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<!DOCTYPE html>
<html lang="en" class="RFC">
<head>
<meta charset="utf-8">
<meta content="Common,Latin" name="scripts">
<meta content="initial-scale=1.0" name="viewport">
<title>RFC 8770: Host Router Support for OSPFv2</title>
<meta content="Keyur Patel" name="author">
<meta content="Padma Pillay-Esnault" name="author">
<meta content="Manish Bhardwaj" name="author">
<meta content="Serpil Bayraktar" name="author">
<meta content="
       
   The Open Shortest Path First Version 2 (OSPFv2) protocol does not
   have a mechanism for a node to repel transit traffic if it is on the
   shortest path.  This document defines a bit called the Host-bit (H-bit). This bit enables a
   router to advertise that it is a non-transit router.  This document also
   describes the changes needed to support the H-bit in the domain.  In
   addition, this document updates RFC 6987 to advertise Type 2 External
   and Not-So-Stubby Area (NSSA) Link State Advertisements (LSAs) 
   (RFC 3101) with a high cost in order to repel traffic effectively. 
    " name="description">
<meta content="xml2rfc 2.42.0" name="generator">
<meta content="non-transit" name="keyword">
<meta content="8770" name="rfc.number">
<link href="rfc8770.xml" rel="alternate" type="application/rfc+xml">
<link href="#copyright" rel="license">
<style type="text/css">/*

  NOTE: Changes at the bottom of this file overrides some earlier settings.

  Once the style has stabilized and has been adopted as an official RFC style,
  this can be consolidated so that style settings occur only in one place, but
  for now the contents of this file consists first of the initial CSS work as
  provided to the RFC Formatter (xml2rfc) work, followed by itemized and
  commented changes found necssary during the development of the v3
  formatters.

*/

/* fonts */
@import url('https://fonts.googleapis.com/css?family=Noto+Sans'); /* Sans-serif */
@import url('https://fonts.googleapis.com/css?family=Noto+Serif'); /* Serif (print) */
@import url('https://fonts.googleapis.com/css?family=Roboto+Mono'); /* Monospace */

@viewport {
  zoom: 1.0;
  width: extend-to-zoom;
}
@-ms-viewport {
  width: extend-to-zoom;
  zoom: 1.0;
}
/* general and mobile first */
html {
}
body {
  max-width: 90%;
  margin: 1.5em auto;
  color: #222;
  background-color: #fff;
  font-size: 14px;
  font-family: 'Noto Sans', Arial, Helvetica, sans-serif;
  line-height: 1.6;
  scroll-behavior: smooth;
}
.ears {
  display: none;
}

/* headings */
#title, h1, h2, h3, h4, h5, h6 {
  margin: 1em 0 0.5em;
  font-weight: bold;
  line-height: 1.3;
}
#title {
  clear: both;
  border-bottom: 1px solid #ddd;
  margin: 0 0 0.5em 0;
  padding: 1em 0 0.5em;
}
.author {
  padding-bottom: 4px;
}
h1 {
  font-size: 26px;
  margin: 1em 0;
}
h2 {
  font-size: 22px;
  margin-top: -20px;  /* provide offset for in-page anchors */
  padding-top: 33px;
}
h3 {
  font-size: 18px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h4 {
  font-size: 16px;
  margin-top: -36px;  /* provide offset for in-page anchors */
  padding-top: 42px;
}
h5, h6 {
  font-size: 14px;
}
#n-copyright-notice {
  border-bottom: 1px solid #ddd;
  padding-bottom: 1em;
  margin-bottom: 1em;
}
/* general structure */
p {
  padding: 0;
  margin: 0 0 1em 0;
  text-align: left;
}
div, span {
  position: relative;
}
div {
  margin: 0;
}
.alignRight.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignRight.art-text pre {
  padding: 0;
}
.alignRight {
  margin: 1em 0;
}
.alignRight > *:first-child {
  border: none;
  margin: 0;
  float: right;
  clear: both;
}
.alignRight > *:nth-child(2) {
  clear: both;
  display: block;
  border: none;
}
svg {
  display: block;
}
.alignCenter.art-text {
  background-color: #f9f9f9;
  border: 1px solid #eee;
  border-radius: 3px;
  padding: 1em 1em 0;
  margin-bottom: 1.5em;
}
.alignCenter.art-text pre {
  padding: 0;
}
.alignCenter {
  margin: 1em 0;
}
.alignCenter > *:first-child {
  border: none;
  /* this isn't optimal, but it's an existence proof.  PrinceXML doesn't
     support flexbox yet.
  */
  display: table;
  margin: 0 auto;
}

/* lists */
ol, ul {
  padding: 0;
  margin: 0 0 1em 2em;
}
ol ol, ul ul, ol ul, ul ol {
  margin-left: 1em;
}
li {
  margin: 0 0 0.25em 0;
}
.ulCompact li {
  margin: 0;
}
ul.empty, .ulEmpty {
  list-style-type: none;
}
ul.empty li, .ulEmpty li {
  margin-top: 0.5em;
}
ul.compact, .ulCompact,
ol.compact, .olCompact {
  line-height: 100%;
  margin: 0 0 0 2em;
}

/* definition lists */
dl {
}
dl > dt {
  float: left;
  margin-right: 1em;
}
/* 
dl.nohang > dt {
  float: none;
}
*/
dl > dd {
  margin-bottom: .8em;
  min-height: 1.3em;
}
dl.compact > dd, .dlCompact > dd {
  margin-bottom: 0em;
}
dl > dd > dl {
  margin-top: 0.5em;
  margin-bottom: 0em;
}

/* links */
a {
  text-decoration: none;
}
a[href] {
  color: #22e; /* Arlen: WCAG 2019 */
}
a[href]:hover {
  background-color: #f2f2f2;
}
figcaption a[href],
a[href].selfRef {
  color: #222;
}
/* XXX probably not this:
a.selfRef:hover {
  background-color: transparent;
  cursor: default;
} */

/* Figures */
tt, code, pre, code {
  background-color: #f9f9f9;
  font-family: 'Roboto Mono', monospace;
}
pre {
  border: 1px solid #eee;
  margin: 0;
  padding: 1em;
}
img {
  max-width: 100%;
}
figure {
  margin: 0;
}
figure blockquote {
  margin: 0.8em 0.4em 0.4em;
}
figcaption {
  font-style: italic;
  margin: 0 0 1em 0;
}
@media screen {
  pre {
    overflow-x: auto;
    max-width: 100%;
    max-width: calc(100% - 22px);
  }
}

/* aside, blockquote */
aside, blockquote {
  margin-left: 0;
  padding: 1.2em 2em;
}
blockquote {
  background-color: #f9f9f9;
  color: #111; /* Arlen: WCAG 2019 */
  border: 1px solid #ddd;
  border-radius: 3px;
  margin: 1em 0;
}
cite {
  display: block;
  text-align: right;
  font-style: italic;
}

/* tables */
table {
  width: 100%;
  margin: 0 0 1em;
  border-collapse: collapse;
  border: 1px solid #eee;
}
th, td {
  text-align: left;
  vertical-align: top;
  padding: 0.5em 0.75em;
}
th {
  text-align: left;
  background-color: #e9e9e9;
}
tr:nth-child(2n+1) > td {
  background-color: #f5f5f5;
}
table caption {
  font-style: italic;
  margin: 0;
  padding: 0;
  text-align: left;
}
table p {
  /* XXX to avoid bottom margin on table row signifiers. If paragraphs should
     be allowed within tables more generally, it would be far better to select on a class. */
  margin: 0;
}

/* pilcrow */
a.pilcrow {
  color: #666; /* Arlen: AHDJ 2019 */
  text-decoration: none;
  visibility: hidden;
  user-select: none;
  -ms-user-select: none;
  -o-user-select:none;
  -moz-user-select: none;
  -khtml-user-select: none;
  -webkit-user-select: none;
  -webkit-touch-callout: none;
}
@media screen {
  aside:hover > a.pilcrow,
  p:hover > a.pilcrow,
  blockquote:hover > a.pilcrow,
  div:hover > a.pilcrow,
  li:hover > a.pilcrow,
  pre:hover > a.pilcrow {
    visibility: visible;
  }
  a.pilcrow:hover {
    background-color: transparent;
  }
}

/* misc */
hr {
  border: 0;
  border-top: 1px solid #eee;
}
.bcp14 {
  font-variant: small-caps;
}

.role {
  font-variant: all-small-caps;
}

/* info block */
#identifiers {
  margin: 0;
  font-size: 0.9em;
}
#identifiers dt {
  width: 3em;
  clear: left;
}
#identifiers dd {
  float: left;
  margin-bottom: 0;
}
#identifiers .authors .author {
  display: inline-block;
  margin-right: 1.5em;
}
#identifiers .authors .org {
  font-style: italic;
}

/* The prepared/rendered info at the very bottom of the page */
.docInfo {
  color: #666; /* Arlen: WCAG 2019 */
  font-size: 0.9em;
  font-style: italic;
  margin-top: 2em;
}
.docInfo .prepared {
  float: left;
}
.docInfo .prepared {
  float: right;
}

/* table of contents */
#toc  {
  padding: 0.75em 0 2em 0;
  margin-bottom: 1em;
}
nav.toc ul {
  margin: 0 0.5em 0 0;
  padding: 0;
  list-style: none;
}
nav.toc li {
  line-height: 1.3em;
  margin: 0.75em 0;
  padding-left: 1.2em;
  text-indent: -1.2em;
}
/* references */
.references dt {
  text-align: right;
  font-weight: bold;
  min-width: 7em;
}
.references dd {
  margin-left: 8em;
  overflow: auto;
}

.refInstance {
  margin-bottom: 1.25em;
}

.references .ascii {
  margin-bottom: 0.25em;
}

/* index */
.index ul {
  margin: 0 0 0 1em;
  padding: 0;
  list-style: none;
}
.index ul ul {
  margin: 0;
}
.index li {
  margin: 0;
  text-indent: -2em;
  padding-left: 2em;
  padding-bottom: 5px;
}
.indexIndex {
  margin: 0.5em 0 1em;
}
.index a {
  font-weight: 700;
}
/* make the index two-column on all but the smallest screens */
@media (min-width: 600px) {
  .index ul {
    -moz-column-count: 2;
    -moz-column-gap: 20px;
  }
  .index ul ul {
    -moz-column-count: 1;
    -moz-column-gap: 0;
  }
}

/* authors */
address.vcard {
  font-style: normal;
  margin: 1em 0;
}

address.vcard .nameRole {
  font-weight: 700;
  margin-left: 0;
}
address.vcard .label {
  font-family: "Noto Sans",Arial,Helvetica,sans-serif;
  margin: 0.5em 0;
}
address.vcard .type {
  display: none;
}
.alternative-contact {
  margin: 1.5em 0 1em;
}
hr.addr {
  border-top: 1px dashed;
  margin: 0;
  color: #ddd;
  max-width: calc(100% - 16px);
}

/* temporary notes */
.rfcEditorRemove::before {
  position: absolute;
  top: 0.2em;
  right: 0.2em;
  padding: 0.2em;
  content: "The RFC Editor will remove this note";
  color: #9e2a00; /* Arlen: WCAG 2019 */
  background-color: #ffd; /* Arlen: WCAG 2019 */
}
.rfcEditorRemove {
  position: relative;
  padding-top: 1.8em;
  background-color: #ffd; /* Arlen: WCAG 2019 */
  border-radius: 3px;
}
.cref {
  background-color: #ffd; /* Arlen: WCAG 2019 */
  padding: 2px 4px;
}
.crefSource {
  font-style: italic;
}
/* alternative layout for smaller screens */
@media screen and (max-width: 1023px) {
  body {
    padding-top: 2em;
  }
  #title {
    padding: 1em 0;
  }
  h1 {
    font-size: 24px;
  }
  h2 {
    font-size: 20px;
    margin-top: -18px;  /* provide offset for in-page anchors */
    padding-top: 38px;
  }
  #identifiers dd {
    max-width: 60%;
  }
  #toc {
    position: fixed;
    z-index: 2;
    top: 0;
    right: 0;
    padding: 0;
    margin: 0;
    background-color: inherit;
    border-bottom: 1px solid #ccc;
  }
  #toc h2 {
    margin: -1px 0 0 0;
    padding: 4px 0 4px 6px;
    padding-right: 1em;
    min-width: 190px;
    font-size: 1.1em;
    text-align: right;
    background-color: #444;
    color: white;
    cursor: pointer;
  }
  #toc h2::before { /* css hamburger */
    float: right;
    position: relative;
    width: 1em;
    height: 1px;
    left: -164px;
    margin: 6px 0 0 0;
    background: white none repeat scroll 0 0;
    box-shadow: 0 4px 0 0 white, 0 8px 0 0 white;
    content: "";
  }
  #toc nav {
    display: none;
    padding: 0.5em 1em 1em;
    overflow: auto;
    height: calc(100vh - 48px);
    border-left: 1px solid #ddd;
  }
}

/* alternative layout for wide screens */
@media screen and (min-width: 1024px) {
  body {
    max-width: 724px;
    margin: 42px auto;
    padding-left: 1.5em;
    padding-right: 29em;
  }
  #toc {
    position: fixed;
    top: 42px;
    right: 42px;
    width: 25%;
    margin: 0;
    padding: 0 1em;
    z-index: 1;
  }
  #toc h2 {
    border-top: none;
    border-bottom: 1px solid #ddd;
    font-size: 1em;
    font-weight: normal;
    margin: 0;
    padding: 0.25em 1em 1em 0;
  }
  #toc nav {
    display: block;
    height: calc(90vh - 84px);
    bottom: 0;
    padding: 0.5em 0 0;
    overflow: auto;
  }
  img { /* future proofing */
    max-width: 100%;
    height: auto;
  }
}

/* pagination */
@media print {
  body {

    width: 100%;
  }
  p {
    orphans: 3;
    widows: 3;
  }
  #n-copyright-notice {
    border-bottom: none;
  }
  #toc, #n-introduction {
    page-break-before: always;
  }
  #toc {
    border-top: none;
    padding-top: 0;
  }
  figure, pre {
    page-break-inside: avoid;
  }
  figure {
    overflow: scroll;
  }
  h1, h2, h3, h4, h5, h6 {
    page-break-after: avoid;
  }
  h2+*, h3+*, h4+*, h5+*, h6+* {
    page-break-before: avoid;
  }
  pre {
    white-space: pre-wrap;
    word-wrap: break-word;
    font-size: 10pt;
  }
  table {
    border: 1px solid #ddd;
  }
  td {
    border-top: 1px solid #ddd;
  }
}

/* This is commented out here, as the string-set: doesn't
   pass W3C validation currently */
/*
.ears thead .left {
  string-set: ears-top-left content();
}

.ears thead .center {
  string-set: ears-top-center content();
}

.ears thead .right {
  string-set: ears-top-right content();
}

.ears tfoot .left {
  string-set: ears-bottom-left content();
}

.ears tfoot .center {
  string-set: ears-bottom-center content();
}

.ears tfoot .right {
  string-set: ears-bottom-right content();
}
*/

@page :first {
  padding-top: 0;
  @top-left {
    content: normal;
    border: none;
  }
  @top-center {
    content: normal;
    border: none;
  }
  @top-right {
    content: normal;
    border: none;
  }
}

@page {
  size: A4;
  margin-bottom: 45mm;
  padding-top: 20px;
  /* The follwing is commented out here, but set appropriately by in code, as
     the content depends on the document */
  /*
  @top-left {
    content: 'Internet-Draft';
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-left {
    content: string(ears-top-left);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-center {
    content: string(ears-top-center);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @top-right {
    content: string(ears-top-right);
    vertical-align: bottom;
    border-bottom: solid 1px #ccc;
  }
  @bottom-left {
    content: string(ears-bottom-left);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-center {
    content: string(ears-bottom-center);
    vertical-align: top;
    border-top: solid 1px #ccc;
  }
  @bottom-right {
      content: '[Page ' counter(page) ']';
      vertical-align: top;
      border-top: solid 1px #ccc;
  }
  */

}

/* Changes introduced to fix issues found during implementation */
/* Make sure links are clickable even if overlapped by following H* */
a {
  z-index: 2;
}
/* Separate body from document info even without intervening H1 */
section {
  clear: both;
}


/* Top align author divs, to avoid names without organization dropping level with org names */
.author {
  vertical-align: top;
}

/* Leave room in document info to show Internet-Draft on one line */
#identifiers dt {
  width: 8em;
}

/* Don't waste quite as much whitespace between label and value in doc info */
#identifiers dd {
  margin-left: 1em;
}

/* Give floating toc a background color (needed when it's a div inside section */
#toc {
  background-color: white;
}

/* Make the collapsed ToC header render white on gray also when it's a link */
@media screen and (max-width: 1023px) {
  #toc h2 a,
  #toc h2 a:link,
  #toc h2 a:focus,
  #toc h2 a:hover,
  #toc a.toplink,
  #toc a.toplink:hover {
    color: white;
    background-color: #444;
    text-decoration: none;
  }
}

/* Give the bottom of the ToC some whitespace */
@media screen and (min-width: 1024px) {
  #toc {
    padding: 0 0 1em 1em;
  }
}

/* Style section numbers with more space between number and title */
.section-number {
  padding-right: 0.5em;
}

/* prevent monospace from becoming overly large */
tt, code, pre, code {
  font-size: 95%;
}

/* Fix the height/width aspect for ascii art*/
pre.sourcecode,
.art-text pre {
  line-height: 1.12;
}


/* Add styling for a link in the ToC that points to the top of the document */
a.toplink {
  float: right;
  margin-right: 0.5em;
}

/* Fix the dl styling to match the RFC 7992 attributes */
dl > dt,
dl.dlParallel > dt {
  float: left;
  margin-right: 1em;
}
dl.dlNewline > dt {
  float: none;
}

/* Provide styling for table cell text alignment */
table td.text-left,
table th.text-left {
  text-align: left;
}
table td.text-center,
table th.text-center {
  text-align: center;
}
table td.text-right,
table th.text-right {
  text-align: right;
}

/* Make the alternative author contact informatio look less like just another
   author, and group it closer with the primary author contact information */
.alternative-contact {
  margin: 0.5em 0 0.25em 0;
}
address .non-ascii {
  margin: 0 0 0 2em;
}

/* With it being possible to set tables with alignment
  left, center, and right, { width: 100%; } does not make sense */
table {
  width: auto;
}

/* Avoid reference text that sits in a block with very wide left margin,
   because of a long floating dt label.*/
.references dd {
  overflow: visible;
}

/* Control caption placement */
caption {
  caption-side: bottom;
}

/* Limit the width of the author address vcard, so names in right-to-left
   script don't end up on the other side of the page. */

address.vcard {
  max-width: 30em;
  margin-right: auto;
}

/* For address alignment dependent on LTR or RTL scripts */
address div.left {
  text-align: left;
}
address div.right {
  text-align: right;
}

/* Provide table alignment support.  We can't use the alignX classes above
   since they do unwanted things with caption and other styling. */
table.right {
 margin-left: auto;
 margin-right: 0;
}
table.center {
 margin-left: auto;
 margin-right: auto;
}
table.left {
 margin-left: 0;
 margin-right: auto;
}

/* Give the table caption label the same styling as the figcaption */
caption a[href] {
  color: #222;
}

@media print {
  .toplink {
    display: none;
  }

  /* avoid overwriting the top border line with the ToC header */
  #toc {
    padding-top: 1px;
  }

  /* Avoid page breaks inside dl and author address entries */
  .vcard {
    page-break-inside: avoid;
  }

}
/* Avoid wrapping of URLs in references */
@media screen {
  .references a {
    white-space: nowrap;
  }
}
/* Tweak the bcp14 keyword presentation */
.bcp14 {
  font-variant: small-caps;
  font-weight: bold;
  font-size: 0.9em;
}
/* Tweak the invisible space above H* in order not to overlay links in text above */
 h2 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 31px;
 }
 h3 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
 h4 {
  margin-top: -18px;  /* provide offset for in-page anchors */
  padding-top: 24px;
 }
/* Float artwork pilcrow to the right */
@media screen {
  .artwork a.pilcrow {
    display: block;
    line-height: 0.7;
    margin-top: 0.15em;
  }
}
/* Make pilcrows on dd visible */
@media screen {
  dd:hover > a.pilcrow {
    visibility: visible;
  }
}
/* Make the placement of figcaption match that of a table's caption
   by removing the figure's added bottom margin */
.alignLeft.art-text,
.alignCenter.art-text,
.alignRight.art-text {
   margin-bottom: 0;
}
.alignLeft,
.alignCenter,
.alignRight {
  margin: 1em 0 0 0;
}
/* In print, the pilcrow won't show on hover, so prevent it from taking up space,
   possibly even requiring a new line */
@media print {
  a.pilcrow {
    display: none;
  }
}
/* Styling for the external metadata */
div#external-metadata {
  background-color: #eee;
  padding: 0.5em;
  margin-bottom: 0.5em;
  display: none;
}
div#internal-metadata {
  padding: 0.5em;                       /* to match the external-metadata padding */
}
/* Styling for title RFC Number */
h1#rfcnum {
  clear: both;
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<table class="ears">
<thead><tr>
<td class="left">RFC 8770</td>
<td class="center">Host Router Support for OSPFv2</td>
<td class="right">April 2020</td>
</tr></thead>
<tfoot><tr>
<td class="left">Patel, et al.</td>
<td class="center">Standards Track</td>
<td class="right">[Page]</td>
</tr></tfoot>
</table>
<div id="external-metadata" class="document-information"></div>
<div id="internal-metadata" class="document-information">
<dl id="identifiers">
<dt class="label-stream">Stream:</dt>
<dd class="stream">Internet Engineering Task Force (IETF)</dd>
<dt class="label-rfc">RFC:</dt>
<dd class="rfc"><a href="https://www.rfc-editor.org/rfc/rfc8770" class="eref">8770</a></dd>
<dt class="label-updates">Updates:</dt>
<dd class="updates">
<a href="https://www.rfc-editor.org/rfc/rfc6987" class="eref">6987</a> </dd>
<dt class="label-category">Category:</dt>
<dd class="category">Standards Track</dd>
<dt class="label-published">Published:</dt>
<dd class="published">
<time datetime="2020-04" class="published">April 2020</time>
    </dd>
<dt class="label-issn">ISSN:</dt>
<dd class="issn">2070-1721</dd>
<dt class="label-authors">Authors:</dt>
<dd class="authors">
<div class="author">
      <div class="author-name">K. Patel</div>
<div class="org">Arrcus</div>
</div>
<div class="author">
      <div class="author-name">P. Pillay-Esnault</div>
<div class="org">PPE Consulting</div>
</div>
<div class="author">
      <div class="author-name">M. Bhardwaj</div>
<div class="org">Cisco Systems</div>
</div>
<div class="author">
      <div class="author-name">S. Bayraktar</div>
<div class="org">Cisco Systems</div>
</div>
</dd>
</dl>
</div>
<h1 id="rfcnum">RFC 8770</h1>
<h1 id="title">Host Router Support for OSPFv2</h1>
<section id="section-abstract">
      <h2 id="abstract"><a href="#abstract" class="selfRef">Abstract</a></h2>
<p id="section-abstract-1">
   The Open Shortest Path First Version 2 (OSPFv2) protocol does not
   have a mechanism for a node to repel transit traffic if it is on the
   shortest path.  This document defines a bit called the Host-bit (H-bit). This bit enables a
   router to advertise that it is a non-transit router.  This document also
   describes the changes needed to support the H-bit in the domain.  In
   addition, this document updates RFC 6987 to advertise Type 2 External
   and Not-So-Stubby Area (NSSA) Link State Advertisements (LSAs) 
   (RFC 3101) with a high cost in order to repel traffic effectively.<a href="#section-abstract-1" class="pilcrow">¶</a></p>
</section>
<div id="status-of-memo">
<section id="section-boilerplate.1">
        <h2 id="name-status-of-this-memo">
<a href="#name-status-of-this-memo" class="section-name selfRef">Status of This Memo</a>
        </h2>
<p id="section-boilerplate.1-1">
            This is an Internet Standards Track document.<a href="#section-boilerplate.1-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-2">
            This document is a product of the Internet Engineering Task Force
            (IETF).  It represents the consensus of the IETF community.  It has
            received public review and has been approved for publication by
            the Internet Engineering Steering Group (IESG).  Further
            information on Internet Standards is available in Section 2 of 
            RFC 7841.<a href="#section-boilerplate.1-2" class="pilcrow">¶</a></p>
<p id="section-boilerplate.1-3">
            Information about the current status of this document, any
            errata, and how to provide feedback on it may be obtained at
            <span><a href="https://www.rfc-editor.org/info/rfc8770">https://www.rfc-editor.org/info/rfc8770</a></span>.<a href="#section-boilerplate.1-3" class="pilcrow">¶</a></p>
</section>
</div>
<div id="copyright">
<section id="section-boilerplate.2">
        <h2 id="name-copyright-notice">
<a href="#name-copyright-notice" class="section-name selfRef">Copyright Notice</a>
        </h2>
<p id="section-boilerplate.2-1">
            Copyright (c) 2020 IETF Trust and the persons identified as the
            document authors. All rights reserved.<a href="#section-boilerplate.2-1" class="pilcrow">¶</a></p>
<p id="section-boilerplate.2-2">
            This document is subject to BCP 78 and the IETF Trust's Legal
            Provisions Relating to IETF Documents
            (<span><a href="https://trustee.ietf.org/license-info">https://trustee.ietf.org/license-info</a></span>) in effect on the date of
            publication of this document. Please review these documents
            carefully, as they describe your rights and restrictions with
            respect to this document. Code Components extracted from this
            document must include Simplified BSD License text as described in
            Section 4.e of the Trust Legal Provisions and are provided without
            warranty as described in the Simplified BSD License.<a href="#section-boilerplate.2-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="toc">
<section id="section-toc.1">
        <a href="#" onclick="scroll(0,0)" class="toplink">▲</a><h2 id="name-table-of-contents">
<a href="#name-table-of-contents" class="section-name selfRef">Table of Contents</a>
        </h2>
<nav class="toc"><ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.1">
            <p id="section-toc.1-1.1.1" class="keepWithNext"><a href="#section-1" class="xref">1</a>.  <a href="#name-introduction" class="xref">Introduction</a><a href="#section-toc.1-1.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.2">
            <p id="section-toc.1-1.2.1" class="keepWithNext"><a href="#section-2" class="xref">2</a>.  <a href="#name-requirements-language" class="xref">Requirements Language</a><a href="#section-toc.1-1.2.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.3">
            <p id="section-toc.1-1.3.1" class="keepWithNext"><a href="#section-3" class="xref">3</a>.  <a href="#name-host-bit-support" class="xref">Host-Bit Support</a><a href="#section-toc.1-1.3.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.4">
            <p id="section-toc.1-1.4.1" class="keepWithNext"><a href="#section-4" class="xref">4</a>.  <a href="#name-spf-modifications" class="xref">SPF Modifications</a><a href="#section-toc.1-1.4.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.5">
            <p id="section-toc.1-1.5.1" class="keepWithNext"><a href="#section-5" class="xref">5</a>.  <a href="#name-autodiscovery-and-backward-" class="xref">Autodiscovery and Backward Compatibility</a><a href="#section-toc.1-1.5.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.6">
            <p id="section-toc.1-1.6.1" class="keepWithNext"><a href="#section-6" class="xref">6</a>.  <a href="#name-ospf-as-external-lsas-nssa-" class="xref">OSPF AS-External-LSAs / NSSA-LSAs with Type 2 Metrics</a><a href="#section-toc.1-1.6.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.7">
            <p id="section-toc.1-1.7.1" class="keepWithNext"><a href="#section-7" class="xref">7</a>.  <a href="#name-iana-considerations" class="xref">IANA Considerations</a><a href="#section-toc.1-1.7.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.8">
            <p id="section-toc.1-1.8.1" class="keepWithNext"><a href="#section-8" class="xref">8</a>.  <a href="#name-security-considerations" class="xref">Security Considerations</a><a href="#section-toc.1-1.8.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.9">
            <p id="section-toc.1-1.9.1" class="keepWithNext"><a href="#section-9" class="xref">9</a>.  <a href="#name-references" class="xref">References</a><a href="#section-toc.1-1.9.1" class="pilcrow">¶</a></p>
<ul class="toc ulEmpty">
<li class="toc ulEmpty" id="section-toc.1-1.9.2.1">
                <p id="section-toc.1-1.9.2.1.1" class="keepWithNext"><a href="#section-9.1" class="xref">9.1</a>.  <a href="#name-normative-references" class="xref">Normative References</a><a href="#section-toc.1-1.9.2.1.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.9.2.2">
                <p id="section-toc.1-1.9.2.2.1" class="keepWithNext"><a href="#section-9.2" class="xref">9.2</a>.  <a href="#name-informative-references" class="xref">Informative References</a><a href="#section-toc.1-1.9.2.2.1" class="pilcrow">¶</a></p>
</li>
</ul>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.10">
            <p id="section-toc.1-1.10.1" class="keepWithNext"><a href="#section-appendix.a" class="xref"></a><a href="#name-acknowledgements" class="xref">Acknowledgements</a><a href="#section-toc.1-1.10.1" class="pilcrow">¶</a></p>
</li>
<li class="toc ulEmpty" id="section-toc.1-1.11">
            <p id="section-toc.1-1.11.1" class="keepWithNext"><a href="#section-appendix.b" class="xref"></a><a href="#name-authors-addresses" class="xref">Authors' Addresses</a><a href="#section-toc.1-1.11.1" class="pilcrow">¶</a></p>
</li>
</ul>
</nav>
</section>
</div>
<div id="sect-1">
<section id="section-1">
      <h2 id="name-introduction">
<a href="#section-1" class="section-number selfRef">1. </a><a href="#name-introduction" class="section-name selfRef">Introduction</a>
      </h2>
<p id="section-1-1">
   The OSPFv2 protocol specifies a Shortest Path First (SPF) algorithm
   that identifies transit vertices based on their adjacencies.
   Therefore, OSPFv2 does not have a mechanism to prevent traffic
   transiting a participating node if it is a transit vertex in the only
   existing or shortest path to the destination.  The use of metrics to
   make the node undesirable can help to repel traffic only if an
   alternative better route exists.<a href="#section-1-1" class="pilcrow">¶</a></p>
<p id="section-1-2">
   A mechanism to move traffic away from the shortest path is
   particularly useful for a number of use cases:<a href="#section-1-2" class="pilcrow">¶</a></p>
<ol start="1" type="1" class="normal type-1" id="section-1-3">
        <li id="section-1-3.1">Graceful isolation of a router, to avoid blackhole scenarios when
       there is a reload and possible long reconvergence times.<a href="#section-1-3.1" class="pilcrow">¶</a>
</li>
<li id="section-1-3.2">Closet switches that are not usually used for transit traffic but need
       to participate in the topology.<a href="#section-1-3.2" class="pilcrow">¶</a>
</li>
<li id="section-1-3.3">Overloaded routers that could use such a capability to temporarily
       repel traffic until they stabilize.<a href="#section-1-3.3" class="pilcrow">¶</a>
</li>
<li id="section-1-3.4">BGP route reflectors, known as virtual Route Reflectors,
       that are not in the forwarding path but are in central locations
       such as data centers.  Such route reflectors are typically used
       for route distribution and are not capable of forwarding transit
       traffic.  However, they need to learn the OSPF topology to
       perform SPF computation for optimal routes and reachability
       resolution for their clients
       <span>[<a href="#BGP-ORR" class="xref">BGP-ORR</a>]</span>.<a href="#section-1-3.4" class="pilcrow">¶</a>
</li>
</ol>
<p id="section-1-4">
   This document describes the functionality provided by the Host-bit (H-bit);
   this functionality prevents other OSPFv2 routers from using the host router by excluding
   it in path calculations for transit traffic in OSPFv2 routing
   domains.  If the H-bit is set, then the calculation of the
   shortest-path tree for an area, as described in <span><a href="https://www.rfc-editor.org/rfc/rfc2328#section-16.1" class="relref">Section 16.1</a> of [<a href="#RFC2328" class="xref">RFC2328</a>]</span>, is
   modified by including a check to verify that transit vertices DO NOT
   have the H-bit set (see <a href="#sect-4" class="xref">Section 4</a>).  Furthermore, in order to repel
   traffic effectively, this document updates <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span> so that Type 2 External and Not-So-Stubby Area (NSSA)
   Link State Advertisements (LSAs) <span>[<a href="#RFC3101" class="xref">RFC3101</a>]</span>
   are advertised with a high cost (see <a href="#sect-6" class="xref">Section 6</a>).  OSPFv3 <span>[<a href="#RFC5340" class="xref">RFC5340</a>]</span> defines an
   option bit, known as the R-bit, for router-LSAs; the H-bit supports similar functionality.<a href="#section-1-4" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-2">
<section id="section-2">
      <h2 id="name-requirements-language">
<a href="#section-2" class="section-number selfRef">2. </a><a href="#name-requirements-language" class="section-name selfRef">Requirements Language</a>
      </h2>
<p id="section-2-1">The key words "<span class="bcp14">MUST</span>", "<span class="bcp14">MUST NOT</span>",
       "<span class="bcp14">REQUIRED</span>", "<span class="bcp14">SHALL</span>",
       "<span class="bcp14">SHALL NOT</span>", "<span class="bcp14">SHOULD</span>",
       "<span class="bcp14">SHOULD NOT</span>",
       "<span class="bcp14">RECOMMENDED</span>", "<span class="bcp14">NOT RECOMMENDED</span>",
       "<span class="bcp14">MAY</span>", and "<span class="bcp14">OPTIONAL</span>" in this document
       are to be interpreted as described in BCP 14
       <span>[<a href="#RFC2119" class="xref">RFC2119</a>]</span> <span>[<a href="#RFC8174" class="xref">RFC8174</a>]</span> when, and only
       when, they appear in all capitals, as shown here.<a href="#section-2-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-3">
<section id="section-3">
      <h2 id="name-host-bit-support">
<a href="#section-3" class="section-number selfRef">3. </a><a href="#name-host-bit-support" class="section-name selfRef">Host-Bit Support</a>
      </h2>
<p id="section-3-1">
   This document defines a new router-LSA bit, known as the Host-bit or
   the H-bit.  An OSPFv2 router advertising a router-LSA with the H-bit
   set indicates that it <span class="bcp14">MUST NOT</span> be used as a transit router (see
   <a href="#sect-4" class="xref">Section 4</a>) by other OSPFv2 routers in the
   area that support the H-bit functionality.<a href="#section-3-1" class="pilcrow">¶</a></p>
<p id="section-3-2">
   If the H-bit is not set, then backward compatibility is achieved, as
   the behavior will be the same as in <span>[<a href="#RFC2328" class="xref">RFC2328</a>]</span>.<a href="#section-3-2" class="pilcrow">¶</a></p>
<span id="name-ospf-router-lsa"></span><div id="ure-ospf-router-lsa">
<figure id="figure-1">
        <div class="artwork art-text alignLeft" id="section-3-3.1">
<pre>
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            LS age             |     Options   |       1       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                        Link State ID                          |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Advertising Router                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     LS sequence number                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |         LS checksum           |             length            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |H|0|0|N|W|V|E|B|        0      |            # links            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          Link ID                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         Link Data                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |     Type      |     # TOS     |            metric             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              ...                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      TOS      |        0      |          TOS metric           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                          Link ID                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                         Link Data                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                              ...                              |</pre>
</div>
<figcaption><a href="#figure-1" class="selfRef">Figure 1</a>:
<a href="#name-ospf-router-lsa" class="selfRef">OSPF Router-LSA</a>
        </figcaption></figure>
</div>
<p id="section-3-4">Bit H is the high-order bit of the OSPF flags, as shown below.<a href="#section-3-4" class="pilcrow">¶</a></p>
<span id="name-ospf-router-lsa-option-bits"></span><div id="ure-ospf-router-lsa-option-bits">
<figure id="figure-2">
        <div class="artwork art-text alignLeft" id="section-3-5.1">
<pre>
                            0 1 2 3 4 5 6 7
                            +-+-+-+-+-+-+-+-+
                            |H|0|0|N|W|V|E|B|
                            +-+-+-+-+-+-+-+-+</pre>
</div>
<figcaption><a href="#figure-2" class="selfRef">Figure 2</a>:
<a href="#name-ospf-router-lsa-option-bits" class="selfRef">OSPF Router-LSA Option Bits</a>
        </figcaption></figure>
</div>
<p id="section-3-6">
   When the H-bit is set, the OSPFv2 router is a host (non-transit)
   router and is incapable of forwarding transit traffic.  In this mode,
   the other OSPFv2 routers in the area <span class="bcp14">MUST NOT</span> use the host router for
   transit traffic but may send traffic to its local destinations.<a href="#section-3-6" class="pilcrow">¶</a></p>
<p id="section-3-7">
   An OSPFv2 router originating a router-LSA with the H-bit set <span class="bcp14">MUST</span>
   advertise all its non-stub links with a link cost of MaxLinkMetric
   <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span>.<a href="#section-3-7" class="pilcrow">¶</a></p>
<p id="section-3-8">
   When the H-bit is set, an Area Border Router (ABR) <span class="bcp14">MUST</span> advertise the
   same H-bit setting in its self-originated router-LSAs for all
   attached areas.  The consistency of the setting will prevent
   inter‑area traffic transiting through the router by suppressing
   advertisements of prefixes from other routers in the area in its
   summary-LSAs.  Only IPv4 prefixes associated with its local
   interfaces <span class="bcp14">MUST</span> be advertised in summary-LSAs to provide reachability
   to end hosts attached to a router with the H-bit set.<a href="#section-3-8" class="pilcrow">¶</a></p>
<p id="section-3-9">
   When the H-bit is set, the host router cannot act as an Autonomous System
   Border Router (ASBR).  Indeed, ASBRs are transit routers to prefixes that are
   typically imported through redistribution of prefixes from other
   routing protocols.  Therefore, non-local IPv4 prefixes, e.g., those
   imported from other routing protocols, <span class="bcp14">SHOULD NOT</span> be advertised in
   AS-external-LSAs if the H-bit is set.  Some use cases, such as an
   overloaded router or a router being gracefully isolated, may benefit
   from continued advertisements of non-local prefixes.  In these cases,
   the Type 2 metric in AS-external-LSAs <span class="bcp14">MUST</span> be set to
   LSInfinity <span>[<a href="#RFC2328" class="xref">RFC2328</a>]</span> to
   repel traffic (see <a href="#sect-6" class="xref">Section 6</a> of this document).<a href="#section-3-9" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-4">
<section id="section-4">
      <h2 id="name-spf-modifications">
<a href="#section-4" class="section-number selfRef">4. </a><a href="#name-spf-modifications" class="section-name selfRef">SPF Modifications</a>
      </h2>
<p id="section-4-1">
   The SPF calculation described in <span><a href="https://www.rfc-editor.org/rfc/rfc2328#section-16.1" class="relref">Section 16.1</a> of [<a href="#RFC2328" class="xref">RFC2328</a>]</span> is
   modified to ensure that the routers originating router-LSAs with the
   H-bit set will not be used for transit traffic.  Step (2) is
   modified to include a check on the H-bit, as shown below.  (Please note
   that all of the sub-procedures of Step (2) remain unchanged and are not included in
   the excerpt below.)<a href="#section-4-1" class="pilcrow">¶</a></p>
<ul class="ulEmpty">
<li class="ulEmpty" id="section-4-2.1">
          <dl class="dlParallel" id="section-4-2.1.1">
            <dt id="section-4-2.1.1.1">(2)</dt>
<dd style="margin-left: 2.5em" id="section-4-2.1.1.2">Call the vertex just added to the
            tree "vertex V".  Examine the LSA
            associated with vertex V.  This is
            a lookup in Area A's link state
            database based on the Vertex ID. If
            this is a router-LSA, and the H-bit
            of the router-LSA is set, and
            vertex V is not the root, then the
            router should not be used for transit
            and Step (3) should be executed
            immediately. If this is a router-LSA
            and bit V of the router-LSA (see
            Appendix A.4.2) is set, set Area A's
            TransitCapability to TRUE. In any case,
            each link described by the LSA gives
            the cost to an adjacent vertex.  For
            each described link (say it joins
            vertex V to vertex W):<a href="#section-4-2.1.1.2" class="pilcrow">¶</a>
</dd>
</dl>
</li>
</ul>
</section>
</div>
<div id="sect-5">
<section id="section-5">
      <h2 id="name-autodiscovery-and-backward-">
<a href="#section-5" class="section-number selfRef">5. </a><a href="#name-autodiscovery-and-backward-" class="section-name selfRef">Autodiscovery and Backward Compatibility</a>
      </h2>
<p id="section-5-1">
   To reduce the possibility of any routing loops due to partial
   deployment, this document defines an OSPF Router Information (RI) LSA
   capability bit <span>[<a href="#RFC7770" class="xref">RFC7770</a>]</span>.  See
   <a href="#sect-7" class="xref">Section 7</a> (<a href="#tab-2" class="xref">Table 2</a>).<a href="#section-5-1" class="pilcrow">¶</a></p>
<p id="section-5-2">The RI LSA <span class="bcp14">MUST</span> be area-scoped.<a href="#section-5-2" class="pilcrow">¶</a></p>
<p id="section-5-3">
   Autodiscovery via announcement of the OSPF Host Router capability
   (<a href="#sect-7" class="xref">Section 7</a>)
   ensures that the H-bit functionality and its associated SPF changes
   <span class="bcp14">MUST</span> only take effect if all the routers in a given OSPF area support
   this functionality.<a href="#section-5-3" class="pilcrow">¶</a></p>
<p id="section-5-4">
   In normal operation, it is possible that the RI LSA will fail to
   reach all routers in an area in a timely manner.  For example, if a
   new router without H-bit support joins an area that previously had
   only H-bit-capable routers with the H-bit set, then it may take some time
   for the RI LSA to propagate to all routers.  While it is propagating, the
   routers in the area will gradually detect the presence of a router
   that does not support the capability and will revert back to the normal SPF
   calculation.  During the propagation time, the area as a whole is
   unsure of the status of the new router; this type of situation can cause temporary
   transient loops.<a href="#section-5-4" class="pilcrow">¶</a></p>
<p id="section-5-5">
   The following recommendations will mitigate transient routing loops:<a href="#section-5-5" class="pilcrow">¶</a></p>
<ul>
<li id="section-5-6.1">Implementations are <span class="bcp14">RECOMMENDED</span> to provide a configuration
      parameter to manually override enforcement of the H-bit
      functionality in partial deployments where the topology guarantees
      that OSPFv2 routers not supporting the H-bit do not compute routes
      resulting in routing loops.<a href="#section-5-6.1" class="pilcrow">¶</a>
</li>
<li id="section-5-6.2">All routers with the H-bit set <span class="bcp14">MUST</span> advertise all of the router's
      non-stub links with a metric equal to MaxLinkMetric <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span> in
      its LSAs in order to prevent OSPFv2 routers (unless a last-resort path)
      that do not support the H-bit from attempting to use the non-stub links for transit
      traffic.<a href="#section-5-6.2" class="pilcrow">¶</a>
</li>
<li id="section-5-6.3">All routers supporting the H-bit <span class="bcp14">MUST</span> check the RI LSAs of all
      nodes in the area to verify that all nodes support the H-bit
      before actively using the H-bit feature.  If any router does not
      advertise the OSPF Host Router capability (<a href="#sect-7" class="xref">Section 7</a>), then the SPF
      modifications described in <a href="#sect-4" class="xref">Section 4</a> <span class="bcp14">MUST NOT</span> be used in the area.<a href="#section-5-6.3" class="pilcrow">¶</a>
</li>
</ul>
</section>
</div>
<div id="sect-6">
<section id="section-6">
      <h2 id="name-ospf-as-external-lsas-nssa-">
<a href="#section-6" class="section-number selfRef">6. </a><a href="#name-ospf-as-external-lsas-nssa-" class="section-name selfRef">OSPF AS-External-LSAs / NSSA-LSAs with Type 2 Metrics</a>
      </h2>
<p id="section-6-1">
   When calculating the path to a prefix in an OSPF AS-external-LSA or
   NSSA-LSA <span>[<a href="#RFC3101" class="xref">RFC3101</a>]</span> with a Type 2 metric, the advertised Type 2 metric
   is taken as more significant than the OSPF intra-area or inter-area
   path.  Hence, advertising the links with MaxLinkMetric as specified
   in <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span> does not discourage transit
   traffic when calculating AS-external or NSSA routes with Type 2 metrics.<a href="#section-6-1" class="pilcrow">¶</a></p>
<p id="section-6-2">
   Consequently, this document updates <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span> so that the Type 2 metric in any
   self-originated AS-external-LSAs or NSSA-LSAs is advertised as
   LSInfinity-1 <span>[<a href="#RFC2328" class="xref">RFC2328</a>]</span>.
  If the H-bit is set, then the Type 2 metric
   <span class="bcp14">MUST</span> be set to LSInfinity.<a href="#section-6-2" class="pilcrow">¶</a></p>
</section>
</div>
<div id="sect-7">
<section id="section-7">
      <h2 id="name-iana-considerations">
<a href="#section-7" class="section-number selfRef">7. </a><a href="#name-iana-considerations" class="section-name selfRef">IANA Considerations</a>
      </h2>
<p id="section-7-1">
   IANA has registered the following value in the 
   "OSPFv2 Router Properties Registry".<a href="#section-7-1" class="pilcrow">¶</a></p>
<span id="name-h-bit"></span><div id="tab-1">
<table class="center" id="table-1">
        <caption>
<a href="#table-1" class="selfRef">Table 1</a>:
<a href="#name-h-bit" class="selfRef">H-Bit</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">Value</th>
            <th class="text-left" rowspan="1" colspan="1">Description</th>
            <th class="text-left" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-left" rowspan="1" colspan="1">0x80</td>
            <td class="text-left" rowspan="1" colspan="1">Host (H-bit)</td>
            <td class="text-left" rowspan="1" colspan="1">RFC 8770</td>
          </tr>
        </tbody>
      </table>
</div>
<p id="section-7-3">
   IANA has registered the following in the "OSPF Router
   Informational Capability Bits" registry.<a href="#section-7-3" class="pilcrow">¶</a></p>
<span id="name-ospf-host-router-capability"></span><div id="tab-2">
<table class="center" id="table-2">
        <caption>
<a href="#table-2" class="selfRef">Table 2</a>:
<a href="#name-ospf-host-router-capability" class="selfRef">OSPF Host Router Capability Bit</a>
        </caption>
<thead>
          <tr>
            <th class="text-left" rowspan="1" colspan="1">Bit Number</th>
            <th class="text-left" rowspan="1" colspan="1">Capability Name</th>
            <th class="text-left" rowspan="1" colspan="1">Reference</th>
          </tr>
        </thead>
        <tbody>
          <tr>
            <td class="text-center" rowspan="1" colspan="1">7</td>
            <td class="text-left" rowspan="1" colspan="1">OSPF Host Router</td>
            <td class="text-left" rowspan="1" colspan="1">RFC 8770</td>
          </tr>
        </tbody>
      </table>
</div>
</section>
</div>
<div id="sect-8">
<section id="section-8">
      <h2 id="name-security-considerations">
<a href="#section-8" class="section-number selfRef">8. </a><a href="#name-security-considerations" class="section-name selfRef">Security Considerations</a>
      </h2>
<p id="section-8-1">
   This document introduces the H-bit, which is a capability feature that
   restricts the use of a router for transit, while only its local
   destinations are reachable.  This is a subset of the operations of a
   normal router and therefore should not introduce new security
   considerations beyond those already known in OSPFv2 <span>[<a href="#RFC2328" class="xref">RFC2328</a>]</span>.  The
   feature introduces the advertisement of host router capability
   information to all OSPFv2 routers in an area.  This information can
   be leveraged for discovery and verification that all routers in the
   area support the capability before the feature is turned on.  In the
   event that a rogue or buggy router incorrectly advertises its
   capability, possible scenarios are as follows:<a href="#section-8-1" class="pilcrow">¶</a></p>
<ul>
<li id="section-8-2.1">The router does not have the capability but sends the H-bit set in
      its LSAs. In this case, a routing loop is possible.
      However, this is mitigated by the fact that this router should be
      avoided anyway.  Moreover, the link metrics cost (MaxLinkMetric)
      of this router will mitigate this situation.  In any case, a
      router advertising the H-bit capability without its link metrics cost
      equal to MaxLinkMetric could be a rogue
      router and should be avoided.<a href="#section-8-2.1" class="pilcrow">¶</a>
</li>
<li id="section-8-2.2">The router has the capability but sends the H-bit clear in its
      LSAs. In this case, the router merely prevents the support of other
      H-bit routers in the area and prevents all the routers from running the modified
      SPF.  Any impacts are also mitigated in this scenario, as other H-bit routers in the
      area also advertise the MaxLinkMetric cost, so they will still be
      avoided unless they are the last‑resort path.<a href="#section-8-2.2" class="pilcrow">¶</a>
</li>
<li id="section-8-2.3">The rogue router is on the only transit path for some destinations
      and sends the H-bit set (for no good/valid reason) in its LSAs, and
      effectively partitions the network.  This case is indistinguishable
      from the normal case where an operator may consciously decide to
      set the H-bit to perform maintenance on a router that is on the
      only transit path.  The OSPF protocol will continue to function
      within the partitioned domains.<a href="#section-8-2.3" class="pilcrow">¶</a>
</li>
</ul>
</section>
</div>
<section id="section-9">
      <h2 id="name-references">
<a href="#section-9" class="section-number selfRef">9. </a><a href="#name-references" class="section-name selfRef">References</a>
      </h2>
<section id="section-9.1">
        <h3 id="name-normative-references">
<a href="#section-9.1" class="section-number selfRef">9.1. </a><a href="#name-normative-references" class="section-name selfRef">Normative References</a>
        </h3>
<dl class="references">
<dt id="RFC2119">[RFC2119]</dt>
<dd>
<span class="refAuthor">Bradner, S.</span>, <span class="refTitle">"Key words for use in RFCs to Indicate Requirement Levels"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 2119</span>, <span class="seriesInfo">DOI 10.17487/RFC2119</span>, <time datetime="1997-03">March 1997</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2119">https://www.rfc-editor.org/info/rfc2119</a>&gt;</span>. </dd>
<dt id="RFC2328">[RFC2328]</dt>
<dd>
<span class="refAuthor">Moy, J.</span>, <span class="refTitle">"OSPF Version 2"</span>, <span class="seriesInfo">STD 54</span>, <span class="seriesInfo">RFC 2328</span>, <span class="seriesInfo">DOI 10.17487/RFC2328</span>, <time datetime="1998-04">April 1998</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc2328">https://www.rfc-editor.org/info/rfc2328</a>&gt;</span>. </dd>
<dt id="RFC6987">[RFC6987]</dt>
<dd>
<span class="refAuthor">Retana, A.</span><span class="refAuthor">, Nguyen, L.</span><span class="refAuthor">, Zinin, A.</span><span class="refAuthor">, White, R.</span><span class="refAuthor">, and D. McPherson</span>, <span class="refTitle">"OSPF Stub Router Advertisement"</span>, <span class="seriesInfo">RFC 6987</span>, <span class="seriesInfo">DOI 10.17487/RFC6987</span>, <time datetime="2013-09">September 2013</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc6987">https://www.rfc-editor.org/info/rfc6987</a>&gt;</span>. </dd>
<dt id="RFC7770">[RFC7770]</dt>
<dd>
<span class="refAuthor">Lindem, A., Ed.</span><span class="refAuthor">, Shen, N.</span><span class="refAuthor">, Vasseur, JP.</span><span class="refAuthor">, Aggarwal, R.</span><span class="refAuthor">, and S. Shaffer</span>, <span class="refTitle">"Extensions to OSPF for Advertising Optional Router Capabilities"</span>, <span class="seriesInfo">RFC 7770</span>, <span class="seriesInfo">DOI 10.17487/RFC7770</span>, <time datetime="2016-02">February 2016</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc7770">https://www.rfc-editor.org/info/rfc7770</a>&gt;</span>. </dd>
<dt id="RFC8174">[RFC8174]</dt>
<dd>
<span class="refAuthor">Leiba, B.</span>, <span class="refTitle">"Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words"</span>, <span class="seriesInfo">BCP 14</span>, <span class="seriesInfo">RFC 8174</span>, <span class="seriesInfo">DOI 10.17487/RFC8174</span>, <time datetime="2017-05">May 2017</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc8174">https://www.rfc-editor.org/info/rfc8174</a>&gt;</span>. </dd>
</dl>
</section>
<section id="section-9.2">
        <h3 id="name-informative-references">
<a href="#section-9.2" class="section-number selfRef">9.2. </a><a href="#name-informative-references" class="section-name selfRef">Informative References</a>
        </h3>
<dl class="references">
<dt id="BGP-ORR">[BGP-ORR]</dt>
<dd>
<span class="refAuthor">Raszuk, R., Ed.</span><span class="refAuthor">, Cassar, C.</span><span class="refAuthor">, Aman, E.</span><span class="refAuthor">, Decraene, B.</span><span class="refAuthor">, and K. Wang</span>, <span class="refTitle">"BGP Optimal Route Reflection (BGP-ORR)"</span>, <span class="seriesInfo">Work in Progress, Internet-Draft, draft-ietf-idr-bgp-optimal-route-reflection-20</span>, <time datetime="2020-01-08">8 January 2020</time>, <span>&lt;<a href="https://tools.ietf.org/html/draft-ietf-idr-bgp-optimal-route-reflection-20">https://tools.ietf.org/html/draft-ietf-idr-bgp-optimal-route-reflection-20</a>&gt;</span>. </dd>
<dt id="RFC3101">[RFC3101]</dt>
<dd>
<span class="refAuthor">Murphy, P.</span>, <span class="refTitle">"The OSPF Not-So-Stubby Area (NSSA) Option"</span>, <span class="seriesInfo">RFC 3101</span>, <span class="seriesInfo">DOI 10.17487/RFC3101</span>, <time datetime="2003-01">January 2003</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc3101">https://www.rfc-editor.org/info/rfc3101</a>&gt;</span>. </dd>
<dt id="RFC5340">[RFC5340]</dt>
<dd>
<span class="refAuthor">Coltun, R.</span><span class="refAuthor">, Ferguson, D.</span><span class="refAuthor">, Moy, J.</span><span class="refAuthor">, and A. Lindem</span>, <span class="refTitle">"OSPF for IPv6"</span>, <span class="seriesInfo">RFC 5340</span>, <span class="seriesInfo">DOI 10.17487/RFC5340</span>, <time datetime="2008-07">July 2008</time>, <span>&lt;<a href="https://www.rfc-editor.org/info/rfc5340">https://www.rfc-editor.org/info/rfc5340</a>&gt;</span>. </dd>
</dl>
</section>
</section>
<div id="acknowledgements">
<section id="section-appendix.a">
      <h2 id="name-acknowledgements">
<a href="#name-acknowledgements" class="section-name selfRef">Acknowledgements</a>
      </h2>
<p id="section-appendix.a-1">
   The authors would like to acknowledge <span class="contact-name">Hasmit Grover</span> for discovering
   the limitation in <span>[<a href="#RFC6987" class="xref">RFC6987</a>]</span>, and <span class="contact-name">Acee Lindem</span>, <span class="contact-name">Abhay Roy</span>, <span class="contact-name">David Ward</span>, <span class="contact-name">Burjiz Pithawala</span>, and <span class="contact-name">Michael Barnes</span> for their comments.<a href="#section-appendix.a-1" class="pilcrow">¶</a></p>
</section>
</div>
<div id="authors-addresses">
<section id="section-appendix.b">
      <h2 id="name-authors-addresses">
<a href="#name-authors-addresses" class="section-name selfRef">Authors' Addresses</a>
      </h2>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Keyur Patel</span></div>
<div dir="auto" class="left"><span class="org">Arrcus</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:keyur@arrcus.com" class="email">keyur@arrcus.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Padma Pillay-Esnault</span></div>
<div dir="auto" class="left"><span class="org">PPE Consulting</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:padma.ietf@gmail.com" class="email">padma.ietf@gmail.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Manish Bhardwaj</span></div>
<div dir="auto" class="left"><span class="org">Cisco Systems</span></div>
<div dir="auto" class="left"><span class="street-address">170 W. Tasman Drive</span></div>
<div dir="auto" class="left">
<span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95134</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:manbhard@cisco.com" class="email">manbhard@cisco.com</a>
</div>
</address>
<address class="vcard">
        <div dir="auto" class="left"><span class="fn nameRole">Serpil Bayraktar</span></div>
<div dir="auto" class="left"><span class="org">Cisco Systems</span></div>
<div dir="auto" class="left"><span class="street-address">170 W. Tasman Drive</span></div>
<div dir="auto" class="left">
<span class="locality">San Jose</span>, <span class="region">CA</span> <span class="postal-code">95134</span>
</div>
<div dir="auto" class="left"><span class="country-name">United States of America</span></div>
<div class="email">
<span>Email:</span>
<a href="mailto:serpil@cisco.com" class="email">serpil@cisco.com</a>
</div>
</address>
</section>
</div>
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