Subject: RE: A question for OSPF-TE

From: "Louis Yun" <[email-masked]>
Date: Mon, 11 Feb 2002 19:54:04 -0800

Cheng-hong,

You can think of the Opaque LSA used for traffic engineering as a natural
extension of the regular OSPF router-LSA.  Just like router-LSAs, then,
OSPF-TE LSAs have area-only scope for scalability reasons.  In fact,
scalability is an even more critical issue for traffic engineering
applications, since any change in link bandwidth or other attributes -- not
only the link going up or down -- will cause the LSA to be reflooded.

One may attempt breaking down the problem of inter-area path calculation
into per area computations, but it's not so simple.  Suppose the Ingress of
a TE tunnel is in a different area then the Egress, and you are trying to
compute a CSPF route.  There may be multiple exit ABRs within the Ingress
area.  There's no way to know beforehand whether a particular exit ABR leads
to the best path, or will even satisfy the path constraints in the
subsequent areas.  The result -- you may have to check all exit ABRs,
raising questions as to how well these computations would scale.
draft-kompella-mpls-multiarea-te-01.txt gives a good discussion about this.
In essence, there's a tradeoff between protocol scalability and optimality
of the computed TE path for a tunnel.

What is needed is an extension of the OSPF summary LSA for inter-area TE
advertisements.  Something like this has in fact been proposed; see
draft-venkatachalam-ospf-traffic-00.txt,
draft-dharanikota-interarea-te-ext-00.txt.

This does not completely solve the problem either.  For intra-area tunnels,
LSP constraints are user-configured and thus only known to the Ingress LSR.
For inter-area LSPs, the LSP constraints have to be signaled to the transit
ABRs along the path, requiring extensions to RSVP-TE or CR-LDP.

regards,
Louis

---

From: [email-masked]
Date: Tue, 12 Feb 2002 11:19:14 +0100

Hi Louis,

For your information, 
-
http://www.ietf.org/internet-drafts/draft-venkatachalam-ospf-traffic-02.txt
-
http://www.ietf.org/internet-drafts/draft-dharanikota-interarea-te-ext-02.txt

were submitted for the last IETF meeting (SLC). 

Regards,
- dimitri.