge switches append DHCP Option 82 (Relay Agent Information) metadata to client requests before forwarding them to the Layer 3 switch. However, client hosts on VLAN 55 fail to obtain IP addresses, and packet drops are logged on the Layer 3 switch for incoming DHCPDISCOVER packets containing Option 82 with a gateway IP address (`giaddr`) of 0.0.0.00.0.0.0. Which configuration change on the Layer 3 switch will resolve this issue?

Show answer & explanation

Answer: Enable DHCP Option 82 trust on the relay agent to allow untrusted ingress packets containing Option 82 data

Answer

The correct action is to enable DHCP Option 82 trust on the Layer 3 relay switch so it accepts and processes incoming client requests pre-tagged with Option 82 information.
When downstream Layer 2 switches append Option 82 information to a client DHCP request before it reaches the Layer 3 relay agent, the request arrives on an untrusted port with a gateway IP address (`giaddr`) of 0.0.0.00.0.0.0. By default security design, many relay agents discard such packets to prevent Option 82 injection attacks. Enabling Option 82 trust allows the Layer 3 switch to accept these packets, insert its SVI IP into `giaddr`, and forward the unicast packet to the DHCP server.

Step-by-Step Solution

1
Analyze the reported symptom and packet behavior
Client broadcast packets contain DHCP Option 82 inserted by downstream Layer 2 switches, but arrive at the Layer 3 relay agent with `giaddr = 0.0.0.0`.
The Layer 2 edge switch appends Option 82 metadata without modifying IP header fields such as `giaddr`.
2
Identify default security rules for DHCP relay agents
Standard DHCP relay security policy dictates dropping packets containing Option 82 if received on untrusted interfaces when `giaddr` is set to 0.0.0.00.0.0.0.
This behavior prevents Option 82 spoofing attacks from untrusted edge ports.
3
Determine the necessary configuration fix
Configuring `ip dhcp relay information trust-all` or enabling interface-level Option 82 trust instructs the relay switch to process pre-existing Option 82 fields and forward the unicast request to 192.168.1.100192.168.1.100.
Explicitly trusting edge Option 82 insertion enables valid cross-VLAN relay functionality in tiered switch architectures.

Key Concept

DHCP Option 82 Untrusted Ingress Processing & Relay Agent Trust Policies
Question 173Question

A core border router receives an IP packet destined for host address 10.20.105.4210.20.105.42. The router's active routing table contains four operational candidate entries that match this target network space:

1. Static route to 10.20.96.0/1910.20.96.0/19 via next-hop 172.16.1.1172.16.1.1 (Administrative Distance: 11)
2. OSPF route to 10.20.104.0/2110.20.104.0/21 via next-hop 172.16.2.1172.16.2.1 (Administrative Distance: 110110, Metric: 2020)
3. Internal EIGRP route to 10.20.104.0/2210.20.104.0/22 via next-hop 172.16.3.1172.16.3.1 (Administrative Distance: 9090, Metric: 1515)
4. External BGP route to 10.20.0.0/1610.20.0.0/16 via next-hop 172.16.4.1172.16.4.1 (Administrative Distance: 2020, Metric: 100100)

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

Show answer & explanation

Answer: 172.16.3.1, because the /22 subnet mask provides the longest matching prefix for the destination address

Answer

172.16.3.1, because the /22 subnet mask provides the longest matching prefix for the destination address.
The fundamental decision rule for IPv4/IPv6 packet forwarding is Longest Prefix Match (LPM). When a router consults its forwarding table, it selects the route with the most specific (longest) subnet mask that covers the target IP address. Here, host address 10.20.105.4210.20.105.42 falls within all four candidate ranges, but /22/22 (22 matching network bits) is more specific than /21/21, /19/19, or /16/16. Consequently, the router routes to next-hop 172.16.3.1172.16.3.1 without evaluating Administrative Distance or protocol metrics.

Step-by-Step Solution

1
Determine which routing entries encompass the destination IP address 10.20.105.42
All four entries match: 10.20.0.0/16 (10.20.0.0 - 10.20.255.255), 10.20.96.0/19 (10.20.96.0 - 10.20.127.255), 10.20.104.0/21 (10.20.104.0 - 10.20.111.255), and 10.20.104.0/22 (10.20.104.0 - 10.20.107.255).
Before comparing routing metrics, all matching candidate prefixes must be identified.
2
Apply the Longest Prefix Match (LPM) rule across all valid matches
The prefix length of /22 (22 matching network bits) is the most specific subnet mask compared to /21, /19, and /16.
Routers always evaluate prefix specificity first; Administrative Distance (AD) and metric are only compared when two identical prefix lengths exist from different sources.
3
Select the next-hop associated with the longest prefix entry
The entry 10.20.104.0/22 points to next-hop 172.16.3.1.
Because /22 has the highest number of matching bits, the router immediately forwards the packet to 172.16.3.1.

Key Concept

Longest Prefix Match (LPM) Routing Precedence
Question 174Question

A network administrator is configuring a GigabitEthernet 0/1 trunk link on switch SW-BuildingA to connect to SW-Core. The link is intended to carry traffic for VLAN 10 (Data), VLAN 20 (Voice), and VLAN 99 (Management). VLAN 99 is configured as the native VLAN on both switches. The administrator executes the command `switchport trunk allowed vlan 10,20` on interface GigabitEthernet 0/1. Immediately after executing the command, remote management access to SW-BuildingA via VLAN 99 drops. Which of the following best explains why management traffic is no longer traversing the trunk link?

Show answer & explanation

Answer: The explicit allowed VLAN list replaces the default allowed range and excludes VLAN 99, preventing its frames from traversing the trunk regardless of native VLAN designation.

Answer

The explicit allowed VLAN list replaces the default allowed range and excludes VLAN 99, preventing its frames from traversing the trunk regardless of native VLAN designation.
On an 802.1Q trunk, the allowed VLAN list controls which VLANs can send and receive frames across the link. When `switchport trunk allowed vlan 10,20` is executed without the `add` keyword, it replaces the entire allowed list with only VLANs 10 and 20. Because VLAN 99 is excluded from this list, all frames associated with VLAN 99 are blocked, causing loss of management access.

Step-by-Step Solution

1
Analyze the command executed on the trunk interface.
The command `switchport trunk allowed vlan 10,20` sets the active allowed list on the interface strictly to VLANs 10 and 20.
By default, trunks allow all VLANs (1-4094). Specifying a list parameter without the `add` keyword overwrites the existing allowed list.
2
Evaluate the interaction between native VLAN configuration and allowed VLAN lists.
VLAN 99 is designated as the native VLAN, but it is not present in the allowed list (10, 20).
An 802.1Q trunk will only forward frames for VLANs that are explicitly present in its allowed VLAN list, even if that VLAN is configured as the native VLAN.
3
Determine the impact on management traffic.
Management traffic belonging to VLAN 99 is pruned and dropped at interface GigabitEthernet 0/1.
Because VLAN 99 is excluded from the allowed list, remote management traffic sent over VLAN 99 cannot traverse the link.

Key Concept

802.1Q Trunk Allowed VLAN Filtering
Question 175Question

A network administrator is provisioning a dedicated management VLAN to connect 14 top-of-rack switch interfaces and 1 central default gateway interface. To minimize wasted IP address space, which of the following CIDR prefix lengths represents the smallest subnet mask that can accommodate all 15 required interfaces?

Show answer & explanation

Answer: /2727

Answer

The prefix length /2727 is the smallest subnet that provides enough usable IP addresses for 15 host interfaces.
The prefix length /2727 allocates 5 host bits (3227=532 - 27 = 5), providing 252=302^5 - 2 = 30 usable IP addresses. This is the smallest valid subnet mask that accommodates all 15 required host interfaces (14 switches + 1 default gateway).

Step-by-Step Solution

1
Determine the total number of required usable host IP addresses.
14 switch interfaces + 1 default gateway interface = 15 usable IP addresses required.
Every active device interface on the subnet requires a unique usable IPv4 address.
2
Calculate usable host capacity using the IPv4 host capacity formula (2n22^n - 2).
For n=4n=4 host bits (/2828), capacity is 242=142^4 - 2 = 14 usable hosts. For n=5n=5 host bits (/2727), capacity is 252=302^5 - 2 = 30 usable hosts.
Two IP addresses in every IPv4 subnet are reserved for the network ID and broadcast address and cannot be assigned to hosts.
3
Select the smallest prefix length that meets or exceeds the required 15 hosts.
The prefix length /2727 provides 30 usable host IP addresses, satisfying the 15 host requirement with minimal address waste.
A /2828 subnet only provides 14 usable addresses, falling short by 1 IP address.

Key Concept

IPv4 Subnet Mask Selection and Usable Host Calculation
Question 176Question

A network administrator is troubleshooting connectivity on an enterprise server host configured with the IP address 10.205.75.142/2110.205.75.142/21. To establish proper firewall routing rules, the administrator needs to identify the complete range of usable host IP addresses within this specific subnet. Which of the following represents the correct usable host IP address range for this subnetwork segment?

Show answer & explanation

Answer: 10.205.72.110.205.72.1 to 10.205.79.25410.205.79.254

Answer

The usable host range for the 10.205.75.142/2110.205.75.142/21 subnet is 10.205.72.110.205.72.1 to 10.205.79.25410.205.79.254.
The CIDR notation /21/21 indicates that the first 21 bits represent the network prefix, corresponding to a subnet mask of 255.255.248.0255.255.248.0. In the third octet, subnets increment by multiples of 88 (256248=8256 - 248 = 8). For an IP address with 7575 in the third octet, the network boundary starts at 7272 (9×8=729 \times 8 = 72), making the network address 10.205.72.010.205.72.0 and the broadcast address 10.205.79.25510.205.79.255. Excluding the network and broadcast addresses yields the usable range of 10.205.72.110.205.72.1 through 10.205.79.25410.205.79.254.

Step-by-Step Solution

1
Determine the subnet mask and third-octet block size for a /21 CIDR prefix.
A /21 prefix leaves 11 host bits (3221=1132 - 21 = 11). The subnet mask is 255.255.248.0255.255.248.0. The third octet increment (block size) is calculated as 256248=8256 - 248 = 8.
The CIDR prefix defines the boundary between the network portion and host portion of the IP address.
2
Find the network address by calculating the nearest lower multiple of the block size (8) for the third octet (75).
Multiples of 8 in the third octet are 0,8,16,24,32,40,48,56,64,72,800, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80. Since 75 falls between 72 and 80, the network ID is 10.205.72.010.205.72.0.
The network ID is the first IP address of the subnet segment.
3
Determine the broadcast address for the subnet segment.
The next subnet begins at 10.205.80.010.205.80.0, making the broadcast address of the current segment 10.205.79.25510.205.79.255.
The broadcast address is the last address in the subnet block immediately preceding the start of the next block.
4
Calculate the usable host IP address range by subtracting network and broadcast addresses.
First usable host IP: 10.205.72.110.205.72.1. Last usable host IP: 10.205.79.25410.205.79.254.
Usable host addresses consist of all IP addresses between the network address and the broadcast address.

Key Concept

Calculating CIDR Subnet Boundaries and Usable Host Ranges for Non-Byte-Aligned Prefixes
Question 177Question

A network technician is setting up a router for a newly expanded office. The organization requires that if an active link goes down, the router should automatically discover alternate paths and update its routing table without requiring manual administrator intervention. Which of the following network technologies should be implemented to fulfill this requirement?

Show answer & explanation

Answer: Dynamic routing protocols

Answer

Dynamic routing protocols provide automatic network discovery and dynamic route recalculation upon link changes.
Dynamic routing protocols enable routers to dynamically advertise reachability, discover topology changes, and update routing tables automatically when network links go down.

Step-by-Step Solution

1
Identify the core network requirement in the scenario.
The router must dynamically recalculate paths and update routing tables upon link failure without manual configuration.
The scenario highlights the need for continuous automated reachability updates.
2
Evaluate the function of dynamic routing protocols.
Dynamic protocols (such as OSPF or RIP) automatically learn remote networks and adjust active paths when topology shifts happen.
Dynamic routing automates path determination and routing table updates.

Key Concept

Dynamic Routing Protocols and Route Convergence
Question 178Question

During a network security audit, an incident response analyst discovers a compromised host with the IPv4 address 172.16.89.138/22172.16.89.138/22. What is the broadcast address for the subnet to which this host belongs?

Show answer & explanation

Answer: 172.16.91.255

Answer

The broadcast address for the subnet is 172.16.91.255.
For an IP address of 172.16.89.138/22172.16.89.138/22, the subnet mask is 255.255.252.0. The third octet increments in multiples of 4 (256252=4256 - 252 = 4). The network containing the third octet value of 89 starts at 88 (172.16.88.0172.16.88.0). The subnet spans up to 172.16.91.255172.16.91.255, making 172.16.91.255172.16.91.255 the broadcast address where all 10 host bits are set to 1.

Step-by-Step Solution

1
Determine the subnet mask and block size from the CIDR prefix /22.
A /22 prefix corresponds to a subnet mask of 255.255.252.0. The block size in the third octet is 256252=4256 - 252 = 4.
The prefix /22 leaves 10 host bits across the third and fourth octets, giving 210=10242^{10} = 1024 total IP addresses (44 blocks of 256).
2
Identify the network address for the host IP address 172.16.89.138.
The third octet boundaries increment by 4 (0,4,8,,84,88,920, 4, 8, \dots, 84, 88, 92). Since 89 lies between 88 and 91, the network address is 172.16.88.0.
The network ID uses the lowest address in the block formed by the multiple of the block size (8888).
3
Calculate the broadcast address of the subnet.
The broadcast address is 172.16.91.255.
The broadcast address has all 10 host bits set to 1, which corresponds to the last IP address in the 172.16.88.0172.16.91.255172.16.88.0 - 172.16.91.255 range.

Key Concept

IPv4 Non-Octet Boundary Subnetting and Broadcast Address Calculation
Estimated Time:2m 0s
Question 179Question

A network administrator is replacing static routes with a dynamic routing protocol across an enterprise network. Which TWO of the following are key operational features provided by dynamic routing protocols? (Select TWO.)

Select all that apply

Show answer & explanation

Answer: Automatically discover reachable remote subnets through neighbor information exchanges; Automatically recalculate alternate paths when a active network link fails

Answer

Dynamic routing protocols automatically discover remote networks by communicating with neighboring routers and recalculate valid backup paths when a link failure occurs.
Dynamic routing protocols automatically discover remote subnets by exchanging network state and path metrics with neighboring routers. In addition, when network topology changes or a link fails, dynamic routing protocols recalculate paths and update routing tables without manual intervention.

Step-by-Step Solution

1
Identify the primary purpose of dynamic routing protocols
Dynamic routing protocols automate route discovery and topology maintenance across routers.
Static routing requires manual route entry, while dynamic protocols continuously update routing tables based on network state.
2
Evaluate the choices against dynamic routing protocol functionality
Discovering remote subnets from neighbor routers and dynamically re-routing traffic around failed links are core features of dynamic routing protocols.
Protocols like OSPF, EIGRP, and RIP continuously share state and metric information to update network topology maps.
3
Distinguish routing from addressing and NAT services
Layer 3 addressing remains essential for routing interfaces, and address translation is handled by PAT/NAT rather than dynamic routing protocols.
Routing protocols direct traffic between subnets using Layer 3 IP headers; they do not remove IP addressing requirements or perform NAT overload functions.

Key Concept

Dynamic Routing Protocol Features and Operations
Estimated Time:1m 0s
Question 180Question

A network administrator is designing and hardening VLAN trunking parameters across enterprise switches to ensure proper frame encapsulation, DTP negotiation, and VLAN prune/allow list configurations. Match each VLAN configuration parameter or frame header attribute on the left with its corresponding operational behavior on the right.

Click a left item, then click its matching right item

Items

switchport trunk native vlan 99
switchport trunk allowed vlan add 10,20
switchport mode dynamic auto
IEEE 802.1Q Tag Structure

Matches

Show answer & explanation

Answer

Each configuration command or 802.1Q frame attribute corresponds to a specific operational mechanism: native VLAN designation handles untagged frame egress, the allowed list 'add' modifier preserves existing allowed VLAN definitions while appending new ones, dynamic auto modes passively negotiate trunking via DTP, and 802.1Q tagging places a 4-byte header containing VLAN ID and CoS fields immediately following the Ethernet Source MAC address.
The matching pairs correctly connect switch configuration commands and frame attributes to their exact operational outcomes: 'switchport trunk native vlan 99' specifies untagged frame handling for VLAN 99; 'switchport trunk allowed vlan add 10,20' preserves existing allowed VLANs while appending VLANs 10 and 20; 'switchport mode dynamic auto' passively waits for DTP requests; and the IEEE 802.1Q Tag Structure inserts a 4-byte header (TPID 0x8100 and TCI) between the Source MAC address and the EtherType field.

Step-by-Step Solution

1
Analyze native VLAN behavior under IEEE 802.1Q standard.
Identified that native VLAN traffic bypasses 802.1Q encapsulation and egresses the trunk port untagged.
802.1Q trunk ports send and expect native VLAN frames without a tag header for backward compatibility with unmanaged switches.
2
Evaluate CLI syntax for trunk allowed VLAN modification.
Determined that using 'switchport trunk allowed vlan add' appends entries without clearing existing allowed VLANs.
Omitting the 'add' parameter overwrites the entire permitted VLAN list, which is a common misconfiguration error.
3
Evaluate Dynamic Trunking Protocol (DTP) operational modes.
Matched dynamic auto to passive trunk negotiation.
Ports in dynamic auto mode respond to incoming DTP request frames from dynamic desirable or trunk ports but will not actively initiate negotiation.
4
Analyze Ethernet frame fields for IEEE 802.1Q encapsulation.
Matched 802.1Q header details to its 4-byte structure containing TPID 0x8100, PCP, DEI, and 12-bit VLAN ID.
The 802.1Q tag is inserted between the Source MAC address and the original EtherType/Length field in Ethernet frames.

Key Concept

VLAN Configuration, DTP Operational Modes, and 802.1Q Frame Header Tagging Mechanics
Estimated Time:2m 30s
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