BGP Community Attribute is an Optional Transitive BGP Path Attribute. With BGP Community Attribute, destinations are grouped and BGP route policy decisions are done special to each of these communities. BGP Community Attributes are assigned to specific prefixes and advertised to the neighbors. Neighbor which receives this advertisements recognize this value and behave according to this Community Attribute. (Advertise again or not).
With BGP Community Attribute, we can apply special policies to special customers. This type of configuration gives customers easy to manage their BGP routes independant form you.
BGP Communities are 32 bits long and can be assigned in three different formats. These 32 bits Community Number range range is from 0 to 4.294.967.295. It can be used with Decimal or Hexadecimal numbers.
There is also a third, new format consit of two 16 bits number. It is used like (0 to 65535):(0 to 65535). The first nubmer is AS Number and the second number is Community Number.
(AS Number : Community Number)
The default BGP Community format is Decimal. This can be changed with a configuration command.
There are four different Well Known BGP Communities. These BGP Well Known Communities are given below with their definitions:
Now, let’s see what are these Special Communities, Well Known BGP Communities one by one.
This Community prevents advertising the routes to outside the Local Autonomus System. The routes coming with this attribute, are not advertised to eBGP peers. So, you can advertise routes to iBGP neigbors with this Well Known BGP Communities. With this BGP No-Export Community, we prevent our Autonomous System also to become a Transit AS.
Think about it. If you are advertising any routes from AS X to AS Y with BGP Community No-Export, AS Y do not advertise these learned routes to any other AS. So, we prevent AS Y to become a Transit AS.
The value of No-Export Community is FFFF:FF01.
This No-Advertise Community prevents advertising the routes to any peer, internal or external. So, the router that gets this type route, it stores it only and does not send it to any iBGP or eBGP neighbor.
The value of No-Advertise Community is FFFF:FF02.
This Community prevents advertising the routes to ouside the Local Autonomous System (AS) or other Confederation peers. It is similar to No export except one point. With this Well Known community, you can advertise the routes only to the routers in the same BGP Confederation. In other words, you can advertise routes inside the same Confederation sub-AS, you can not advertise the routes to the other Confederation Sub-AS.
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