1. Which layer of the OSI model is responsible for logical addressing and determining the best path to route packets between networks?
- a.Data Link (Layer 2)
- b.Network (Layer 3)✓
- c.Transport (Layer 4)
- d.Session (Layer 5)
The Network layer (Layer 3) handles logical addressing, such as IPv4 and IPv6, and performs routing to select the best path across internetworks. Routers operate at this layer. The Data Link layer uses physical MAC addresses within a single segment, while the Transport layer manages end-to-end delivery.
2. A host is configured with the IPv4 address 172.16.45.10/20. What is the network address of the subnet it belongs to?
- a.172.16.45.0
- b.172.16.40.0
- c.172.16.0.0
- d.172.16.32.0✓
A /20 mask (255.255.240.0) makes the third octet increment in blocks of 16 (256 - 240 = 16). The subnets in the third octet are 0, 16, 32, 48, so 45 falls in the 32 block, giving a network address of 172.16.32.0. The host portion is zeroed to identify the subnet.
3. On a Cisco switch trunk link using IEEE 802.1Q, how are frames belonging to the native VLAN handled by default?
- a.They are dropped for security
- b.They are tagged with the highest VLAN ID
- c.They are sent untagged✓
- d.They are tagged with VLAN ID 1
With 802.1Q, frames on the native VLAN traverse the trunk untagged by default, while all other VLANs are tagged with a 4-byte VLAN identifier. The native VLAN must match on both ends of the trunk to avoid a mismatch. For security, administrators often change the native VLAN away from the default VLAN 1.
4. In a Spanning Tree Protocol topology, which switch becomes the root bridge?
- a.The switch with the lowest bridge ID✓
- b.The switch with the most ports
- c.The switch with the highest MAC address
- d.The switch configured last
STP elects the root bridge as the switch with the lowest bridge ID, which is the combination of bridge priority and MAC address. If priorities are equal (default 32768), the lowest MAC address breaks the tie. Administrators lower the priority on the desired switch to control root placement.
5. A router has two routes to 10.1.1.0/24: a static route with administrative distance 1 and an OSPF route with AD 110. Which route is installed in the routing table?
- a.Both routes, load-balanced equally
- b.The static route, because it has the lower AD✓
- c.Neither, they conflict and both are rejected
- d.The OSPF route, because OSPF is dynamic
Administrative distance measures the trustworthiness of a routing source, and the route with the lower AD is preferred. A static route (AD 1) is preferred over an OSPF route (AD 110) to the same destination. The less-trusted OSPF route is kept in the OSPF database but not installed while the static route is valid.
6. Which command correctly configures a default static route that forwards all unknown-destination traffic to next-hop 203.0.113.1?
- a.ip route 0.0.0.0 0.0.0.0 203.0.113.1✓
- b.ip route 203.0.113.1 0.0.0.0 0.0.0.0
- c.ip route 203.0.113.1 255.255.255.255 0.0.0.0
- d.ip default-gateway 203.0.113.1
A default route uses 0.0.0.0 0.0.0.0 to match any destination, followed by the next-hop address, so 'ip route 0.0.0.0 0.0.0.0 203.0.113.1' is correct. It is the gateway of last resort used when no more specific route matches. The 'ip default-gateway' command only applies to a device that is not routing (ip routing disabled).
7. In OSPFv2, two routers on the same Ethernet segment fail to form an adjacency. Which mismatch is a common cause?
- a.Different serial numbers
- b.Different interface descriptions
- c.Different hostnames
- d.Different hello/dead timers or mismatched area IDs✓
OSPF neighbors must agree on parameters including hello and dead intervals, area ID, authentication, subnet mask, and MTU to reach the FULL adjacency state. A mismatch in any of these prevents the neighbor relationship from forming. Hostnames and descriptions are cosmetic and do not affect adjacency.
8. A network uses Port Address Translation (PAT). How does the router distinguish return traffic for multiple internal hosts sharing one public IP address?
- a.By using different MAC addresses
- b.By using VLAN tags
- c.By assigning each host a separate public IP
- d.By using unique source port numbers✓
PAT (NAT overload) maps many private addresses to a single public IP by translating and tracking unique source port numbers for each session. The router records these port-to-host mappings in its translation table so return traffic is delivered to the correct inside host. This conserves scarce public IPv4 addresses.
9. A standard IPv4 ACL is applied to filter traffic. On which criterion can a standard ACL match?
- a.Application-layer payload
- b.Source and destination IP plus port
- c.Source IP address only✓
- d.Destination MAC address
Standard IPv4 ACLs (numbered 1-99 and 1300-1999) filter based only on the source IP address. Extended ACLs are required to match source and destination addresses, protocols, and port numbers. Because standard ACLs are less granular, they are typically placed close to the destination to avoid blocking too much traffic.
10. In a controller-based (SDN) network architecture, which plane does the centralized controller primarily assume from the individual network devices?
- a.Data (forwarding) plane
- b.Management plane only
- c.Control plane✓
- d.Physical layer
SDN separates the control plane from the data plane, centralizing control-plane decisions such as routing and topology in a controller. The controller programs the devices, which continue to forward traffic in their data plane. A southbound interface (for example, OpenFlow or NETCONF) carries instructions from the controller down to the devices.