Another good strategy for large sites is to crosslink pages very low in the hierarchy to each other. This is one of the reasons that large ecommerce retailers will have Related Products and Customers Also Bought links on their products pages (the main reason, of course, is that it’s good for sales).
Since Googlebot crawls in from links (rather than strictly crawling top-down) this kind of cross linking can provide an additional path for the pages very low in a site’s hierarchy to get crawled.
It is usually not necessary to crosslink pages higher in the site hierarchy: by the very nature of being high in the site hierarchy, there will be lots of paths for Google to crawl to them (and they will have lots of authority flowing to them). It’s also worth noting that smaller sites with only a few hundred pages usually do not need to worry about this kind of crosslinking at all.
Crosslinking Gone Wild!
I have seen plenty of sites that get out of control with crosslinking: every page has a giant list of dozens (or hundreds!) of links at the bottom. An inexperienced SEO figured that crosslinking would help flow authority around, and they wanted to get as much authority to as many pages as possible.
At first glance, it seems like a nice theory, but them problem is you are essentially removing the hierarchy of the site and creating a flat structure. Yes, you are getting more authority to all those low hierarchy pages, but at the cost of lowering the authority of your most important pages.
I’ll explain how this works in detail in the PageRank Flow section. But for now just understand that you want a hierarchal site structure. Crosslinking the bottom of your hierarchy to a few other pages on the bottom can be good for large sites, but going too far will hurt your overall ranking ability.