The Network Layer
This layer is responsible for logical addressing and routing.
Network Layer (Layer 3)
What is the PDU used at the network layer?
A packet
What protocol maps IPv4 addresses to MAC addresses?
ARP.
Command to enter global configuration mode.
configure terminal
Command to display the routing table.
show ip route
The primary protocol used at the network layer for communication.
IP (Internet Protocol)?
What is the purpose of the IPv4 “Protocol” field?
It identifies the upper-layer protocol (e.g., TCP = 6, UDP = 17).
What type of message is an ARP Request?
Broadcast.
Command to assign an IP address to an interface.
ip address [IP] [mask]?
Meaning of code “C” in a routing table.
Directly connected network.
The process of dividing large packets into smaller ones for transmission.
Fragmentation
What happens if the “Don’t Fragment” (DF) bit is set but the packet is too large for the next link?
The router drops the packet and sends an ICMP Fragmentation Needed message.
How does a switch handle an ARP Request when it knows the destination MAC?
Unicasts it to that port.
Command that shows interface IPs and statuses.
show ip interface brief?
What does the router prefer if multiple routes exist?
Lowest administrative distance.
When a packet traverses multiple routers, which two fields in the IPv4 header are changed at each hop?
The TTL and the Header Checksum
If two packets arrive on different interfaces but share the same destination IP, how does the router decide which to forward?
It checks the routing table for the longest prefix match.
Why might communication fail after replacing a NIC?
Stale ARP entries until refreshed.
Command that saves the current configuration.
copy running-config startup-config
What happens if no matching or default route exists?
Packet is dropped; ICMP unreachable sent.
A router receives an IPv4 packet with a TTL of 1. Explain what happens and what message is returned.
The router drops the packet and sends an ICMP Time Exceeded message back to the source.
What command displays a router’s active IPv4 interfaces and addresses?
show ip interface brief
What command on a router clears the ARP table?
clear ip arp
Command that sets an encrypted enable password.
Command that sets an encrypted enable password.
What does the route code “S” mean in S 192.168.10.0/24 via 10.1.1.2?
Static route.