Subject: Next Hop Change..

From: "chetan kumar s" <[email-masked]>
Date: Mon, 23 Jul 2001 15:05:22 +0530 (IST)

Hi Eric,
	
	I was just going thro' the mailing archives, got into some
confusing things. Please refer to the archives for this mail..

http://cell.onecall.net/mhonarc/mpls/2001-May/msg00352.html

U have mentioned that LSR need to withdraw the labels when operating in
Ordered Control Mode..

" d) if operating in ordered control mode, LSR-A would withdraw labels it
has previously provided for FEC F to its peers - especially if
it is not currently capable of establishing a new binding to a label
issued by the new next hop; note that this will result in tearing down
the LSP for FEC F fom each upstream peer all the way to however many
ingress LSRs there are. "

Now if refer to RFC, A.1.7,, page 115, NH.17. This statement is reached
only when the LSR is in DoD OR the peer is in DoD. If operating in un
solictated order control mode this statement is never reached.

In a way I think ur explanation seems OK. If a LSR sends label mapping
due the the fact that it is order control mode, then it *has* to withdraw
the mapping. On the other hand if the mapping is sent from a independent
control mode LSR, it need not withdraw the label. 

Any comments...

Thanks
Chetan S

---

From: Eric Gray <[email-masked]>
Date: Mon, 23 Jul 2001 12:16:55 -0400

Chetan S,

    At least somebody reads my posts! :-)

    So, to address your comments: the DU, Independent control
mode is slightly strange in the case under discussion.  If an LSR
is 'comfortable' with distributing labels even though it may not
have labels for LSPs downstream (the essence of independent
control), then the ability to forward packets without labels to the
routing next hop is implied.

    This SHOULD be the case in such an implementation.  It is
not very likely that you will see DU distribution being used to
distribute labels for a FEC that the distributing LSR does not
know how to forward.  Also, merely doing so effectively breaks
routing in a sense similar to the 'MPLS framework' notion of an
MPLS node (an LSR with no unlabeled packet forwarding ability)
- such an LSR cannot have both outstanding route advertisements
and no LSP in effect for those advertisements since it does not
have 'reachability' if it does.

--
Eric Gray