Sometimes you realize you made a mistake instantly, sometimes it takes a little while. I botched a basic technical question about BGP route advertisements today. So basic that if I’d thought about it for just a minute before responding I would have caught myself and made sure to answer it correctly.

“There are four routers in ASN 65001, in a full mesh. One of these routers is connected to two other ASNs (100 and 200 respectively). When this router hears a new prefix from ASN 100, will it advertise the route to the router in ASN 200?”

The answer is yes, it will.

I got myself wrapped up in a different scenario and said it would not. The scenario that I had in mind was this: If outer A in ASN 65001 receives a route form ASN 100, it will advertise it to all its IBGP peers (B,C, and D) however those peers will not advertise the EBGP route to each other.

I didn’t realize the mistake until well after the conversation, while I was driving across town. It became very obvious to me that I’d screwed up.

I’ve been kicking myself over this all afternoon.