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Given for instance,

interface Tunnel200
tunnel destination 110.153.1.1

There's a static route for 110.153.1.1, pointing to physical interface Gi0/0. Gi0/0 is only used for tunnel200.

Under show interface summary,

the TxBS/RxBS of tunnel200 is always slightly lower than the physical interface Gi0/0. Shouldn't it be the other way round as there is overhead for the GRE tunnel?

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    The overhead is only visible (and countable) outside the tunnel...
    – Zac67
    Mar 14 at 11:47
  • @Zac67 oh thanks for clarifying! Is there a reason why the TxBS/RxBS is always slightly lower for the tunnel rather than equal to the physical interface?
    – iridescent
    Mar 14 at 12:22
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(from comment) The overhead is only visible (and countable) outside the tunnel. Roughly you should see (outer packet header + GRE header) * number of packets more traffic on the outside (physical) interface than on the tunnel interface.

Depending on where you're watching, a physical interface might also be counting L2 traffic (add the L2 header size * number of packets), and additionally might be receiving non-tunnel traffic (CDP/LLDP, BPDUs, OSPF, ...) - low-volume unless the interface has more uses.

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