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Router vs. Switch vs. Bridges

Posted on 2003-11-10
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Last Modified: 2007-12-19
I know Router is L3(Layer 3) device, switch is L2 device

my Q:

(1) What is the difference between switches and bridges?
(2) What is the difference between a L3 switch and a Router ?

thx
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Question by:codenamecharlie
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11 Comments
 
LVL 7

Expert Comment

by:NicBrey
ID: 9714029
Bridges and switches basicly perform the same function. Think of a switch as a bridge with much more ports.

A layer 3 switch perform the routing function (layer 3) in hardware, where a router would use CPU cycles and an operating system of some sort. The term "route one, switch many" apply with layer 3 switches. Only the first packet of a stream is passed to a router module or external router. The switch learn the destination address from the router and adds an entry to a table in memory. All packets of the same stream after that is switched between the input and output ports of the switch, completely bypassing the router.

Have a look at this link...
http://computer.howstuffworks.com/lan-switch.htm
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Expert Comment

by:sunray_2003
ID: 9714570
Check here ... you can know the difference

http://www.practicallynetworked.com/networking/bridge_types.htm

Sunray
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Expert Comment

by:praflik
ID: 9716796
In my opinion a bridge and switch have a major difference beyong the number of ports.

A bridge isolates traffic on a network segment.

A,B,C are ports on a bridge.  A1,A2,B1,B2,C1,C2 are workstations plugged into the network.

A1,A2===(A)Bridge(B)====B1,B2
                     (C)                
                       |
                       |
                    C1,C2

If A1 is sending a packet to A2 then the bridge will ignore that packet and not forward it because it knows that A1 & A2 are on the same segment.

If A1 is sending to B1 then the bridge will broadcast the packets to all other ports (B & C in this example).

In other words...if I'm  a bridge I will either ignore a packet or forward it to all ports.

A Switch will complete a circuit from sender to reciever instead of broadcasting on all ports when traffic crosses a segment.

A1,A2===(A)Switch(B)====B1,B2
                     (C)                
                       |
                       |
                    C1,C2

As with the bridge if A1 is sending a packet to A2 then the Switch will ignore that packet and not forward it because it knows that A1 & A2 are on the same segment.

If A1 is sending to B1 then the Switch will forward the packer to port (B) only.

Fundamentally the Bridge is only concerned with the source while the Swtches is concerned about the Source and Destination.

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LVL 3

Expert Comment

by:bartt
ID: 9816837
Hi codenamecharlie,

NicBrey is right about the difference between switches and bridges and the link he send in was good. Praflik doesn't the point really.

Both a bridge and a switch are working by the same principle and work with the following states:

- learning : the device is ready to accept packets
- flooding : the device broadcasts the packets because he doesn't know to which port the host is attached
- forwarding : the device knows which port
- filtering : just sending to traffic to right ports and if the hosts are on the same port the traffic will not go through the switch
- aging : delete old addresses from the MAC-table.

So IMHO a switch and a bridge have the same function except a switch has more ports.

Cheers
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LVL 1

Expert Comment

by:praflik
ID: 9819644
I thought the point was "What is the difference between switches and bridges?"

You say there is none I say there is, otherwise why not just call it a 12 Port Bridge.

Bridges don't forward they flood.  I think that is a significant differance.

The only time they would work the same is if you had a two port switch and were comparing it to a two port bridge.
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Expert Comment

by:praflik
ID: 9825301
One last thing.

Bridges are often used to connect unlike topologies (Ethernet-to-Token Ring) (Ethernet-Serial/T1).

Ethernet Switches only connect Ethernet Segements togther.
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Accepted Solution

by:
NicBrey earned 50 total points
ID: 9831278
praflik,

I do not agree with you. Bridges also learns which MAC addresses are on different ports and forward only out the port where the MAC addresses are located. They do not flood out all ports.

Here is a link
http://www.cisco.com/en/US/about/ac123/ac147/ac174/ac203/about_cisco_ipj_archive_article09186a00800c836c.html

Consider a 12 port bridge...   does that not do pretty much what a switch does??  Only difference is a switch do it in hardware (chip speed)while a bridge in software.  
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Assisted Solution

by:praflik
praflik earned 50 total points
ID: 9852431
NiceBrey,

I don't think that there neccessarily is one answer for this question.  As is often the case in this business there is no one standard we can look to and get an exact answer to the question.  There have been many companies that have developed what they consider to be a bridge and maybe some of them had the same capabilities as a switch at some point in their evolution.  Maybe the 3Com bridges works like I said and the Cisco bridges worked like you say.

I was simply trying to get to the essence of codenamecharlies's question.  He wanted to know the difference between the devices.

By your own most recent posting there are a couple, switches work in hardware and process in parallel so there is a difference beyond the number of ports.

Also, I have never heard of a switch that joins two unlike topologies like a bridge will so yes I believe there is a difference and that is what codenamecharlie wanted to know.
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