Network Engineering 101- How packet travels through the network

 

ARP Table : Mapping of IP Address to MAC address

Mac Address Table : Mapping of Switchports to Mac Address

Routing Table : Mapping of IP Networks to Interfaces

Steps

1.)   Host A has data for Host D

-        Host already knows Host D’s IP Address

-        Host  A creates a L3 header

-        Host A needs to learn Default Gateway’s Mac address

2.)   Host A sends an ARP request for 11.11.11.1

3.)   Switch X Receives Frame

-        Switch X learns MAC address Mapping on Port 2

4.)   Host B discards frame

5.)   Router receives ARP request – Router learns Host A’s ARP mapping

6.)   Router generates ARP response

7.)   Switch X receives frame

-        Switch X learns MAC address mapping on port 3

-        Switch X forwards frame out port 2

8.)   Host A receives ARP response

-        Host A learns Router’s ARP mapping

9.)   Host A creates L2 header and sends packet

10.)  Switch X receives frame

-        Switch X already knows mapping for port2

-        Switch X forwards frame out port 3

11.)  Router receives packet and strips L2 Header

12.)  Router consults routing table

-        22.22.22.0/24 network exists on eth2 interface

-        Router needs to learn MAC address for 22.22.22.40

13.)  Router sends as ARP request for 22.22.22.40

14.) Switch Y receives frame

-        Switch Y learns mapping address on port 4

-        Switch Y floods frame out all ports

15.) Host C discards frame

16.) Host D receives ARP request

-        Host D learns router’s ARP mapping

17.)  Host D generates ARP response

18.)  Switch Y receives frame

-        Switch Y learns Mac address on port 5

-        Switch Y forwards frame out port 4

19.) Router receives ARP response

- Router learns Host D’s ARP mapping

20.)  Router creates L2 header and sends packet

21.) Switch Y receives frame

- Switch Y already knows mapping for port 4

- Switch Y forwards frame out port 5

22.) Host D receives packet

- Host D strips L2 header

- Host D strips L3 header

- Host D receives data ( finally !)

23. Host D responds with data for Host A

- Host D already knows Host A’s IP address’

- Host D creates a L3  header

- Host D needs to send the packet to its default gateway

- Host D already knows the default gateway’s Mac address

24. ) Host D creates a L2 header and sends packet

25.) Switch Y receives frame

- Switch Y already knows mapping for port 5

- Switch Y forwards frame out port 4

26.) Router receives packet and strips L2 header

27.) Router consults routing table

- 11.11.11.0/24 networks exists on eth1 interfaces

- Router already knows Host A’s Mac address

28.) Router creates a L2 header and sends packet

29.) Switch X recives frame

- Switch X knows mapping for Port 3 –

Switch forwards frame to port 2

30.) Host A receives packet

- Host A strips L3 header

Host A strips L3 header

-        Host A receives data