Hello Diafeked
BGP communities are additional piece of information attached to prefixes that tell BGP routers how to handle those prefixes. Communities are shared between iBGP and eBGP peers alike. BGP communities are transitive BGP attributes, which means they are shared between different Autonomous Systems.
Now there are two types of communities. The first consists of four “well known” communities which are predefined for all BGP routers. These are described in the lesson. The second type consists of extended communities, that is, those that are not well known. These are custom communities that you can define yourself. So when we say type, there really isn’t any difference in the way the communities are actually managed, it’s just a set of predefined communities that all BGP routers know about. All communities are managed in the same way.
Now some vendors, like Cisco have created extended BGP communities that are predefined within their own devices. One such extended community is indeed the cost community. Cisco has predefined this extended community to function in a specific way, and can be enabled by using predefined commands. Specifically, the set extcommuhity cost command can be used to create this community, along with all of the parameters that Cisco has predetermined. This specific extended community is a configured to be a non transient extended community, and this does indeed mean that it is passed between iBGP peers but not between eBGP peers.
You can find out more info about the cost extended BGP community at the following Cisco documentation:
I hope this has been helpful!
Laz