BGP

Border Gateway Protocol, the path-vector protocol that holds the internet together and increasingly runs inside enterprise data centers. Articles tagged BGP cover fundamentals, neighbor states, path attributes, best-path selection, traffic engineering, and Cisco IOS XE configuration.

36 posts

BGP

BGP Best Path Selection Algorithm — All 13 Steps

Cisco BGP picks the best path with a 13-step algorithm. Most real decisions resolve in steps 1 to 5, but troubleshooting unexpected path selection means knowing every step. A complete walkthrough with examples.

Mar 28, 2026 5 min read

BGP

BGP Session Authentication: MD5 and TCP-AO

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.

Mar 28, 2026 4 min read

BGP

Originating a Default Route in BGP

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.

Mar 28, 2026 4 min read

BGP

BGP Route Aggregation and Summarization

The IPv4 BGP table has over 950,000 prefixes and growing. Route aggregation lets you advertise a summary instead of many specifics, reducing the routing table burden on your peers and the internet. Configuration and gotchas.

Mar 28, 2026 4 min read

BGP

Cisco BGP Weight Attribute: Local Path Preference

Weight is the first step of Cisco BGP best-path selection, Cisco-proprietary, never advertised, and local to the router. The most targeted BGP traffic engineering tool when you need one router to override AS-wide policy.

Mar 28, 2026 3 min read

BGP

BGP AS-Path Prepending: When It Works and When It Doesn't

AS-path prepending is the go-to tool for influencing inbound BGP traffic: artificially lengthen the AS-path so remote ASes prefer your other exit. It is blunt, global, and has real security implications. When it works, and when it does not.

Mar 28, 2026 5 min read