Switch

Today, I learned about the switch. 

What is switch?? 🤨

 

Features:

• Hosts have dedicated direct connection to switch.
• Switches buffer packets.
• Ethernet protocol used on each incoming link, but no collisions; full duplex.
  • Each link is its own collision domain.
• Example: 'A-to-A' and 'B-to-B' can transmit simultaneously, without collision.

Here is the question: how does switch know A' reachable via interface 4, B' reachable via interface 5?

Answer is "each switch has a switch table, each entry": 

• (MAC address of host, interface to reach host, time stamp)
• It looks like a forwarding table!

 

Now, we are gonna talk about the self-learning of switch.

Switch learns which hosts can be reached through which interfaces.

• When frame received, swich learns location of sender: it is reachable through the incoming interface.
• Switch records (sender, interface) pair in its table like this.

 

When a frame arrives at switch:

• Switch compares the frame's Source MAC address to the switch's MAC address table.
• If the source is unknow Switch adds (Source MAC address, incoming port# the frame was received on switch) to the switch MAC address table.

 

• Switch compares the frame's Destination MAC address to the switch's MAC address table.
  • If there is an entry, switch forwards the frame out the associated port (through a unicast message).
  • If there is no entry, switch floods (it forwards the frame out all its ports except the port that the frame was received on).

Note: a switch has a switch table and a node has an APR table.

 

Here is the example of self-learning, forwarding.

 

 

Now, we are gonna talk about the interconnecting switches.

• A switch can be connected to other switches

Question: sending from A to G - how does S1 know to forward frame destined to G via S1, S4 and S3?

Answer: self learning! (works exactly the same as in single-switch case!)

 

This is the table figuring out the interconnecting switches.

Switches vs routers

Both are stored-and-forward:

• Routers: network-layer devices (examine network-layer headers)
• Switches: link-layer devices (examine link-layer headers)

Both have forwarding tables:

• Routers: compute forwarding table using routing algorithms, IP addresses
• Switches: learn forwarding table using self-learning, flooding, MAC addresses