The post by Ethan Banks reminded me of a “mystery” I was trying to solve years ago when developing my first BGP course. The Cisco documentation has always claimed there were four well-known communities (the Internet community being one of them), while the RFC 1997 lists three well-known values. Unfortunately, most people blindly copy the IOS documentation (including the authors of the latest revision of the Cisco’s BGP course) without asking themselves “what the heck is the Internet community”.
I don’t remember when exactly I’ve created the BGP community chapter of that BGP course, but I was able to fetch a very old BGP course description from the Internet Archives … and by that time, the course was in its fifth or sixth revision. It must have been 10 years ago.It was time to revisit the mystery. I’ve tried applying the Internet community to a network originated by the BGP routing process to see what its value is:
router bgp 65000
network 192.168.1.0 route-map SetInternet
set community internet additive
While the router obediently attached the Internet community to the IP prefix, I was no wiser … all show outputs converted the community value into its symbolic name. I had to use Wireshark and analyze the actual routing updates between BGP neighbors to figure out that the Internet community has an illegal value 0:0. Obviously it’s not a well-known community.
If you're looking for more in-depth BGP knowledge, try our Configuring BGP on Cisco Routers e-learning solution. If you just need to enhance your hands-on skill, the BGP Remote Lab Bundle is the perfect choice.
Digging through old materials finally gave me the answer I was looking for: sometimes you need a permit all at the end of the ip community-list (like access-lists, the community-lists have an implicit deny all at the end) and someone decided that permit internet makes more sense than the familiar permit any (yes, that’s correct … you use the keyword internet to match any community in the ip community-list).
And just for the sake of completeness, let me conclude with a ten year old slide explaining this phenomenon: