This number simple represents how many hop counts is the router away. We would require to use ebgp-multihop keyword with neighbor statement so that neighbors which are not directly connected can form relationship with each other. if directly connected, no need to use it. If you are using a loopback interfaces to form neighbor relationshipsbetween diectly connected routers, then you need to use hop of 1.
I am not understanding how the IGP might cause a black hole if it is not implemented correctly. Perhaps you can clarify this risk?
If you configure ebgp multihop for an external neighbor that is directly connected then it will have these effects: 1) if the neighbor peer address is not the physical interface address (perhaps the neighbor address is its loopback) then you will be able to send packets with hop count greater than the default of 1 and successfully peer with the neighbor; 2) if the link to the directly connected neighbor goes down and if you have a path to that neighbor through some other interface (and if you have a route to the neighbor address through the other interface) then you will be able to maintain the BGP neighbor relationship.
Note that for a directly connected neighbor specifying a hop count of 255 might seem a bit high. Generally you might want to specify a reasonable number of hops for a realistic alternate path to the neighbor and it is hard to imagine a realistic alternate path to a directly connected neighbor that would take 255 hops.
Maybe I should clarify this, the situation I am describing has to do these two BGP peering routers has the same IGP domain also. Let's assume two routers are connected via the same ethernet, one side of the switch port went down, the otherside will stay up. If they are in the same IGP domain and a different eBGP domain (with the same IGP domain) in the middle. Instead a hard down, traffic will be black holed.
This is a weird scenario but it would happen, and depend on the design.
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