iBGP Local-AS Details
Did you know you could use the neighbor local-as BGP functionality to fake an iBGP session between different autonomous systems? I knew Cisco IOS supported that monstrosity for ages (supposedly “to merge two ISPs that have different AS numbers”) and added the appropriate tweaks1 into netlab when I added the BGP local-as support in release 1.3.1. Someone couldn’t resist pushing us down that slippery slope, and we ended with IBGP local-as implemented on 18 platforms (almost a dozen network operating systems).
I even wrote a related integration test, and all our implementations passed it until I asked myself a simple question: “But does it work?” and the number of correct implementations that passed the test without warnings dropped to zero.

