The MPLS WG Archive

["Vijayanand C - CTD, Chennai." <vijayc@ctd.hcltech.com>]

I also agree with abhijit that the problem can be addressed at the routing
protocol level. If a link goes down the routing protocol would generally
give a next hop change to the LDP module to trigger a request along the new
path. In other words, by the time LDP detects it routing protocol would have
sensed it and found and alternate path.If that were not the case, ie, if
alternate next hop is not immediatly available, the routing entity in
upstream(A in this case) would also be aware of that the dest is unreachable
and would have  informed the LDP entity about it. Subsequently when next hop
becomes available at B, routing protocol at A would intimate LDP at A. The
case described where the ingress has to re-request would seldom arise with a
good routing protocol implementation.

Please correct me if I am wrong

Regards,
Vijay

-----Original Message-----
From: Abhijit Gadgil [mailto:gabhijit@ee.iitb.ac.in]
Sent: Friday, March 29, 2002 5:56 PM
To: Manoj Dutta
Cc: mpls@UU.NET
Subject: Re: LDP Label WIthdraw doubt !!



Manoj,

Consider here the events that would lead to such a behavior. One cannot
neglect the role played by routing protocol here. The first event would
cause B to withdraw and A to release label(s). However the next event when
B sees D as next hop for some fec F, would in turn cause the routing
protocol to indicate the reachability for fec F through D. This event
would generate an internal trigger at LSR A for LDP to issue request
(implementation dependent). I dont think it is required for A to do
something different becuase it is ingress LSR.

LDP working alone without any routing protocol cannot obviously catch the
dynamic effects.

-abhijit.

Manoj Dutta wrote :

>Hi,
>
>I have a doubt regarding label wihdraw procedures described in RFC 3036
>section A.1.5.
>Consider the following topology :-
>
>
>    A-------B---------C--------D
>            |                  |
>             -----D------------
>
>
> Now, lets suppose that for a given FEC F,
>
>a) C is B's nexthop for FEC F
>b) B is A's nexthop for FEC F
>
>All the routers are using Downstream on demand with ordered control. So,
>A send request to B (for FEC F) and gets a label
>L (assuming B has a label from C and so on..). Now, for some reason the
>link between B and C goes down and consequently
>B withdraws label L from A. So, now A has no label for FEC F. After some
>time, B chooses D as the nexthop for FEC F and
>gets a label from D subsequently. But, A's nexthop still points to B for
>FEC F and so A never sends a request to B to get a label
>for FEC F. This behaviour is due to LWd.5 in section A.1.5 in the RFC.
>Is there a reason for this check. So, basically A would send
>a request to B if it were in DOD Independent control. According to me,
>if two LDP peers are using DOD then, in case of a label withdrawal the
>upstream should always send a request to nexthop for theFEC in question.
>
>Does it make sense ??
>
>
>Thanks in advance
>Manoj