The MPLS WG Archive

[Abhijit Gadgil <gabhijit@ee.iitb.ac.in>]

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