Re: MPLS Explicit Null Label question

http://www.networkworld.com/community/node/24143

Here's a good article by Jeff Doyle to compliment Ben's explanation.

On Thu, Jan 28, 2010 at 2:04 PM, ben edgar <bedgar2500_at_gmail.com> wrote:

> this is used in the case of Penultimate Hop Popping.
>
> so the second last router can either pop the label and send just the
> IP packet on (which loses the EXP bits inside the label packet)
> or it can send the IP Packet with a Label with the Value of 0 which
> indicates to the last router to pop the top. This forces the last
> router to do 2 look ups - the label look up and once the top has been
> popped the IP look-up. The benefit is, because the last router get's a
> label, it can look inside the label for the EXP bits.
>
> Ben
>
>
>
> On Thu, Jan 28, 2010 at 1:37 PM, Syed Khalid Ali
> <khalid_khursheed_at_hotmail.com> wrote:
> > 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
> >
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> --
> Ben Edgar
> CCIE #24646
>
>
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Received on Thu Jan 28 2010 - 14:33:34 ART