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I'm new to networking and was wondering if switches modify frames as they pass through?

I've heard that routers modify the contents of their packets as they pass through (which I believe is true), but I can't find a concrete answer about what modifications switches make to the frames that pass through it, if any.

Help would be appreciated!

Thanks

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  • I could swear this has been asked before.
    – Ricky
    Jan 16, 2015 at 4:14

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In general, layer-2 switching does not modify the frames.[1] A VLAN tag may be added or removed as necessary for frames between tagged and untagged ports.

A layer-3 switch is both a router (l3) and switch (l2). When it switches traffic, it's just like any other layer-2 switch. When it routes traffic between segments, then it's routing and will modify packets like any router (namely decrement ttl.)

[1] Not to confuse matters, but "carrier ethernet" hardware can do VLAN translation, which does, technically, modify frames.

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    Tagging/untagging a frame would change the checksum also.
    – richardb
    Jan 16, 2015 at 8:55
  • The FCS lives as "layer-1" and isn't something any NIC (or OS) ever (normally) passes around; much like the preamble, inter packet gap, etc., they aren't something the OS or driver deals with; it's handled by the layer-1 interface of the NIC chipset. (ethernet diagnostic hardware uses chips that can be set to a "raw" mode so they can see everything; errors and all.)
    – Ricky
    Jan 16, 2015 at 22:54
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    Also worth noting that like any other router it will strip the src/dst mac address and replace the source with its own mac addr and the dst with the mac addr of the destination host or next hop.
    – Mark
    Aug 5, 2016 at 13:06

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