In this and next posts, we will learn about L2 (Ethernet) switching and L3 (IP) routing. This post will cover:

• ARP and IP packet
• changes in Ethernet header during switching/routing process
• entry changes in the tables for server, switch and router (i.e. routing, ARP and MAC tables)

Switching and Routing

Please take a close look at the figure above, especially server/router MAC address, IP address, and switch/router port (interface) numbers in it, as it illustrates the configuration of a network to be discussed here and in the next post.

As seen in the left side of the figure above, if both sender (e.g. SVR1 in the figure) and receiver (e.g. SVR2) are located in the same network (LAN), the two communicate with each other directly through the switch between them (e.g. S1) without having to go through the router (e.g. R1). At this time, each packet consists of the following fields:

On the other hand, if the two are not located in the same network (LAN), they communicate through the router (R1). In this case, Ethernet header fields have different values on this and the other side of the router.

Sending packets from SVR1 to R1

Sending packets from R1 to SVR3

Ethernet Switching

1. SVR1 sends ARP Request

• SVR1 at IP address of 1.1.1.10 is trying to send a packet to SVR2 at IP address of 1.1.1.20.
• To send a packet, a server must refer to the routing table first. Through routing table lookup, the server (e.g. SVR1 here) finds out that the destination address is located in the same network as itself (having no gateway at the routing entry 1.1.1.0/24 indicates both sender and receiver are in the same network), and that the Outgoing Interface (OIF) is lan1 (note that there is just one server port in the figure, lan1).
• Now, SVR1 refers to its ARP table to find the MAC address of the destination IP address 1.1.1.20, but finds the table empty (having no related entry is called "ARP miss").
• So, SVR1 sends an ARP request to port lan1 to obtain the MAC address of SVR2 (1.1.1.20). At this time, the ARP request packet consists of the following fields:

• Now, this packet is received by S1 (Switch 1), and the source MAC address of the received packet (the switch does not care whether the received packet is IP or ARP) is learned. Then, the following is recorded in the MAC table at S1: {MAC address m1 is connected to fe1 port}.
• Immediately after the source MAC is learned, S1 checks the destination MAC address of the received packet, and finds out it is broadcast-type. So, it performs flooding to all the ports except for the receiving port (or to all the ports belonging to the same VLAN if VLAN support is provided). That way, the ARP request packet is received by SVR2 and Router (R1).
• From the target IP address of the received ARP request packet, R1 notices the address does not match any of the addresses that it has (i.e.1.1.1.1 or 2.1.1.1). So, it discards it.​

2. SVR2 responds with ARP Reply

• SVR2 also checks the target IP address of the received ARP request packet, and finds out the address matches its own address. So, it enters the MAC address corresponding to 1.1.1.20 in the Sender MAC field of an ARP reply packet, and sends the ARP reply packet to lan1 port. At this time, the ARP reply packet includes the following fields:

• S1, upon receiving the packet, learns the source MAC and records the following in the MAC table: {MAC address m2 is connected to fe2 port}.
• Then, it sends (unicasts) the packet to fe1 port by referring to the MAC table where m1, the destination MAC address of the received packet, is mapped to fe1.
• Next, SVR1 records the received value (m2, the MAC address assigned for 1.1.1.20) in its ARP table.

3. SVR1 sends IP Packet to SVR2

• SVR1, now ready to send an IP packet to SVR2, sends one with the following fields to lan1 port:

• Upon receiving the packet, S1 (Switch 1) attempts to learn the source MAC address, and finds out the address m1 has already been learned. So, no MAC learning is needed. now, it checks for m2, the destination MAC address, in the MAC table, and finds out it is mapped to fe2 port.
• Accordingly, the packet is sent to fe2 port and received by SVR2.

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