[rbridge] C Bit Clarification

[Donald Eastlake]

Hi David,

On Tue, May 11, 2010 at 4:03 PM, David Michael Bond
<david.bond at iol.unh.edu> wrote:
> Hello Everyone,
>
> I have a question regarding the draft. It’s just a minor clarification.
>
>
>
> Section 4.1.1 says:
>
> The "C" bit shown in Figure 4.2 is not used in TRILL. It MUST be set to zero
> and is ignored by receivers.
>
>
>
> In a transit RBridge situation if an RBridge receives a TRILL data frame
> with the C bit set to one in either the Outer or Inner VLAN should it
> transparently forward the frame or clear the C bit?

I think this is mostly talking about the Inner.VLAN.

Although TRILL specifies the Outer VLAN ID and priority (which affects
queuing) to be used if a frame is sent tagged, it runs above 802.1Q
ports. For 802.3, which is what the standard focuses on, the C bit
should be clear, but I suspect the bit would just be ignored on
receipt, if it were a 1, by most existing 802.1Q/802.3 ports. However,
for more exotic link types, like 802.5, which I don't know much about
and which are not covered by the TRILL standard, I think you might
have to set the C (actually CFI) bit in the Outer.VLAN.

For the Inner.VLAN, I think transit RBridges should just transparently
forward it.

> Also, I believe “[802.1Qad] S-tags, …” Should be “[802.1ad] S-tags, …”

Yes. Only [802.1ad] appears in the References, so this should be caught later.

Thanks,
Donald

> Thanks,
>
> David
>
>
>
>                     David Bond
>
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