Default BGP Scanner Behavior

[cisc0.ameer]

in this link Link it is described about   Default BGP Scanner Behavior

BGP monitors the next hop of installed routes to verify next-hop reachability and to select, install, and validate the BGP best path. By default, the BGP scanner is used to poll the RIB for this information every 60 seconds. During the 60 second time period between scan cycles, Interior Gateway Protocol (IGP) instability or other network failures can cause black holes and routing loops to temporarily form.

that line i change to blue color i am not understanding what does meaning by cause black holes

regard

[Jon Marshall · Accepted solution]

In a network context when someone talks about traffic being black holed they usually mean packets being dropped by the network. 

So the BGP scanner checks the IP routing table for a next hop IP and finds it available but before the next run of the BGP scanner the next hop becomes unreachable but BGP still thinks it is reachable and so still uses routes with that next hop IP so traffic would be black holed until the BGP scanner was run again and the routing information was updated.

Jon

[cisc0.ameer]

thanks  for your replying .

[Joseph W. Doherty]

Just to add a bit to Jon's information, a network "black hole" resembles a stellar black hole in that once a packet enters it, it's not seen again. It's lost, often without any explicit indication that traffic has been lost beyond if the receiver informs the sender (as would TCP) that traffic wasn't received. (BTW, the receiver's information that traffic has been lost might also be lost too as a "black hole" might impact traffic in both directions.)

Sometimes you can config a router to intentionally "black hole" some traffic, for example a static routing statement to a NULL interface or an ACL that drops some selected packets.

Networks during convergence often "black hole" (i.e. lose or drop) packets until they re-converge (and it's not just BGP issue). For example, two routers interconnected via a switch, if one of the routers "dies" the other will still "see" the link being up because of the switch. If a static route points to that link, it will continue to "black hole" traffic (unless a destination tracking feature is used). A dynamic routing protocol will continue to drop traffic until the routing protocol notices the neighbor router is no longer reachable (which can a fraction of a second to large fraction of a minute [or even more]).