Ethernet bridging questions

- D
- David Goodenough
  
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posted
17 years ago

Sun, May 7, 2006 4:24 PM

I'm planning on setting up ethernet bridging on a Windows XP system, in order to get a connection with OpenVPN working. From what I understand, when I bridge the two ethernet adapters in XP (*), it makes some sort of announcement that the bridge has been set up.

(*) One being the physical NIC, the other the "virtual" OpenVPN NIC.

What is the nature of this announcement? Is there an RFC that covers the details? Is it routable, or does it only go as far as the local subnets attached to the two interfaces?

Enquiring minds want to know.

Thanks in advance.

- D
- Dom
  
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posted
17 years ago

Sun, May 7, 2006 5:19 PM

The most obvious announcement comes in the form of STP, which XP will enable by default. An IP host may announce its presence in the form of a gratuitous ARP. Neither is routable.

- A
- Albert Manfredi
  
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posted
17 years ago

Sun, May 7, 2006 9:13 PM

If you're really bridging, and not routing, then in principle there's no need for any announcement. A new host connecting via an Ethernet bridged through the Windows machine would go about its business the usual way, e.g. beginning with a DHCP broadcast request, and the bridge sends on the broadcast to the VPN side. And the reply would come back the same way, because the switches between the new host and the router will have learned the way back.

If the Windows machine is running STP or RSTP, then Bridge Protocol Data Unit (BPDU) frames are exchanged between this switch and other switches in the catenet, as described in IEEE 802.1D, which you can download for free from the IEEE web site.

Bert