generic question about L3 switches

Replies

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  Rahul Jamgade

  durga

  Network devices till now have been very loclised to the fucntionalities they perform. for example the L2 devices are called switches(MAC address) , L3 devices are called routers (IP routing), can anyone specify why L3 switches have come into existence or reason out why we need L3 devices at all?
  
  This is only to trigger a discussion, no specific requirement to answer

  Basically Switches are called L2 devices reason being they work at layer 2 of OSI model and its basic function is to work just like hub except for the fact that it provides separate collision domain so that the bandwidth is dedicated between any two nodes that are communicating. However when you have two different logical network segment (ip network address range of node A is different from that of Node B) and hence intelligent device is required to do this routing which is done by Layer 3 (IP works at Layer 3).
  However when you need to have very fast traffic flow between two different logical network (two different IP address range) then layer 3 devices such as router can fundamentally address the issue, but it requires very fast data transfer that is where router cannot work. However switches can work much fast as compared to the router because of its ASIC capabilities (ASIC : Application Specific Integrated Circuits).Hence the combination of router and switches are used to have routing as well as fast data transfer facility. This functionality is known as Route Once Switch many.This is what you called as L3 switching. L3 switches are used at high level of network topology i.e. between end point distribution switches and Router at the top level and generally used in big enterprises where the nodes concentration is higher.
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  durga ch

  Right,it's faster hardware based switching which makes them better choice, (at least in recent times), though many might argue routing is also a form of switching . Anyone might well say that routing is at L3 and switching is at L2,
  it can be counter argued that, routing is switching using IP addresses (ie l3) andswitching is switching using mac addresses (l2). The generic functionalities of routers like IP look up, policy based routing, flow based operations,
  usage of routing protocols are basically due to the L3 nature of routers , these functions cannot be applied to mac addresses.now coming to L3 switches, they use the the better of both worlds- hardware fastness of L2 switch but switch using Ip addresses, that actually leads them to have routign protocols run even EGPs like BGP and can do QoS same like Routers. Going further into the discussion , are the ASRs. Also, if noticed L3 switches tend to have more NICs than normal L3 router and thus can handle data from more logical LANs . To route packets form one VLANs to another in normal L2 switches we generally apply .1q, but with l3 switches, it really makes it easy to move from L2 to l3 routing.