Forwarding issues related to MACs starting with a 4 or a 6 (Was: [c-nsp] Wierd MPLS/VPLS issue)

Mike Jones mike at mikejones.in
Tue Dec 6 12:50:38 UTC 2016


> MACs that didnt make it through the switch when running 4.12.3.1:
>
>     4*:**:**:**:**:**
>     6*:**:**:**:**:**
>     *4:**:**:**:**:**
>     *6:**:**:**:**:**
>     **:**:*B:**:6*:**
>     **:**:*F:**:4*:**

Can anyone explain the last 2 for me?

I was under the impression that this bug was mainly caused by some
optimistic attempt to detect raw IPv4 or IPv6 payloads by checking for
a version at the start of the frame. This does not explain why it
would be looking at the 5th octet.

I also would assume that there must be something else to the last 2
examples beyond just the B or F and 4 or 6 because otherwise it would
match way too many addresses to have not been noticed before. Perhaps
the full MAC address looks like some other protocol with a 4 byte
header?

Thanks,
Mike


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