routing & switching

[jaye1234]

Can anyone explain to me the principles of routing & switching & their key differences? I am confused over these two aspects.
Also what steps does a switch take in building a forwarding database or switching table & how is that used to forward packets?

This is really driving me nuts. Any help appreciated.

Thanks in advance.

[Prometheus_Fire]

Put simply:

• Routing operates at layer 3 of the OSI model and is concerned with IP addressing and getting IP packets to remote networks. This is where IP addresses work.
  
• Switching works at layer 2 of the OSI model and is concerned with flooding, forwarding or filtering data frames to connected devices on the same network. This is where MAC addresses work.

Hope this helps. If you feel your question has been answered, don't forget to mark it as SOLVEd.

[jaye1234]

Thanks prometheus fire for your reply.
I understand that a router is layer 3 & works on the ip address & that a switch is layer 2 & works using MAC addresses.
What i was concerned with is how the router bases its forwarding decisions compared to how a switch works. You say a switch is concerned with flooding, I thought it was a hub that flooded data out all ports. If a switch can flood, forward or filter data frames how does it make the decision to do either & how does it decide on the exit port?

[Prometheus_Fire]

That's only partially correct.

Switching:

1. When a switch receives a frame it looks at the source MAC address first.
2. If it doesn't have an entry in its Content Addressable Memory (CAM or Area which the MAC address table lives) it adds the source MAC address and port into its CAM.
   
3. Flooding: The switch then looks at the destination MAC address and if it doesn't have an entry in its MAC address table it treats the frame as an unknown unicast and it floods it out all switchports EXCEPT (This is important) the one it received it on.
   
4. Forwarding: If the switch does have an entry in its MAC address table for the destination MAC address it then forwards the frame out of the switchport that the destination MAC address is mapped to in its MAC address table.
   
5. Filtering: If a switch receives a frame with a MAC address that it knows exists on the switchport it received the frame on it will filter or drop the frame.

Routing

As for how a router makes forwarding decisions, it depends in part on the routing protocol you use. Basically though where switching only cares about host based hardware addressing routing only cares about the network portion of IP addressing.

For instance: If you have 2 routers and they share a common network segment on 192.168.0.0/30. Router1 serial 0/0 will have an external IP address of 192.168.0.1 and Router2 serial 0/0 will have an external IP address of 192.168.0.2. This way they can communicate with each other.

On top of this Router1 has a directly connected network of 172.16.1.0/24 (It's Ethernet 0/1 internal IP address is 172.16.1.254) and Router2 has a directly connected network of 10.2.2.0/24 (It's Ethernet 0/1 internal IP address is 10.2.2.254). The routers only advertise the network portions of the IP addressing scheme to each other via their routing updates across their serial interfaces. Neither of them care about the last octet.

This is a messy explanation, but using the information above network traffic will basically be passed like so:

1. ComputerA (172.16.1.10) wants to request a web page from ServerB (10.2.2.2).
2. ComputerA sends an ethernet frame via a switch to its default gateway Router1 (172.16.1.254)
3. Router1 strips off the ethernet header and trailer and adds a layer 3 header.
4. Router1 knows that the 172.16.1.0/24 network is attached to Router2 so it sends the packet over its serial 0/0 interface (192.168.0.1) to the Router2 serial 0/0 interface (192.168.0.2).
5. Router2 receives the packet and strips off the layer 3 information.
6. Router2 then sends the packet over it's internal interface (10.2.2.254) via an ethernet switch on the 10.2.2.0/24 network to ServerB 10.2.2.2.
7. ServerB then sends the requested data via a switch back to its default gateway: Router2 (10.2.2.254).
8. Router2 strips off the ethernet header and trailer and adds a layer 3 header.
9. Router2 knows that network 172.16.1.0/24 is attached to Router1 so it sends the packet to it over their serial interfaces.
10. Router1 receives the packet and strips off the layer 3 information.
11. Router1 sends the frame via a switch to ComputerA (172.16.1.10).

NB: During this process while the source and destination IP addresses don't change, the source and destination MAC addresses are altered to reflect the sending and next devices.

Clear as mud?