The MPLS WG Archive

[Curtis Villamizar <curtis@workhorse.fictitious.org>]

In message <3ACA41AB.2A3ACB59@sandburst.com>, Eric Gray writes:
> Curtis,
> 
>     As I understand it, LDP is the preferred protocol for use
> with MPLS BGP when BGP peers are not adjacent with
> respect to an MPLS domain.  Suppose that MPLS BGP is
> used to establish many VPN services which are possibly
> being used to provide a label switching service to enterprise
> customers.
> 
>     For that matter, suppose that LDP is being used to signal
> and maintain <your favorite> VPN technology's LSPs.


Eric,

You wouldn't be using LDP on the outer tunnel, just the inner VPN
tunnel.  The outer tunnel is a RSVP-TE tunnel.  We are already lab
testing this at a customer and it interoperates nicely with other
people's BGP/VPN LDP implementations.

Because the TE tunnel is rerouted around a failure the LDP adjacency
that ride inside the TE tunnels never go down.  The LDP TCP connection
sees some temporary loss at worst and then continues as if no topology
change occurred because the logical adjacency never went down.  This
works as long as there is a brief holddown on the TE tunnel and it
reroutes before the holddown expires.  In practice works great.

Where you want to maintain LDP (and IGP) adjacency is where a router
is reloaded for something like software upgrade and nothing else in
the IGP changes during the reload time.  If you have two route
processors on the router that was reloaded, that should be a switch in
well under a second.  Rumor has it that some routers with one route
processor take a few minutes to reload so for those, if the topology
changes you'd rather route around them.

So please remind me, exactly what was the point of your objection.
:-)

Curtis


> You wrote:
> 
> > In message <3ACA3769.1E1C006F@sandburst.com>, Eric Gray writes:
> > >
> > >     I also suspect that you may be understating the advantages
> > > of maintaining session context.  Consider the impact to the
> > > network if LSPs are dropped lightly and these LSP are in turn
> > > being used to carry (stacked) LSPs.
> > >
> > > --
> > > Eric Gray
> >
> > You wouldn't be using LDP in that case.  ;-)
> >
> > Curtis