routing vs bridging question

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cpthk
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routing vs bridging question

Post by cpthk » Wed Mar 30, 2011 10:02 pm

In openVPN, we learn that routing does not transmit non-IP protocols like broadcast, IPX. The advantage of bridging is that it does transmit those protocols. But I am confused that why the routers we buy at out local computer store, which should follow the routing mode, can transmit non-IP protocols? Aren't they suppose to act like openVPN that it just ignores those packets? What's the reason?


Thanks.

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janjust
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Re: routing vs bridging question

Post by janjust » Thu Mar 31, 2011 7:34 am

This is a common misconception , largely caused by the names used : "routing" vs "bridging".

A tun-style setup (--dev tun) is often referred to as a routed setup. Tun adapters can forward only IP based traffic, by design.

OpenVPN also supports a tap-style setup (--dev tap) without resorting to bridging. In this case, full Ethernet frames are forward via the tunnel, so you can forward IPX/AppleTalk/whatever type of traffic over such a link. Your typical home router resembles this.

Bridging mode is a special case of a tap-style setup where the tap adapter and a local ethernet adapter (or another tap-adapter, for that matter) are bridged to form a single adapter. All traffic coming in on one interface is automatically re-broadcasted on the other interface(s). This can be handy to bridge two networks together (like a virtual hub) but it does have a serious impact on performance and scalability.

PS there's no need to thank me for doing your homework :mrgreen:

cpthk
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Re: routing vs bridging question

Post by cpthk » Thu Mar 31, 2011 11:44 am

hmm..., that's interesting

so the home "routers" we buy are actually doing the bridging, not the routing, that's kind of confusing. I should probably say our home routers are routing + bridging

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maikcat
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Re: routing vs bridging question

Post by maikcat » Thu Mar 31, 2011 1:07 pm

hi cpthk,

just curious...

what router are you using?


michael.
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janjust
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Re: routing vs bridging question

Post by janjust » Thu Mar 31, 2011 3:57 pm

some home routers do routing+bridging, mostly when buying a wireless router.
Most home routers don't do bridging, but they do routing+switching:
- all traffic which is forwarded out the WAN port (to your cable/DSL provider) is routed; it is, generally speaking, quite hard to route IPX traffic via your provider.
- your local PCs which are all connected via wires to the router form a single subnet, so the router acts as a switch here.

As I said, some home routers make the wired and wireless networks appear as one, which is usually done using bridging.

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