is-label-imposed-in-case-of-penultimate.html

---
date: 2008-08-21T07:54:00.000+02:00
tags:
- MPLS
- you've asked for it
title: Is a label imposed in case of Penultimate Hop Popping?
url: /2008/08/is-label-imposed-in-case-of-penultimate/
---

Shivlu Jain sent me an interesting question: <blockquote class="cite">I'm wondering whether a router performing penultimate hop popping (PHP) imposes an IGP label or not.The value of implicit null is 3; does it mean the router imposes this label (and adds four bytes to the packet)?</blockquote>
The penultimate router does not impose the IGP label (that's why this behavior is called penultimate hop <i>popping</i>). However, the egress router has to signal to its upstream neighbor (the penultimate router) that it should NOT impose a label, so it uses "implicit null" label (= 3) in TDP/LDP updates to signal that the top label should be popped, not rewriten.<div class="more">This article is part of <a href="/2007/01/youve-asked-for-it-series/">You've asked for it</a> <a href="/tag/youve-asked-for-it/">series</a>.</div>