In v6 you could set set-bgp-prepend (https://wiki.mikrotik.com/wiki/Manual:R ... ng_filters) to prepend AS Path with router's own AS number in both input and output filters.

In v7 you have settings:
1. bgp-path-peer-prepend - prepends with peer's AS, works only in input
2. bgp-path-prepend - prepends with router's own AS, works only in output

I need to prepend AS Path of imported prefixes with router's own AS. How to do it? Why bgp-path-prepend doesn't work in input (v7.11.2)?

Because BGP adds its own ASN only when sending routes to remote peers (so prepending own as is possible only in output).
By adding local as in input you are deliberately "saying" that those routes are looped, you might as well just reject the routes.

Do you suggest that the solution is impossible, even though it works on Linux and Mikrotik v6?

I don't know the internals of Mikrotik v7 BGP, but it lacks functionality.

The problem is that in AS Path "AS3 AS2 AS2 AS2 AS1" you can't tell which of "AS2 AS2 AS2" was added by the AS2 router in output and which by the AS3 router in input, so basically, you don't know what router prepends AS Path.

Because BGP adds its own ASN only when sending routes to remote peers (so prepending own as is possible only in output).
By adding local as in input you are deliberately "saying" that those routes are looped, you might as well just reject the routes.

No, the purpose of prepending own AS on received routes is to artificially increase the number of hops for one of the paths.
So the prepending is to be done AFTER it has been checked to be a loop.

the purpose of prepending own AS on received routes is to artificially increase the number of hops

For that bgp-path-peer-prepend exists

The problem is that in AS Path "AS3 AS2 AS2 AS2 AS1" you can't tell which of "AS2 AS2 AS2" was added by the AS2 router in output and which by the AS3 router in input, so basically, you don't know what router prepends AS Path.

Don't get your point, how prepending in input would allow to determine which router prepended AS2? AS path is a list of ASNs there is no extra info to determine such things.

To me prepending with own AS is normal action.

To me AS Path "AS3 AS2 AS2 AS2 AS1" means that AS2 router prepended itself 3 times, either in the input or output, so that's the router I need to check to change prepend settings. That's how I use Linux (bird) and RouterOS v6. That doesn't work on RouterOS v7, because on v7 prepending in the output on AS2 router results in same AS Path as prepending in the input on AS3 router.

I don't understand the idea of prepending with peer AS but if someone uses it that's OK. Just why "bgp-path-prepend" does nothing in input filters?

Also "bgp-path-peer-prepend" works with "+", while "bgp-path-prepend" doesn't.

I request fixing "bgp-path-prepend" so it can be set in input filters and it accepts values with "+" (relative to current default setting which is "bgp-path-prepend 0" in input and "bgp-path-prepend 1" in output).