Session IDs are very common with Magento and they can cause severe duplicate content issues, as there’s no limit to the number of duplicated URLs that can be generated. Session IDs are used to track a user’s session and they’re usually generated via the checkout, to track what items have been added to cart. A session ID will be appended to the end of URLs, here’s an example: domain.com/page?__SID=df23n54jtklg.
The best way to deal with this is to properly fix the issue, which will require development resource. If you’re not in a position to fix this, I would recommend blocking the URLs via the robots.txt or assigning meta robots rules (noindex, follow) – I would also recommend providing instructions to Google via the parameter handling options in Google Webmaster Tools.
You should also block you checkout pages so that crawlers won’t access pages with the session ID appended to the end of the URL.
In the SEO configuration settings (catalog > search engine optimisation) you can choose whether to serve category paths in product URLs – this is the main thing you need to consider for product URLs. I would recommend using top-level product URLs (eg: domain.com/product.html rather than domain.com/category/sub-cat/product.html) as this will prevent the product being duplicated if it’s featured in more than one category.
I personally really like BazaarVoice, because it provides a lot of customisation options and has some really advanced functionality. That said, BazaarVoice is quite expensive and it’s subscription-based.
The default Magento module is actually quite good, although you might want to use additional module or work with your developer to get more from it.
BazaarVoice also has schema support, making it easier to get the ratings schema appearing on your product listings.
The trailing slash canonical issue is one of the most talked about technical SEO issues with the Magento platform, as most URLs on Magento sites render with or without a trailing slash – meaning there are two versions of each page. This generally becomes a problem when large sites have links pointing to both versions, which can lead to both pages ranking and also means the link value is split between two pages.
The obvious choice with the trailing slash issue is to apply a rewrite rule, however whenever I’ve tried this I’ve always had an issue somewhere (checkout, admin etc). I use MageSEO to assign a primary URL suffix (either / or no /), which then ensures that the canonical tag points to that suffix on all pages across the website.
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