Subject: LDP Label WIthdraw doubt !!

From: Manoj Dutta <[email-masked]>
Date: Thu, 28 Mar 2002 12:58:20 -0800

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

---

From: Abhijit Gadgil <[email-masked]>
Date: Fri, 29 Mar 2002 17:56:28 +0530 (IST)

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.