The MPLS WG Archive

[Eric Gray <eric.gray@sandburst.com>]

Eric,

    I agree that LDP is not a routing protocol.  I never said it was.  Check
the thread a little more closely.  Of course, you may not have been saying
that I did say this - even though your comments below were in reply TO
me.  :-)

    However, you do not have to use BGP - and that is what I did say.  In
fact, for some types of 'virtual private' services, you do not have to use a
routing protocol at all.

    Assuming you do use a routing protocol (for the task of ditributing routes
at least), there is no reason why <your favorite> label distribution protocol
could not be used to distribute labels for those routes.  Doing so with LDP,
just for instance, would require adding a new (set of) FEC(s) that include
some sort of VPN identifier (or route discriminator) along with the prefix.
IF I were going to define such a thing, I would define a new FEC object/TLV
that includes an 8 byte field for VPN-ID (or RD) and any of the currently
defined FECs as a sub-TLV (object).  In addition, it would be necessary to
add some verbiage around how the ingress selects an LSP for forwarding,
and - it seems to me - the big part of the task is over.

--
Eric Gray

You wrote:

> Distributing routes is easy enough.  The  tricky part comes when you need to
> decide whether  to use  a particular route.   It's especially tricky  if you
> happen  to have  received two  routes to  the same  destination.   It's even
> trickier if you want to make sure  that everyone's choice of a route is made
> in such a way that no routing loops are formed.
>
> That's why when you specify a  routing procedure, you don't just specify the
> messages used to distribute the  routes, you specify a decision procedure, a
> set of attributes used to compare routes, etc., etc., etc.
>
> That in turn is why it makes  no sense to say "let's distribute routes using
> LDP".   Route   distribution  doesn't   necessarily  require  BGP,   but  it
> certainly requires a routing protocol, and LDP is not a routing protocol.