Lesson 9 - Bridging/Switching Learning Process

In the previous lesson we looked at the Ethernet and a hub operation. We classified hub to be a layer 1 device as it does not understand any headers used by upper layers of our networking model. It simply forwards the bits it receives out all remaining ports. Even though, they do provide basic connectivity to our hosts, they also reveal a lot of weaknesses. For details look at the lesson 8.

A more intelligent and robust device that can replace a hub is a layer 2 switch. You've already learned how to navigate in IOS, and provide the switch with a simple configuration. With this lesson we begin a more serious exploration of layer 2 through layer 4 functions starting with Layer 2. This is good enough excuse to brush upon encapsulation/de-encapsulation process, and the structure of the headers.

Enapsulation Process Re-Visited
This is a quick review of encapsulation process I would present to my nine year old son (if he wanted to learn more).

1. Assuming that TCP session is already complete, the application is forming the request (data) which is sent down to the layer 4. Layer 4 process places source and destination port numbers in the header (Pic. 1).

Pic. 1 - Application sends data to the Transport layer.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

2. Transport layer sends the segment down to the layer 3 for processing. This payload ends up encapsulated in an IP header with the source and destination IP addresses added in the header (Pic. 2).

Pic. 2 - Transport layer sends the segment to the Internet layer.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

3. The layer 3 sends the packet down to the layer 2 (Data-Link) for processing. Layer 2 adds its own header (here the Ethernet header) with the source and the destination MAC addresses (Pic. 3).

Pic. 3 - Internet layer sends packet down to Data-Link layer.

Then, this whole 'thing' is converted into bits and put onto the wire.

Now, we can see what happens when the bits are sent to the port of the switch to reach the destination computer. Let's bring back the Ethernet header to see what we find in it (Pic. 4).

Pic. 4 - Ethernet Header

Icons designed by: Andrzej Szoblik - http://www.newo.pl

In this header there are two pieces of information that switches use to build their mac-address-table (CAM) and make forwarding decisions. Those are: source and destination MAC addresses. And here is how it works.

Initially, the mac-address-table (aka CAM) is completely empty (Pic. 5).

Pic. 5 - Content Addressable Memory (CAM) initially is empty.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Sooner or later, some computers begin to transmit something across the network. In my example, the PC1 (source MAC address: 1111.1111.1111) begins transmission to PC3 (destination MAC address: 3333.3333.3333). Below is the sequence of events.

1. PC1 with the source MAC address of 1111.1111.1111 sends the Ethernet frame to the destination MAC address of 3333.3333.3333 (PC3).

2. SW1 receives the frame on port F0/1. It 'reads' the source MAC address and maps it to the receiving port in its CAM (like shown in the Pic. 6).

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NOTICE
Switches learn MAC addresses by reading the SOURCE MAC from the INCOMING frames (going towards the switch) only. They do not learn anything when the frame leaves the switch.

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Pic. 6 - SW1 learns dynamically 1111.1111.1111 by reading the source MAC address from the incoming frame.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

3. SW1 then, reads the destination MAC address and will try to find the outbound port for that destination. Since 3333.3333.3333 has not yet been mapped to any port, the switch will performflooding (Pic. 7).

Flooding is the act of sending a frame out of all active ports except the port where the frame arrived. 

There are few reasons why switch decides to flood a frame:

• Switch does not know where the destination host is = unknown MAC address.
• The destination MAC address is broadcast: FFFF.FFFF.FFFF.
• The destination MAC address is multicast.

 Pic. 7 - SW1 Floods unknown destination MAC address 3333.3333.3333.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

4. Computer with MAC address other than 3333.3333.3333 drop the incoming frames. PC3 is the destination of the frame so it further processes it (de-encapsulation). Meanwhile, the SW2 learns the source MAC address on the receiving port F0/12 and maps it in its CAM. Since, as of right now, it does not know where 3333.3333.3333 resides, it also floods the frame (Pic. 8).

Pic. 8 - SW2 is learning 1111.1111.1111 on F0/12 and flooding the frame.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

5. PC3 (3333.3333.3333) responds the PC1 (1111.1111.1111). SW2 receives the frame sourced with 3333.3333.3333 on port F0/2. It puts this in its mac-address-table (CAM), mapping it to the inbound port F0/2. Next, it will read the destination MAC address in the frame (1111.1111.1111) and consults it with its CAM entries. It finds the outbound port F0/12 where this address has already been learned from the incoming frame. This time, the transmission is not flooded as SW2 has the mapping in the table. SW1 receives the frame on its port F0/12. It reads the source MAC address (3333.3333.3333) and maps it to the receiving port F0/12 in its CAM table. Then it looks at the destination MAC address (1111.1111.1111) and finds the outbound port which is F0/1. NO FLOODING THIS TIME on either switch! All illustrated in the Pic. 9.

Pic. 9 - Port-to-Port transmission in the reply as all MAC addresses in question have already been learned.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

Since all computers 'speak' now and then, the switches will learn their MAC addresses from the incoming frames by reading the source MAC address field in the headers. They will populate the CAM and like depicted in Pic. 10.

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NOTICE
All entries in the CAM table (mac-address-table) have a default aging timer which is 300 seconds (5 minutes). If the host do not refresh those entries by sending frames toward the port, the entries will be removed after 5 minutes. Of course, if the host transmits the frames again the incoming ports will map them again.

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Pic. 10 - Content Addressable Memory (CAM) full.

Icons designed by: Andrzej Szoblik - http://www.newo.pl

In the next lesson we will look at Cisco Discovery Protocol.In lesson 11, we're going to put all the pieces together to show you the practical applications of what you have learned.