The MPLS WG Archive

[Bob Thomas <rhthomas@cisco.com>]

Igor,

> How does LDP spec/rfc3036 address race condition of simultaneous Label 
> Request and Label Mapping being sent for the same prefix in unsolicited 
> label advertisement mode?
> 
> Section "A.1.6. Recognize New FEC" says we need to send label request to 
> downstream peer regardless of configured label advertisement mode. New FEC 
> was detected immediately after LDP was configured, so we have not got label 
> yet at this moment.

A thing that is not as clear as it should be in rfc3036 is that
Downstream Unsolicited Liberal Retention mode implies LSR Request Procedure
"Request Never" as per rfc3031 Section 5.1.2.1:

  5.1.2.1. RequestNever
  
     Never make a request.  This is useful if the downstream LSR uses the
     PushConditional procedure or the PushUnconditional procedure, but is
     not useful if the downstream LSR uses the PulledUnconditional
     procedure or the the PulledConditional procedures.
  
     This procedure would be used by an LSR when unsolicited downstream
     label distribution and Liberal Label Retention Mode are being used.

Therefore, an LSR operating in DU/Liberal Retention mode would not
send a Label Request when it recognizes a new FEC.

Bob


> At the same time peer sends us Label Mapping with label for the same prefix 
> according to configured unsolicited advertisement mode.
> 
> This scenario ends up with two labels for the same prefix, which can be 
> quite a waste of label space for big number of routes.
> 
> I know, LDP spec is stable for big period of time, and may be I just missed 
> something.