Understanding Low Visibility Prefixes

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BGP.Tools aims to provide a “real time” view of the internet. However since bgp.tools reports on how other networks see the internet, there are going to be differences in propagation speeds between networks.

In rare cases these propagations (especially with prefix withdrawals) can take a long time (up to days) to flush out of everyone’s routing tables.

Rather than display prefixes that only a few sessions can see on bgp.tools, the site will automatically hide prefixes that only visible from less than 300 different feeds (or 15%), unless the user chooses to see them.

In rare cases routes can get “stuck” and will persist until the router that is stuck in reboots or flaps. Investigations have been done on this issue in the past and some mitigations are in progress, but the long tail of fixing everyones routers will result in this problem being present for a long time.

Sometimes a low visibility route may mean that a prefix is being filtered for most peers, or is more specific prefix only visible to customers. These may automatically get put into low visibility prefixes depending on how visible (how many sessions can see the prefix) they are.