Subject: cosntrained routing and MPLS

From: ravi panikkar <[email-masked]>
Date: Thu, 21 Feb 2002 14:17:11 -0800 (PST)

Hi
  I 'm an MPLS newbie and i'm currently looking at how
MPLS interacts with the underlying L3 
routing protocols.
    My understanding of how a QoS constrained LSP is
setup is as follows.The L3 routing protocols  uses QoS

routing to populate the routing table( example QoS
extensions to OSPF , Guerin ). MPLS then uses this
routing table to populate its LFIB which can 
be then used by CR-LDP to setup a constrained LSP
using  lets say an ER-LSP
     If this process is used,  an additional time
overhead will occur while the constrained LSP is being

setup  because of the following 1. the routes need to
be calculated once again 2. The MPLS data 
structures need to be populated again .Is this
overhead  permissible , is this how it actually
happens or am 
i totally lost here ??

Thanks
Ravi

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From: "Arun Kumar Dheena" <[email-masked]>
Date: Fri, 22 Feb 2002 17:37:57 +0530

Hi,
      The RFC 2547bis uses BGP to distribute the VPN routes. But, we can
modify LDP itself to carry the information carried in BGP like the VPN
routes, route targets and VPN labels etc (by defining, new TLVs and new LDP
messages).

The motivation for this is, firstly, the LDP-VPNs can provide the same
scalability as in mentioned RFC2547, as the intermediate LSR's (P routers)
need not maintain the VPN-Lables and routes. (This can be achieved by
maintaining LDP sessions between the PE LSR's). Secondly, we can avoid using
BGP for MPLS-VPNs.

Will this apparach be an alternate for BGP/MPLS VPN?
Or am I missing something  fundamental?
I want to know whether any work has been done in this direction.

Thanks in advance.
regards
arun

Wipro-CDC

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From: "Miguel Angel Chana" <[email-masked]>
Date: Fri, 22 Feb 2002 16:24:21 +0100

> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]On Behalf Of Arun
> Kumar Dheena
> Sent: Friday, February 22, 2002 1:08 PM
> To: [email-masked]
> Subject: LDP - VPN
> 
> 
> Hi,

LDP distributes LABELS bindings to FECs, those being IPv4, VPNv4, 
MAC addresses, VC's or whatever else. Imagine a situation where 
PEs are separated by LSR routers that do not have any notion of 
the FEC for which you are binding the label. Since they don't
understand the FEC, they can not give you a label binding. This
is why LDP directed sessions are used for anything else than
IPv4 (for example, Martini). Either that or you have to do all
the LSR in the path "aware" of the FEC. The idea of MP-BGP is 
pretty much the same, you are using BGP sessions between all the
peers to exchange VPNv4 information.  

> maintaining LDP sessions between the PE LSR's). Secondly, we 

Sure you can, but in this case remember you will have to maintain
a LDP session for each of the PEs (full mesh) I'd like to see you
maintaining a couple hundreds PEs with full meshing requirements, 
without all the handy mechanisms BGP has (for example, Route Reflectors)
Or maybe we can add the notion of Route Reflectors to LDP...

> I want to know whether any work has been done in this direction.

Ask the folks at PPVPN

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From: "Arun Kumar Dheena" <[email-masked]>
Date: Mon, 25 Feb 2002 18:25:36 +0530

Hi,
       Thanks for your replies.

The draft-martini-l2circuit, does establish point-to-point tunnel between
PE-LSR's. It maps the incoming L2 traffic onto the corresponding
point-to-point tunnel.

Suppose, if we establish direct LDP session between PE-LSRs as in
martini-drafts  but instead of  exchanging L2 information (as in L2VPNs) we
exchange L3 VPN related information as in BGP/MPLS VPN (by defining
RouteTarget-TLV, VPN-Label, VPN-IPv4 TLV to the LDP etc), we (might) get a
Layer 3 VPN without using BGP.  With filtering capabilities, LDP-VPN can
support the features as in BGP/MPLS VPNs. The PE-LSR - CE routing has to be
explored in this approach.

 What are the flaws/disadvantages in this approach. Is this worthwhile?

As Miquel has mentioned, the PE-LSR's has to be fully-meshed but only in the
"worst case" (because,  a PE-LSR will establish a LDP session with another
PE-LSR only if they have a VPN-site in common). Yes, this is a limitation
but then there is no FEC reflector(!) concept in LDP.

Thanks in Advance

regards
arun

---

From: Giles Heron <[email-masked]>
Date: Mon, 25 Feb 2002 13:05:20 +0000

But why would we bother?

draft-martini is for point-to-point circuits.   Targetted LDP is ideal 
for this because it sets up point-to-point "signalling" sessions between 
the two PEs that share the circuit.  No other PE would want to know this 
information, so using BGP and route-reflectors would seem an inefficient 
way to distribute this information (though I guess you could do some 
sort of filtering based on extended communities?)

draft-lasserre-vkompella extends draft-martini for multipoint L2VPNs 
using Ethernet.  This also fits targetted LDP well since the MAC 
learning requirement of the L2 VPN requires that a PE distribute a 
distinct VC label to each peer PE.

For an L3 VPN, however, a PE will generally want to distribute an 
identical label to each peer PE.  BGP and route reflectors would seem 
like a better way to do this than using LDP (in the absence of a "FEC 
reflector").

Giles

Arun Kumar Dheena wrote:

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-- 
=================================================================
Giles Heron    Principal Network Architect    PacketExchange Ltd.
ph: +44 7880 506185              "if you build it they will yawn"
=================================================================

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From: Robert Raszuk <[email-masked]>
Date: Mon, 25 Feb 2002 14:02:06 +0100

Arun,

>  What are the flaws/disadvantages in this approach. Is this worthwhile?

I would ask an opposite question:

Please indicate one advantage of turning your directed LDP into IBGP ???

[Just don't say an obvious one - that some vendors don't know how to
write a bgp code.]

R.

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From: Eric Rosen <[email-masked]>
Date: Mon, 25 Feb 2002 11:26:03 -0500

Providing a  layer 3 VPN  requires the distribution of  routing information.
To distribute routing information, one  must use a routing protocol.  LDP is
not a routing protocol.  BGP is. 

For example, BGP has things like a decision process, to ensure that for each
address prefix, a  non-looping set of routes is chosen.   If you think about
sites that are  multihomed, have backdoor connections, etc.,  etc., the need
for this becomes apparent.