Packet flow in routing/switching

Created by Tran Anh Tai

Situation 1: CPE1 talks with CPE2

- Assume at CPE1: ping 192.168.0.4

CPE1 checks its routing table to see where to send packet to. It detects that CPE2 is on same network.

Header of sending packet from CPE1.
SIP = 192.168.0.3, DIP = 192.168.0.4
SA = m1, DA=? (unknown yet)

CPE1 broadcasts ARP to know MAC address of 192.168.0.4. After this step it knows DA as m2. Consequently, Switch1 updates its learning table
MAC (m1) ------ Port (fe1)
MAC (m2) ------ Port (fe2)

At this time, CPE1 just sends packet to CPE2 with below header.
SIP = 192.168.0.3, DIP = 192.168.0.4
SA = m1, DA=m2

Situation 2: CPE1 talks with CPE3
- Assume at CPE1: ping 192.168.11.5

CPE1 checks its routing table to see where to send packet to. It detects that CPE3 is not on same network. It has to send packet to its gateway (192.168.0.1) at interface R1(Lan1)

Header of sending packet from CPE1.
SIP = 192.168.0.3, DIP = 192.168.11.5
SA = m1, DA=? (unknown yet)

CPE1 broadcasts ARP to know MAC address of 192.168.0.1. After this, CPE1 know MAC of gateway as a1. Simultaneously, Switch1 updates its learning table.
MAC (m1) ------ Port (fe1)
MAC (m2) ------ Port (fe2)
MAC (a1) ------ Port (fe0)

CPE1 then sends packet to gateway with below header
SIP = 192.168.0.3, DIP = 192.168.11.5
SA = m1, DA=a1


Next, Router1 receives the packet, it check DIP (192.168.11.5) in its routing table to see where to forward packet. Ok, it detects that the packet should be forwarded to interface R2(Lan2, 192.168.11.1).

Router1 creates below packet.
SIP = 192.168.0.3, DIP = 192.168.11.5
SA = a2, DA=?

Now, it broadcasts ARP to know MAC address of 192.168.11.5. After this, Router1 knows MAC of 192.168.11.5 as m3. Switch2 updates learning table as
MAC (m3) ------ Port (fe1)
MAC (a2) ------ Port (fe0)

Finally, Router1 sends packet to CPE3 as below header.
SIP = 192.168.0.3, DIP = 192.168.11.5
SA = a2, DA= m3

Take note
- Switch just works at L2 (MAC address)
- Router is to transmit packets between different networks based on routing table (L3, IP address).

Routing in router:
+ Static routing (route add ...)
+ Dynamic routing (using protocol such as RIP, which broadcasts periodically its routing table to neighborhood and update the best routes).

Q&A
1. Why Home Gateway doesn't need to use RIP protocol?