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TOPIC: Source MAC address

Source MAC address 9 years 2 days ago #23329

  • skepticals
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I wanted to clarify something.

What would the source mac address be when a frame leaves a router with two interfaces? Let's say that the frame entered S0 and left E1. Would it have S0's or E1's MAC address? I took a simulator test and it said S0, but that doesn't seem right.

Thoughts?
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Re: Source MAC address 9 years 1 day ago #23332

  • toddwoo
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Thats not right at all...

Fire up ethereal, (wireshark) then ping your local gateway, then ping something on the other side of the gateway.

Your notice that the source mac address is the same, even though the packets are coming from the gateway, or a device beyond the gateway.

Todd
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Re: Source MAC address 9 years 1 day ago #23333

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Humm, maybe I did not explain my question well enough or I just don't understand your response.

To my knowledge - between hops - the MAC addresses are stripped from the frame. So, if I send a frame to my gateway router and my router sends it to the next hop, my gateway router would strip my computer's MAC address out of the frame and put its MAC address in for the source. Is this incorrect?
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Re: Source MAC address 9 years 1 day ago #23335

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Yes... the router would put the MAC address of the outgoing interface as the new source address.

Strictly speaking the frame is discarded once it enters the incoming interface, and a new frame is created on the outgoing interface.

One "sticky" point is that most serial interfaces don't use MAC on layer 2.
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Re: Source MAC address 9 years 1 day ago #23337

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If most Serial connects don't use MAC at layer 2, what would the source MAC address be? The MAC address of the Ethernet interface that originally received the packet? or would it be the sending computer?
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Re: Source MAC address 8 years 11 months ago #23358

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Ok my boss said my visio skills are lacking... so to pratice I made up a qukck visio doc...



Here is the docs on a frame relay frame.. notice no mac address.

http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/frame.htm#wp1020669
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