The MPLS WG Archive

[George Swallow <swallow@cisco.com>]

> > No, if the packet arrives unlabeled then you necessarily had 
> > to have given out an explicit null label for that FEC.  
> 
> Seems to me there are a lot of potential reasons for an unlabelled packet
> turning up at the downstream LSR besides the downstream LSR offering
> implicit NULL.

Yes!  But the only valid one is that you gave out an explicit null.
So you check that you did in fact do that.  If not it's an error.

> > The reference to BGP was just for VPNs.  I haven't given a 
> > lot of thought to CSC yet.
> 
>  I have to admit I have trouble envisioning applicability to other than LDP
> or RSVP.

The applicability to other things comes with the FEC stack.  I've also
suggested that the error reporting procedures use the error subcode to
point to where in the stack the error occurred.  So far my co-authors
have not gotten back to me.  So I'll post my suggestions to the list
later today.

> > No, I meant that you're neighbor gave you an implicit null 
> > label so that instead of your normal swap operation of remove 
> > and replace the top label, you just remove the label.
> 
> I get the feeling we're crossing up explicit and implicit NULLs in this
> dialog. my understanding is that implicit null would never appear in the
> dataplane. 

Right!

> Your reference above to appending an implicit NULL to the stack

That's why appending an implicit null results in a PoP!

> runs counter to my understanding.

...George

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George Swallow             Cisco Systems                  (978) 936-1398
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