The MPLS WG Archive

[George Swallow <swallow@cisco.com>]

Dave -

> Hi George:
> 
> > <snipped>
> > 
> > 
> > Excellent point you raise.  One that I've been worried about. 
> 
> OK
> 
> >  I've sent some new procedures and error conditions to my 
> > co-authors on lsp-ping.  One change that I have requested is 
> > is that you must check whether the protocol that would have 
> > distributed the FEC you have is actually associated with the 
> > interface that you received the packet on.  
> 
> OK , that will catch a discontinuity when you've neglected to configure LDP
> on a link and its a direct adjacency. That's one scenario.... 
> 
> > And that that 
> > protocol distributed a label (including NULL) for that FEC.  
> 
> So we're inferring correctness here. If I got a ping on an interface, and
> there is a control adjacency that handed out a FEC label binding on that
> interface and the FEC seems legit, then everything must be OK? 

No, if the packet arrives unlabeled then you necessarily had to have
given out an explicit null label for that FEC.  

> IMHO only an e2e ping that uses the 127/8 can actually catch a real problem
> when the unroutable packet comes up for air and the receiving LSR trys to
> forward it further...
> 
> > Note, I say associated since and LDP session can pertain to 
> > multiple interfaces and BGP can pertain to all core facing interfaces.
> 
> Are you envisioning self test for CSC...? 

The reference to BGP was just for VPNs.  I haven't given a lot of
thought to CSC yet.

> > 
> > Another change, intended to help diagnose the problem, is 
> > that when you do a trace with a Downstream Mapping TLV, you 
> > MUST include null labels in the stack. That way, if you reach 
> > the PHP router and it is doing a PoP and forward because it 
> > is the end of the LSP (got no label from it's neighbor) it 
> > will send back an empty stack, whereas if it was appending an 
> > implicit NULL label (still a PHP) it would send back a stack 
> > with just the null label as the first entry.
> 
> Did you mean "appending an explicit NULL (still a PHP)"?

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.

Cheers,

...George


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George Swallow             Cisco Systems                  (978) 936-1398
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