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Ethernet loops and dual protocols

By Bayk ·
I have the same PC connected to the same Ethernet segment (one hub) via two NIC's.
One NIC has TCP/IP loaded onto it the other has the IPX/SPX protocol loaded onto it. I know that loops are not allowed on Ethernet, and this is a physical loop configuration. However, it has been suggested that it is not a logical loop. In other words an Ethernet frame containing an IPX/SPX addressed code cannot go through OSI layer 3 to the application level on the card running TCP/IP and therefore no harm is done.
On the other hand I suspect the constraint is an Ethernet one because the damage done is at OSI layers 1 and 2. My problem is that I cannot find anywhere the reason why looping is not allowed on an Ethernet segment.

Can anyone help please? Ineed a solid argument to validate my hunch.
Thank you.

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Ethernet loops and dual protocols

by schmrand In reply to Ethernet loops and dual p ...

There is no loop. There would not even be one if both cards were speaking the same protocol. Think of what is happening to the packets - they are being taken in by the card at the physical layer and translated up to the protocol layer to be handed to the protocol stack. There is nothing in the card, the stack or the application that would push those packets back out through the other card. A hub with that configuration would be repeating the packets back onto the same segment[1], so that would be a loop, but your PC just looks at the packets and either uses or discards them.

[1]and would repeat the repeated packets, ad infinitum and a wedged repeater.

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Ethernet loops and dual protocols

by Bayk In reply to Ethernet loops and dual p ...

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Ethernet loops and dual protocols

by NetTek In reply to Ethernet loops and dual p ...

What you associate with ethernet is not a consideration at layers 3 through 7 of the OSI model. The factors associated with an ethernet network are only present at layer 1, the physical layer (in the form of ethernet hardware), and layer 2, the datalink layer (because ethernet uses MAC addresses). It cannot really be said that "looping is not allowed on an ethernet segment" because there is really no correlation. What you consider as a "loop" (sending a packet from one nic on a host out on the network, and coming back in to the host on a second nic card) would involve protocols, and ethernet is not protocol dependant (you can use NetBEUI, IPX/SPX, TCP/IP, Banyon Vines, etc.).

The problem with looping externally is that in using IPX or IP, a host is identified with a MAC address. Any packets that leave a host, and are destined for that same host, always return on the same MAC address from which it was sent. Actually, those packets never even leave the NIC card and go out on thenetwork. It really has nothing to do with ethernet.

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Ethernet loops and dual protocols

by Bayk In reply to Ethernet loops and dual p ...

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Ethernet loops and dual protocols

by GIJoe In reply to Ethernet loops and dual p ...

Is this still a problem?

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