Layer2 and Layer3 Switches

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Aug 14th, 2008
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What is the basic difference between Layer2 and Layer3 switch?


If we say that layer3 switch does the work of both switching and routing then why do we use routers?


Secondly, if we say layer2 switch is used for peer to peer communication and layer3 switch is for process to process communication.

What exactly peer to peer and process to process communications are in short?

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Joseph W. Doherty Thu, 08/14/2008 - 05:35
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"What is the basic difference between Layer2 and Layer3 switch? "


One knows how to forward L2 traffic, e.g. Ethernet, the other L3 traffic, e.g. IP.


"If we say that layer3 switch does the work of both switching and routing then why do we use routers? "


Often L3 switches provide much higher forwarding rates than pure routers. This is usually accomplished by dedicated hardware for the former. However, the hardware also often limits other capabilities, both for hardware support, like types of interfaces, and software feature support beyond just forwarding L3 traffic.


For example compare the features of a 3560 L3 switch with any of the ISRs.


The distinction between the two, L3 switch vs. router, can blur on some equipment. Compare the 6500 with the 7600.


"Secondly, if we say layer2 switch is used for peer to peer communication and layer3 switch is for process to process communication.

What exactly peer to peer and process to process communications are in short?"


Sorry, don't really understand this question. Peer-2-peer and process-2-process can be conducted at either L2 or L3. Both L2 and L3 provide transport, L3 though, normally is routable. Support for peer-2-peer or process-2-process is usually accomplished in a higher "layer" than L2 or L3.


Faizan Shaikh Thu, 08/14/2008 - 06:42
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Joesph,


Does it mean that a cisco router can be replaced by a layer 3 switch for routing?


If not, what are the reasons?


Thanx a ton!

Joseph W. Doherty Thu, 08/14/2008 - 09:57
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There's confusion about peer-2-peer and process-2-process. For instance, mention is made of process-2-process using IP port numbers, but there are no IP port numbers, their likely thinking of IP's UDP or TCP protocols which do support port numbers.

Joseph W. Doherty Thu, 08/14/2008 - 09:52
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"Does it mean that a cisco router can be replaced by a layer 3 switch for routing?"


In some cases, yes.


"If not, what are the reasons? "


When you have a need for features not available on a comparable L3 switch. For instance, could be as simple as interface type. Most small L3 switches only support Ethernet interfaces, but even the small ISRs routers support, serial, ISDN, frame-relay, ATM, VPN, etc.

Faizan Shaikh Thu, 08/14/2008 - 11:19
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Thanx a ton friends!


Is it true that layer 3 switch doesn't support wan connection?


Is this the other reason not to replace router with layer 3 switch?

Joseph W. Doherty Thu, 08/14/2008 - 15:39
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"Is it true that layer 3 switch doesn't support wan connection? "


No, some do. Many don't

Joseph W. Doherty Fri, 08/15/2008 - 02:51
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"is it possible to convert a layer 2 switch to layer 3 switch?"


Usually no, unless their software and/or hardware can be upgraded. Within the Cisco line, the 4500 chassis have supervisor modules that are L2 or L3.

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