RE: MPLS Explicit Null Label question

Thank you all for the help!

regardsKhalid
Date: Thu, 28 Jan 2010 14:33:34 -0500
Subject: Re: MPLS Explicit Null Label question
From: shaw38_at_gmail.com
To: bedgar2500_at_gmail.com
CC: khalid_khursheed_at_hotmail.com; ccielab_at_groupstudy.com

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
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