Configure BGP on Cisco IOS: Peering, Path Selection, and Route Manipulation
Working reference for BGP on Cisco IOS. eBGP vs iBGP and the iBGP full-mesh problem, peering setup over physical interfaces vs loopbacks (with update-source and next-hop-self), the network statement and its requirement that the prefix be in the IP routing table, the nine-step path-selection process (Weight - Local Pref - AS-Path - Origin - MED - eBGP/iBGP - IGP cost - Router ID), the four most-used manipulations (local-pref for outbound preference, AS-Path prepend for inbound, MED for same-peer multilink, communities for ISP-coordinated traffic engineering), prefix-list filtering on all neighbors, peer groups, soft vs hard reset, and pitfalls (missing IP route, iBGP next-hop, communities not sent, outbound prefix-list omitted).