The MPLS WG Archive

["Andrew G. Malis" <Andy.Malis@vivacenetworks.com>]

Giles,

> > >MPLS in IP or MPLS in GRE is a tunnel as far as I'm concerned - so you
> > >shouldn't withdraw the labels.
> > >
> > >But yes, I take your case about backup tunnels.  Really this is the
> > >standard issue of layering and holddown timers isn't it?  IF the backup
> > >tunnel doesn't kick in within x seconds THEN withdraw the advertised
> > >labels?
> >
> > It's not just that.  Another problem the current language is, for example,
> > if A is advertising VC labels to B using DU, A needs to somehow figure out
> > if B has a tunnel (of any sort) back to A before it can advertise the label
> > to B.  I think that's impossible in the general case.
>
>I'm ahead of you here (I think!)
>
>Luca and I had just this debate - I was trying to persuade him to take
>the text out on this very pretext.  Somehow he persuaded me that you
>should still withdraw the label if you don't have a tunnel.  My memory
>is rubbish, but I think the argument is that if A OR B doesn't have a
>tunnel then it will withdraw its label and the (bidirectional) circuit
>will go down.  Also of course in the (non-general) case of RSVP-TE
>tunnels A and B should both know the status of tunnels in either
>direction?
>
>Giles

I think it's impossible to make this check in the general case.  Here are 
some reasons:

1. The IP addresses used in RSVP and LDP signaling may be 
different.  Imagine an LSR using a distributed architecture over a number 
of line cards.  The RSVP tunnels are anchored in the line cards of their 
associated interfaces, and use a unique IP address for each line card in 
the RSVP sender object.  However, the LDP signaling for VC labels is 
anchored in a central processor card, and uses its own IP address.

If you're using OSPF or ISIS within an area, then presumably you can look 
at the RSVP and LDP IP addresses and confirm in the link state database 
that the RSVP and LDP addresses are from the same router ID.  However, that 
is not possible across areas, between ASes, or when using non-link-state 
routing protocols.

2. Even if an incoming TE tunnel's sender address matches that used for the 
LDP signaling, the receiving LSR has no way of knowing whether or not the 
sending LSR will choose to use that particular TE tunnel to carry the VCs; 
it may be restricted to IP traffic engineering.

So the sentence really has to go, since I think it's not implementable.

Cheers,
Andy