Prefix-Lists & the Route Filtering Stack

Route filtering needs to match both address and mask length — ACLs don’t. Prefix-lists do: ip prefix-list PL seq 20 permit 10.0.0.0/8 ge 16 le 24 Matches any 10/8 subnet with mask between /16 and /24. Exact match = no ge/le. 0.0.0.0/0 le 32 = everything. Top-down, sequential numbers, implicit deny at the bottom. The […]

Advanced BGP — Route Reflectors, Protection, Policy

Advanced BGP is three things: scale, protect, control. Scaling iBGP — Route Reflectors iBGP split horizon forces a full mesh (N·(N−1)/2 sessions). A Route Reflector breaks the rule deliberately: neighbor x route-reflector-client. Loop safety comes from two extra attributes: Originator_ID and Cluster_List. Protection — maximum-prefix neighbor 10.1.1.2 maximum-prefix 1000 80 Warns at 80 %, shuts […]

BGP Basics — Attributes & Aggregation

Path-vector protocol between autonomous systems, over TCP 179, AD 20 (eBGP) / 200 (iBGP). No multicast, no auto-discovery — peers are configured. Messages: OPEN, UPDATE (NLRI + attributes), KEEPALIVE, NOTIFICATION. Keepalive 60 / hold 180. Best-path order (know it cold) Weight (Cisco-local) 2. Local Preference 3. Locally originated 4. Shortest AS-path 5. Origin IGP>EGP>? 6. […]