This week I was mindlessly going around “the internet” and stumbled upon an interesting IETF RFC draft from the Inter Domain Routing Working Group (IDR WG). The draft is for Large BGP Communities and is currently its 6th version.

What is it why do we need it?

You probably heard of BGP communities. This BGP attribute is defined in RFC 1997 is one of the most used attributes to help service providers apply specific routing policies to a group of prefixes sharing some common property.

BGP Communities are 4 Byte (32bit) values represented as A:B, where A is the decimal representation of the first 2 Bytes and B the decimal representation of the lower 2 Bytes. It is common practice to use the first 2 Bytes as an AS number and the last 2 Bytes to convey information to upstream routers (such as a Local Preference value to be set).

But there is a problem with this. RFC 1997 has been in use since 1996 and since then a lot has changed. One of these changes is the RFC6793 which defined the capability of BGP speakers to use 4 Byte (32bit) AS numbers instead of the shorter 2 Byte (16 bit) AS numbers. Continue reading →