Network Layer Lesson-2

Network Layer (Protocol Data Unit)

Network Layer, Protocol Data Unit

Network Layer, Protocol Data Unit
Network Layer (CCNA) Lesson-2


Network Layer Protocol Data Unit:-

send packet frame form one device into another throw switch

When we want to send packet frame from host1 to host2

Host 1 need mac address of host2

So host1 generate an address resolution request (broadcast) requesting mac address

Address resolution request (ARP) :- is a broadcast sent to destination address all networks.

So host2 reply to the ARP broadcast, reply unicast to host1

Protocol Data Unit


Destination IP

Source IP



What is the ping: - testing utility, work with Internet control messaging protocol (ICMP)

Ping has two parts: - echo, echo reply

Address resolution protocol -function: - resolve IP addresses to their relevant mac address and put them in a table

Host1 encapsulates the original ICMP packet and put them inside frame (inside data portion of frame)

Host 2 will de-encapsulate the frame, extract the original packet and match destination ip with its ip then send reply

By default encapsulation is HDLC (high-level data link controller) cisco proprietary protocol for sending data over synchronous serial connection.

Serial connection:-

router has serial connection to CSU DSU

Router has serial connection to CSU DSU which is termination point or demarcation point for the ISP (internet service provider)

CSU – DSU (channel service unit, data service unit) also called DCE –data connection equipment

CSU –DSU connects to ISP cloud and in the other end there will be another CSU-DSU connecting to your remote side router

CSU-DSU provides function to your router:-

Its provide clocking or physical bit transfer rate between two end devices (routers)

IP routing theory:-

Routing is process going from one broadcast domain into another

routing process from one broadcast domain into another

Ex: Host-1 can’t ping Host-2

Host1 create ICMP echo message with source ip address and Destination ip

Then host1 create an Ethernet frame for transmission on local data link (layer2)

Host 1 does not have mac address of host2, so send ARP request to requesting mac address of device host2 IP

This ARP request reach router 1, router1 see that host2 IP not connected to it

Router 1 reply’s back to host 1 with mac address port1 (own address)

Now host 1 create Ethernet frame with its own source mac address and with destination mac for port1 and encapsulate the original ICMP inside it

Router 1 receives the frame and matches destination mac then de-encapsulation the frame and extracts the packet.

Inside each router (IP routing table) has list of all remote destination and compare with IP routing table.

Router 1 fined that host2 network is available throw serial and encapsulate on serial with DHLC

Router 1 encapsulates the original echo packet into DHLC encapsulation and push it throw port1

Router2 de-encapsulation it and extract the original echo packet

Router2 compare destination address with its IP routing table, router 2 want to create an Ethernet frame to transmission.

Router 2 send ARP broadcast to know mac address of host 2 IP.

Host 2 receive ARP and unicast to router 2 reply.

Has mac address and create frame to destination mac of host 2

Host 2 receive and match mac address and de-encapsulation the frame and extract the packet

Host 2 sends back echo reply.

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