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.
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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: