The BGP RFC 4271 reads :
In the context of this document, we assume that a BGP speaker advertises to its peers only those routes that it uses itself (in this context, a BGP speaker is said to "use" a BGP route if it is the most preferred BGP route and is used in forwarding). All other cases are outside the scope of this document.
When a BGP speaker receives a path for a given destination that it will not use (for example, because it already knows a smaller path), according to this, this means it will not "re-advertise" it to its peers. Is that mandatory ? Can the BGP speaker save this path, even if it is not re-advertising it, so it has a backup, for example in the case the path it already knows disappears for some reason ?
I tried looking whether this was the case or not in HE looking glasses, and for every destination I tried, I only found a single path. Is it because these (possibly existing or not) alternative paths are not displayed in looking glasses, but are they still recorded in the local BGP speaker "view" of the Internet ?