BGP peering does not auto-discover. Every session is deliberately configured, runs over TCP port 179, and walks a strict finite state machine before a single route flows. The foundation you need before touching BGP config.
Every time you load a webpage, your traffic crosses networks owned by different organizations. BGP, the Border Gateway Protocol, is what stitches 75,000+ autonomous systems into a single working internet. Here is how, and why.
Prefix lists are the preferred BGP filtering tool: faster than ACLs, easier to read, and built for matching networks with specific prefix lengths. Every production BGP deployment needs them on inbound and outbound.
BGP confederations scale iBGP by dividing the AS into sub-ASes that speak confederation eBGP internally, while appearing as a single AS to the outside world. An alternative to route reflectors with tradeoffs you need to know.
Ten routers need 45 iBGP sessions. Fifty need 1,225. Route reflectors break the iBGP split-horizon rule in a controlled way, and are the standard scaling tool for iBGP in enterprise and service provider networks.
One of the most common iBGP problems is routes that show up in the BGP table but never hit the routing table because the next-hop is unreachable. Here is when next-hop-self is the fix, and when it actually makes things worse.
iBGP has rules eBGP does not: split-horizon, full-mesh, and next-hop preservation. Ignore any of them and routes vanish silently from your network. A practical guide to configuring iBGP neighbors and full mesh.
A single mismatch in neighbor IP, AS number, or filtering keeps a BGP session down without a helpful error. A practical Cisco IOS XE eBGP configuration walkthrough including dual-homed peering and source/TTL considerations.
BGP does not auto-advertise connected interfaces like OSPF. Routes must be injected via network, redistribution, or aggregation. Understand NLRI, withdrawn routes, and the Adj-RIB pipeline to control what you advertise.
BGP evaluates 13 steps in order and stops at the first one that produces a winner. Here is the full order, with steps 1 and 2 proven on real show ip bgp output from a three-AS CML lab where the best marker moves between two paths as one attribute changes.
An unauthenticated BGP session is vulnerable to TCP-based attacks and spoofed RSTs. MD5 has been the standard for decades; TCP-AO is the modern replacement. A practical guide to BGP session authentication.
A BGP default route tells a peer to send anything it does not have a more specific path for. Different injection methods, default-originate, network, redistribute, have different behaviors and failure modes. Pick carefully.