A sitemap index is an XML file whose job is to point to other sitemaps. A single sitemap file is limited in size, so a big site splits its pages across several files, such as one for posts and one for pages, and publishes an index that lists them. Content management systems and SEO plugins commonly produce one automatically, which is why the address you see is often sitemap_index.xml rather than sitemap.xml.
<sitemapindex>
<sitemap><loc>https://example.com/post-sitemap.xml</loc></sitemap>
<sitemap><loc>https://example.com/page-sitemap.xml</loc></sitemap>
</sitemapindex>Why it matters
A tool that read only the index would find no pages at all, because an index contains no page addresses. To see the site, it has to open each sitemap listed and read the pages inside. It matters when you give an address to a scanner too: an index is as good a starting point as an ordinary sitemap, but the results depend on the files beneath it being readable.
How Link Signals treats it
Link Signals follows an index to every sitemap it lists, and reads compressed .xml.gz files as well. For a scan it takes the pages your plan allows; if the sitemap holds more than that, it reads a sample spread across the whole site. When it is given a site address rather than a sitemap address, it looks in robots.txt for Sitemap: lines, and then tries the usual locations, including /sitemap.xml, /sitemap_index.xml, /sitemap-index.xml and /wp-sitemap.xml. At most 10 sitemap files from one robots.txt or request are read. See sitemap discovery.
In monitoring, a file that cannot be read on a given day does not make findings disappear. The check is tried again a few hours later.
Common misunderstandings
- An index is not a list of pages, so “0 pages” from an index usually means the files under it could not be read, not that your site is empty.
- The dates in an index belong to the sitemap files. The dates that matter for monitoring are the lastmod values on the pages inside them.
- A page missing from every sitemap is not read at all, however well it is linked.
Questions
Which address should I give Link Signals?
Either the index or any single sitemap works. If you are not sure of the address, find your sitemap URL.
Can a sitemap list other sitemaps from another domain?
An index can point anywhere, but search engines expect the files to be on the same site. Link Signals reads what the index lists, so check a surprising entry before you trust it.
Run a scan and find where these show up on your own site.
