--- title: What Is BGP? Border Gateway Protocol Explained description: What is BGP? Learn how Border Gateway Protocol enables BGP routing and directs internet traffic between autonomous systems. image: https://blog.blazingcdn.com/hubfs/Gemini-Blog/image-Nov-03-2026-03-10-41-7740-PM.jpeg --- [![BlazingCDN](https://blog.blazingcdn.com/hubfs/Logo/blog-logo-w.png)](https://blog.blazingcdn.com?hsLang=en-us) [📘 Learn ▾](https://blog.blazingcdn.com/cdn-learn?hsLang=en-us) [CDN Fundamentals](https://blog.blazingcdn.com/cdn-fundamentals?hsLang=en-us) [By Content Type](https://blog.blazingcdn.com/by-content-type?hsLang=en-us) [Advanced Concepts](https://blog.blazingcdn.com/advanced-concepts?hsLang=en-us) [📖 Glossary](https://blog.blazingcdn.com/glossary?hsLang=en-us) [🎬 Video & Streaming ▾](https://blog.blazingcdn.com/video-streaming-cdn?hsLang=en-us) [🔴 Live Streaming](https://blog.blazingcdn.com/live-streaming?hsLang=en-us) [📺 VOD & OTT](https://blog.blazingcdn.com/vod-ott?hsLang=en-us) [🎞️ Encoding & Players](https://blog.blazingcdn.com/encoding-players?hsLang=en-us) [🎧 Audio & Podcasts](https://blog.blazingcdn.com/audio-podcasts?hsLang=en-us) [💰 Bandwidth & Costs](https://blog.blazingcdn.com/bandwidth-costs?hsLang=en-us) [🏗️ Infrastructure ▾](https://blog.blazingcdn.com/cdn-infrastructure?hsLang=en-us) [🗄️ Storage & Origin](https://blog.blazingcdn.com/storage-origin?hsLang=en-us) [📦 Large File Delivery](https://blog.blazingcdn.com/large-file-delivery?hsLang=en-us) [💸 Cloud Costs & Egress](https://blog.blazingcdn.com/cloud-costs-egress?hsLang=en-us) [🌐 Networking & ISP](https://blog.blazingcdn.com/networking-isp?hsLang=en-us) [⚙️ DevOps & Cloud Infra](https://blog.blazingcdn.com/devops-cloud-infra?hsLang=en-us) [⚡ Web Performance](https://blog.blazingcdn.com/web-performance?hsLang=en-us) [🏭 Industries ▾](https://blog.blazingcdn.com/cdn-industry-insights?hsLang=en-us) [📺 Media & Broadcasting](https://blog.blazingcdn.com/media-broadcasting?hsLang=en-us) [💾 Software & SaaS](https://blog.blazingcdn.com/software-saas?hsLang=en-us) [📣 AdTech & Advertising](https://blog.blazingcdn.com/adtech-advertising?hsLang=en-us) [🎮 Gaming & Esports](https://blog.blazingcdn.com/gaming-esports?hsLang=en-us) [📱 Mobile Apps & Developers](https://blog.blazingcdn.com/mobile-apps-developers?hsLang=en-us) [🤖 AI & Machine Learning](https://blog.blazingcdn.com/ai-machine-learning?hsLang=en-us) [🏛️ Enterprise & Corporate](https://blog.blazingcdn.com/enterprise-corporate?hsLang=en-us) [📚 E-Learning & EdTech](https://blog.blazingcdn.com/e-learning-edtech?hsLang=en-us) [🏟️ Sports & Live Events](https://blog.blazingcdn.com/sports-live-events?hsLang=en-us) [💰 Pricing & Compare ▾](https://blog.blazingcdn.com/cdn-pricing-and-cdn-costs?hsLang=en-us) [Provider Pricing](https://blog.blazingcdn.com/provider-pricing?hsLang=en-us) [Cost Optimization](https://blog.blazingcdn.com/cost-optimization?hsLang=en-us) [Decision Support](https://blog.blazingcdn.com/decision-support?hsLang=en-us) [⚡ Compare](https://blog.blazingcdn.com/cdn-comparison?hsLang=en-us) [Provider Comparisons](https://blog.blazingcdn.com/provider-comparisons?hsLang=en-us) [Strategy Comparisons](https://blog.blazingcdn.com/strategy-comparisons?hsLang=en-us) [📊 Benchmarks](https://blog.blazingcdn.com/cdn-ratings-and-benchmarks?hsLang=en-us) [🔒 Security ▾](https://blog.blazingcdn.com/cdn-security?hsLang=en-us) [Attack Protection](https://blog.blazingcdn.com/attack-protection?hsLang=en-us) [Encryption & Access](https://blog.blazingcdn.com/encryption-access?hsLang=en-us) [Video & DRM Security](https://blog.blazingcdn.com/video-drm-security?hsLang=en-us) [🛠️ Tools](https://blog.blazingcdn.com/cdn-tools?hsLang=en-us) [Get Started](https://blazingcdn.com/sign-up-contact-form/) [Infrastructure](https://blog.blazingcdn.com/en-us/tag/infrastructure) [Infrastructure - Networking & ISP](https://blog.blazingcdn.com/en-us/tag/infrastructure-networking-isp) # What Is BGP? Border Gateway Protocol Explained Vadym Borovskyi Nov 3, 2026, 4:12:27 PM ![](https://blog.blazingcdn.com/hubfs/Gemini-Blog/image-Nov-03-2026-03-10-41-7740-PM.jpeg) BGP (Border Gateway Protocol) is the routing protocol that connects the internet's independent networks, called autonomous systems, by letting each one announce which IP prefixes it can reach and through which chain of networks. Every BGP router keeps one best path per prefix, chosen by policy rather than latency, and advertises it to neighbors over TCP port 179. Border Gateway Protocol version 4, standardized in RFC 4271 in 2006, carries roughly 1 million IPv4 prefixes and about 200,000 IPv6 prefixes between more than 75,000 active autonomous systems as of 2025. It is the only interdomain routing protocol in production use, so every packet that crosses a network boundary follows a path BGP chose. ## **How BGP routing works between autonomous systems** BGP routing works by exchanging reachability, not topology. Each autonomous system (AS), identified by a 16-bit or 32-bit AS number (32-bit per RFC 6793), opens TCP sessions to its neighbors, announces the prefixes it originates or transits, and records the full AS path each route has traveled. Routers then apply local policy to choose one best route per prefix. A session moves through a small state machine: Idle, Connect, OpenSent, OpenConfirm and Established. Once established, peers exchange four message types. OPEN negotiates capabilities such as 4-byte ASNs and multiprotocol support for IPv6. UPDATE carries new routes and withdrawals. KEEPALIVE proves liveness. NOTIFICATION reports an error and tears the session down. Timing is where operators get surprised. RFC 4271 suggests a 90 second hold time with keepalives at one third of it, and many vendor defaults use 90 or 180 seconds. A router that dies silently can therefore keep attracting traffic for up to three minutes unless Bidirectional Forwarding Detection (BFD) runs alongside the session. The suggested minimum route advertisement interval (MRAI) of 30 seconds for external peers also batches updates, so a withdrawal can take tens of seconds to cross several AS hops. ### How a BGP router chooses the best path The decision process is a tie-breaking ladder, evaluated in order until one route wins: 1. Highest local preference (the operator's own business policy, usually customer over peer over transit). 2. Shortest AS path. 3. Lowest origin type. 4. Lowest multi-exit discriminator (MED), compared between routes from the same neighbor AS. 5. External (eBGP) route over internal (iBGP) route. 6. Lowest interior cost to the next hop, often called hot-potato routing. 7. Lowest router ID as the final tiebreaker. Notice what is missing: round-trip time, packet loss and link utilization. BGP is blind to performance, which is why a route that is four AS hops long can be faster than a two-hop route that crosses an ocean. ## **Where BGP sits in the network stack** BGP is an application-layer protocol that runs over TCP, yet it programs the forwarding tables that move every IP packet. It sits above interior gateway protocols such as OSPF or IS-IS, which find paths inside one network, and below DNS-based traffic steering, which decides which IP address a client resolves in the first place. **BGP vs. OSPF:** OSPF is a link-state protocol that floods the full topology of one administrative domain and computes shortest paths by link cost, converging in under a second when tuned. BGP is a path-vector protocol that crosses administrative boundaries, hides internal topology, and trades convergence speed for policy control and scale. **eBGP vs. iBGP:** eBGP runs between different autonomous systems and prepends the local ASN to the path. iBGP distributes external routes inside one AS without changing the AS path, which is why it needs a full mesh, route reflectors or confederations to avoid loops. **BGP vs. DNS steering:** DNS-based steering picks an IP address per resolver, can react to measured latency, and is limited by resolver caching and TTLs. BGP anycast picks a destination per network path, reacts only to reachability and policy, and needs no client-side state. ## **Anatomy of a BGP route announcement** A route as a router stores it is a prefix plus attributes. The example below uses documentation prefixes and private-use ASNs: an origin network, AS 64500, announces 203.0.113.0/24 to its transit provider, AS 64501, which passes it to a peer, AS 64510. | Attribute | Value seen at AS 64510 | What it controls | | --- | --- | --- | | Prefix | 203.0.113.0/24 | Longest match wins in forwarding, before any BGP attribute is compared | | AS path | 64501 64500 | Loop prevention and path length; the rightmost ASN is the origin | | Next hop | Peering address of AS 64501 | Where packets are actually forwarded | | Local preference | 100 (set locally, never sent to eBGP peers) | First step of the decision ladder | | Communities | Tags such as "do not export to peers" | Signals policy to upstreams without bilateral negotiation | Prefix length beats every attribute in the table, which is exactly what makes more-specific hijacks so effective. Practical limits follow from this. Most networks filter IPv4 announcements longer than /24 and IPv6 announcements longer than /48, so those are the smallest blocks you can expect to propagate globally. ## **BGP hijacks and route leaks** A BGP hijack is an announcement of a prefix by an AS that is not authorized to originate it. A route leak is a legitimate route propagated beyond its intended scope, for example a customer re-announcing one provider's routes to another provider. Both happen because BGP, by default, trusts whatever neighbors say. The textbook hijack is February 2008, when a national telecom announced a /24 inside YouTube's /22 to block the site domestically. The more-specific route leaked upstream and pulled most of YouTube's global traffic for roughly two hours. Leaks are more common and often larger: a single misconfigured optimizer or a missing export filter can redirect thousands of prefixes through a small regional network that collapses under the load. The defenses are layered. RPKI Route Origin Validation (RFC 6811) lets networks reject routes whose origin AS or prefix length contradicts a signed Route Origin Authorization; roughly half of IPv4 routes were covered by ROAs as of 2025. ROV does not stop path manipulation, so leaks still need prefix filters, max-prefix limits, BGP roles with the Only-to-Customer attribute (RFC 9234) and, as it matures, ASPA path validation. BGPsec (RFC 8205) signs full paths but has almost no deployment. ## **How anycast CDNs use BGP to route users** An anycast CDN announces the same prefix from many locations at once. Each client network then forwards packets to whichever announcement its own BGP policy prefers, which usually means the topologically nearest site. No DNS logic is involved: the routing system itself does the load distribution. Failover is a withdrawal. When a site stops announcing, upstreams fall back to the next-best path, and the speed of that fallback is pure timer arithmetic. Our worked estimate: with a 90 second hold time and no BFD, detecting a silent failure takes up to 90 seconds, plus one or two MRAI cycles of 30 seconds as the withdrawal crosses AS hops, for a worst case near 150 seconds. With BFD at a 300 ms interval and a multiplier of 3, detection drops to 0.9 seconds and propagation dominates. Health-checked withdrawal, where the site pulls its own announcement before the host dies, beats both. Operators steer anycast with AS path prepending, community tags that tell upstreams where to announce, and announcing different prefixes from different site groups. The trade-off is that BGP optimizes for policy, so some clients land on a distant site even when a closer one exists. BlazingCDN delivers through [**Anycast routing with built-in failover**](https://blazingcdn.com/anycast-cdn/), measured June 2026 at 24 ms and 1.11 Gb/s in North America on a 100 MB HTTPS download. ## **Common misconceptions about BGP** **"BGP picks the fastest path."** It picks the most preferred path by policy and AS path length. Latency is invisible to the protocol unless an operator encodes it in local preference. **"Anycast breaks TCP."** Path changes mid-session can reset connections, but routes for a given client are stable for long periods, and large-scale anycast delivery of long downloads works in production every day. **"RPKI solved hijacking."** ROV blocks wrong-origin announcements only where both a ROA exists and the receiving network validates. A forged path with the correct origin AS still passes. ## **FAQ: BGP and Border Gateway Protocol** ### What is BGP hijacking and how is it different from a route leak? BGP hijacking is an unauthorized network originating someone else's prefix, while a route leak is a valid route spread past its intended scope. Hijacks are origin problems that RPKI Route Origin Validation can catch. Leaks keep the correct origin, so they need prefix filters, max-prefix limits and BGP roles per RFC 9234 to contain them. ### What port does BGP use? BGP uses TCP port 179 for every session, both external and internal. Because it rides on TCP, it inherits reliable delivery and flow control, but also exposure to TCP resets. Operators commonly protect sessions with TCP-AO or MD5 authentication and the Generalized TTL Security Mechanism, which rejects packets that did not originate one hop away. ### How long does BGP take to converge after a failure? BGP convergence ranges from under a second to several minutes, depending on failure detection and propagation. A silent failure with a 90 second hold time and no BFD can take 90 seconds just to notice. Each AS hop then adds batching delay from advertisement intervals, with 30 seconds suggested for external peers in RFC 4271. ### Why do anycast CDNs use BGP instead of DNS for routing users? Anycast CDNs use BGP because it steers every packet by network path without depending on resolver location or DNS caching. Failover becomes a route withdrawal rather than waiting for TTLs to expire. The cost is that BGP ignores latency, so operators combine prepending, communities and selective announcements to correct suboptimal catchments. ## **Check your own BGP exposure this week** Run three checks on your stack. First, confirm every prefix you originate has a valid RPKI Route Origin Authorization with a max length matching what you actually announce, since a loose max length invites more-specific hijacks. Second, compare the AS paths toward your service from two or three public looking glasses against the path you expect, and note any surprise transit networks. Third, time a controlled withdrawal of one anycast site or origin route and measure how long traffic takes to shift, then decide whether BFD or health-checked withdrawal belongs in your runbook. For heavy static delivery on anycast, the [**BlazingCDN features overview**](https://blazingcdn.com/features/) lists what every plan includes, from origin shield and request coalescing to Prometheus metrics, so you can compare it against your current setup. 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All rights reserved. ```json { "@context" : "https://schema.org", "@id" : "https://blazingcdn.com/#organization", "@type" : "Organization", "description" : "High-volume CDN for video, live streaming, OTT/IPTV, software, games, SaaS and large-file delivery.", "logo" : { "@id" : "https://blazingcdn.com/#logo", "@type" : "ImageObject", "height" : 560, "url" : "https://blazingcdn.com/wp-content/uploads/2024/08/logo-560-560.png", "width" : 560 }, "name" : "BlazingCDN", "sameAs" : [ "https://www.linkedin.com/company/blazingcdn", "https://x.com/BlazingCdn", "https://www.reddit.com/r/BlazingCDN/", "https://www.wikidata.org/wiki/Q141711074" ], "url" : "https://blazingcdn.com/" } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/#website", "@type" : "WebSite", "inLanguage" : "en-US", "name" : "BlazingCDN Blog", "publisher" : { "@id" : "https://blazingcdn.com/#organization" }, "url" : "https://blog.blazingcdn.com/" } ``` ```json { "@context" : "https://schema.org", "@id" : "https://blog.blazingcdn.com/en-us/what-is-bgp-border-gateway-protocol-explained#article", "@type" : "BlogPosting", "author" : { "@id" : "https://blog.blazingcdn.com/en-us/author/blazingcdn#person", "@type" : "Person", "description" : "Vadym Borovskyi is Chief Product and Marketing Officer at BlazingCDN, where he has led product strategy and growth since 2021. 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