MPLS Explicit Null Label question

Group:
I am trying to understand the MPLS reserved label. Below is the text from RFC
3032A value of 0 represents the "IPv4 Explicit NULL Label".This label value is
only legal at the bottom of the label stack. It indicates that the label
stack must be popped, and the forwarding of the packet must then be based on
the IPv4 header.
I am confused if the top label is removed and EXP bit are either copied to IP
packet or (in case there are more label) to later label and the egress LSR has
only one lookup to do for IP packet forwarding.
RFC did not mention any thing about saving the EXP bits but the book MPLS
fundamentals indicates
that.http://www.ciscopress.com/articles/article.asp?p=680824&seqNum=2

Thanks,Khalid
Received on Thu Jan 28 2010 - 23:37:15 ART