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Suppose we want to connect a local area network (Ethernet) with a wide area network, the question arises is, can we connect a local area network with wide area network?

If we directly connect the LAN segment to the packet switch of the wide area network, then obviously every signal on the segment will be forwarded to the switch, to prevent this, can be use a bridge to connect this segment to the network switch (because bridge will only forward the packets if necessary)?

Another question arises is, every WAN Technology uses a specified frame format, suppose the WAN is using a frame format that is different from Ethernet frame format then what we will do?

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i think that you want to look for a router –  johnh Jul 19 '10 at 17:52
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@johnh: sometimes you don't want a router but you do want a single VLAN to be in 2 places at once. Not often, and probably not in this case, but it does happen. –  chris Jul 19 '10 at 19:40
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4 Answers

I recommend against doing this in most setups as:

  • every single broadcast packet on that Ethernet segment will be sent to the WAN link. This will cause a lot of unnecessary traffic on the (slower) WAN link and degrade its performance.
  • WAN links are generally point-to-point, whereas Ethernet is a broadcast medium. Not suitable for bridging.
  • There are valid reasons to do so, but they are specific use cases where you actually need layer 2 connectivity (see comments). In most applications, this isn't required.

If you're just looking to expand your corporate network, what you should do instead is:

  • Hook up your WAN to your router/firewall/whatever and assign a different network to your WAN link.
  • Then route packets appropriately over that link.

(regarding your point about frame formats - frame format conversion is one of the functions of a bridge. It Just Happens.)

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... and by "recommend" I mean "for $DIETY's sake, don't do it!" –  MikeyB Jul 19 '10 at 17:57
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This used to be common years ago. Products like the Shiva LanBridge, Ascend Pipline, and Cisco IOS would let you bridge Ethernet and/or Token Ring across dialup and ISDN links. It was a lot easier to bridge in a mulitprotocol environment (IPv4, IPX, DECnet, AppleTalk, Vines) than to set up routing for each protocol. Also, bridging doesn't require as much CPU power as routing. It usually doesn't make sense nowadays. –  Gerald Combs Jul 19 '10 at 18:18
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People still do this with mpls to bridge 2 remote networks together for things like vmotion. $$$ but if you gotta do it... –  chris Jul 19 '10 at 19:33
    
Looks like I mis-remembered the Shiva product. NetModem, maybe? –  Gerald Combs Jul 19 '10 at 19:43
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@chris,@Gerald: Great examples of older and newer reasons why you'd still want to do this. I was writing under a few unstated assumptions :) –  MikeyB Jul 19 '10 at 20:29
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There are a couple of issues here:

  • Purely physical/electrical - What's your WAN handoff? If you've got a T1 incoming, you can't plug it into an ethernet switch and have it work, despite them both perhaps using the same kind (RJ-45) of plug. T1s are ancient analog technology and run at +/- 12V. Ethernet is newer digital tech and runs at like +/- 3V. And the signaling is completely different.

  • Security - You usually don't want to directly connect your LAN to the WAN because you don't want to let just anyone out on the WAN have full access to your internal fileshares, printers, etc. Also, you don't want all your broadcast traffic to be broadcast across the want - a the very least it's going to eat bandwidth unnecessarily, and at worst if you've got some kind of metered link you could end up spending a lot of money for nothing.

Usually you put a router in the way as the interface - both physical and security - between WAN and LAN.

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Ethernet voltages: 10Mb = 2.2v; 100Mb = 3.15v; 1Gb = 4v. All are differential. –  Chris S Jul 19 '10 at 18:07
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If we directly connect the LAN segment to the packet switch of the wide area network, then obviously every signal on the segment will be forwarded to the switch

1: Read up the definition of a SWITCH

2: realize how REALLY wrong you are.

Seriously, SWITCHES are what SELECTS pacets, HUBS and BRIDGES are NOT doing this, unless it is a learning bridge.do not,BRIDGES only do when learning, which means efficiently they are switches.

It WORKS with a switch, EXCEPT if you have qa lot of broadcast traffic. It is STRONGLY adviced to use a router to further isolate traffic.

Another question arises is, every WAN Technology uses a specified frame format, suppose the WAN is using a frame format that is different from Ethernet frame format then what we will do?

Because your computer does not care. The computer sends a higher level packet (IP that is encapsulated in an ethernet frame if using an ethernet card. NATURALLY a WAN equipment that uses a different wire level format and accepts / turns out ethernet frames HAS to transpose them.

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A switch and a bridge are logically the same thing, it's just that a switch usually has more ports (and is newer). –  Ronald Pottol Jul 19 '10 at 18:28
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What do you mean by WAN?

Do you mean an internet connection? Do you mean that you're going to get a L2 circuit from your ISP to connect you to some other site in your metro area?

You possibly haven't described exactly what you want here...

In short -- you can use Ethernet as a wan technology. I've seen in plenty of places where you get "ethernet" from your ISP and packets you put into one port come out on some port in some remote location. Behind the scenes, it is likely either an MPLS or PBT-TE network of some sort. You don't need to know the magic.

There are other situations where you can connect 2 devices to a dedicated T1 circuit and ethernet goes in one device, gets converted to some internal T1 protocol (or dropped), and comes out the other device as ethernet.

Do you have some specific reason why you want to bridge your network directly to a WAN? What's at the other end of that WAN? Is it just "the internet" or are you just using the wan as an expensive / slow bridge to some other network somewhere else?

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