A Made IT project


Layer 3 Switching

The reason
The idea of switching on layer 3 was first proposed by a company called Ipsilon. Different standards emerged and at the moment of writing this document there is still no definitive standard. There are some solutions though that are incorporated in a standard (like ATM). For detailed descriptions see the vendors Web-Sites mentioned.

The problems, or network history in an eye-wink
Before looking at the solutions it is good to see what is causing the problems.
First, when networks emerged there was a need to extend the maximum distances, so repeaters were invented. After that, people wanted to connect LANs over Wide Area Networks (WANs). This problem was solved by building bridges. When more and more networks were linked together routers were needed to make the whole thing managable.
And when we had that we wanted more speed. More speed meant creating switches. The first were just called switches and switched on OSI layer 2, so they were actually switching bridges and now we need more speed and have real time traffic and need 3rd layer switches, which are actually switched routers.
But since a router is situated on layer 3 it adds delay and there is another problem with routers, they route. This means that data can follow different routes to their destination with as a result that some packages will arrive earlier than packedges send before them. This makes it impossible to run time sensitive applications.

The solutions
The problems that need to be solved is the different routes for packages from the same job and the delay added by the routers.
Solving different routes seems to be the easiest one. Just take a fixed path and the problem is solved, but implementing this is the real problem.
Solving the delay factor, once there is a fixed path, is actually a piece of cake, because with a fixed path bridging is all that is needed.

The propositions for call setup
The solution for a fixed path is a call setup, where there is a detection of the destination and an agreement on the path that is used for the time the link is used. The technics used sound like ATM and that is were everyone agrees on; ATM will be the future. But in the meantime we have an installed base of several thousands of routers which are connected through dedicated lines and no ATM. So there is a need for a temporary different solution.

Let's see what the posibilities are on the 3rd and 2nd layer of the OSI reference model:

Layer 3Route everywhere
Route once, switch afterwards
Layer 2/3Switch here you can, route where you must
Layer 2Switch everywhere

The first option, route everywhere, is the actual router as we know it today. And the last one, switch everywhere, is a real bridge. So those two options are not our concern in this document.

Route once, switch afterwards VS Switch where you can, route where you must
Those two options have a basic different view of the network. The switch where you can device sees the network as being flat and is mainly a bridge. Only when it isn't able to determine where the data must go it will use it routing capabilities.
The otherone is mainly concerned about routing. It accepts the network as a must route network. Only when it has the knowledge of the path the data ought to go it will switch.
Later on we will see the importance of those two different views of the network.

The technologies
The now known architectures can basically be split up into three different functions:

Conventional Router

The Conventional Router
This one is added to give you a good comparison. The route calculation and packet processing take place in the software on layer 3. This means that packets need to be moved from the layer 2 hardware interface to layer three.
The information exchange between the two routers are the standard routing protocols.

Routing Switch

The Routing Switch
Routing calculations takes place at layer 3 in hardware or software, while the actual packet processing takes place at layer 2. They are mostly just like ordinary routers. The speed gain is accomplished by reducing the amount of features supported and moving as much logic as possible into hardware.
The information exchange between the two router switches are the standard routing protocols.
Supported by: e.g. Ascend, Bay Networks, Cisco, Extreme, Foundry, IBM, Intel, Madge.

Flow switch

The Flow switch
Route calculation and packet processing takes place at layer 3 until a flow is detected. The flow is then switched at layer 2 through the network. This way of handling data traffic much looks like ATM or frame relay with a VC for long lived traffic.
The information exchange between two flow switches is accomplished through flow management protocols.
Supported by: e.g. Ipsilon, ATM-forum.

Switched router

The Switched router
Route calculation and packet processing takes place at layer 3. By adding a tag to the packed information the amount of layer 3 processing is reduced. According to the Cisco view you need tag-edge routers that add the tag to non-taged packets and in the rest of the network you need tag-switches which actually give you the speed gain.
The information exchange between two switched routers is accomplished through the tag distribution protocol.
Supported by: e.g. Cisco, Ascend, IBM, 3Com.

None of the above are a world wide standard and none are interchange able with another.

For a more close description on the different schemes please contact the vendors and read the specs given with a certain product carefully!!!

Ipsilon Technical White Paper on IP Switching