--- title: Use Disaggregated Prefixes to Select the Primary Link «... description: A hands-on BGP lab exercise demonstrating prefix disaggregation as a last-resort technique for controlling inbound traffic flow. When an ISP (X2) uses BGP... canonical: https://daily.dev/posts/use-disaggregated-prefixes-to-select-the-primary-link-bgp-labs-j8ovgczqv twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Use Disaggregated Prefixes to Select the Primary Link « BGP Labs | daily.dev og:description: A hands-on BGP lab exercise demonstrating prefix disaggregation as a last-resort technique for controlling inbound traffic flow. When an ISP (X2) uses BGP... og:url: https://daily.dev/posts/use-disaggregated-prefixes-to-select-the-primary-link-bgp-labs-j8ovgczqv og:image: https://api.daily.dev/og/posts/J8OvgczqV.png og:image:alt: Use Disaggregated Prefixes to Select the Primary Link « BGP Labs og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Use Disaggregated Prefixes to Select the Primary Link « BGP Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 6 min read · 0 upvotes · 0 comments ## Summary A hands-on BGP lab exercise demonstrating prefix disaggregation as a last-resort technique for controlling inbound traffic flow. When an ISP (X2) uses BGP local preference to prefer direct customer routes and ignores AS-path prepending or BGP communities, the only way to force traffic via the primary link (X1) is to advertise more-specific prefixes (/23s) to X1 while advertising only the aggregate (/22) to X2. The lab walks through configuring prefix origination and outbound prefix-list filters on EBGP sessions, with verification steps using FRRouting or Arista EOS via the netlab toolchain. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How can I force a backup ISP to route traffic through my primary ISP when the backup uses local preference and ignores AS-path prepending? Advertise two more-specific prefixes (e.g., /23s) to the primary ISP and only the aggregate prefix (e.g., /22) to the backup ISP. Because more-specific routes always win in BGP route selection, the backup ISP will prefer the path learned via the primary ISP for the more-specific prefixes. This technique is called prefix disaggregation and should be a last resort, as it adds entries to global routing tables. _Engineers navigating stubborn ISP routing policies track BGP techniques like this on daily.dev._ ### Why doesn't AS-path prepending work to shift inbound traffic away from a link when the ISP uses local preference? AS-path prepending loses to local preference in BGP's decision process. If an ISP assigns a higher local preference to directly connected customer routes (e.g., localpref 200 vs. the default 100), the local preference attribute is evaluated before AS-path length, so prepending extra AS hops has no effect on that ISP's path selection. _Keeping up with BGP policy edge cases like this is easier when the right posts surface on daily.dev._ ## Similar posts on daily.dev - [Enforcing the First AS in BGP AS\_PATHs](https://daily.dev/posts/enforcing-the-first-as-in-bgp-as-paths-yx1xiadkj) · Cloudflare · 0 upvotes · 0 comments - [Multivendor SR-MPLS « ipSpace.net blog](https://daily.dev/posts/multivendor-sr-mpls-ipspace-net-blog-qekokwduk) · ipSpace.net Blog · 0 upvotes · 0 comments - [The Never-Ending IPv6 Loopback Prefix Saga « ipSpace.net blog](https://daily.dev/posts/the-never-ending-ipv6-loopback-prefix-saga-ipspace-net-blog-4nlhgh10l) · ipSpace.net Blog · 0 upvotes · 0 comments --- Tags: [#bgp](https://daily.dev/tags/bgp) [View this post on daily.dev](https://daily.dev/posts/use-disaggregated-prefixes-to-select-the-primary-link-bgp-labs-j8ovgczqv) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Use Disaggregated Prefixes to Select the Primary Link « BGP Labs","url":"https://daily.dev/posts/use-disaggregated-prefixes-to-select-the-primary-link-bgp-labs-j8ovgczqv","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/use-disaggregated-prefixes-to-select-the-primary-link-bgp-labs-j8ovgczqv"},"datePublished":"2026-08-11T08:52:17.340Z","dateModified":"2026-08-11T08:54:06.770Z","description":"A hands-on BGP lab exercise demonstrating prefix disaggregation as a last-resort technique for controlling inbound traffic flow. 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Because more-specific routes always win in BGP route selection, the backup ISP will prefer the path learned via the primary ISP for the more-specific prefixes. This technique is called prefix disaggregation and should be a last resort, as it adds entries to global routing tables. Engineers navigating stubborn ISP routing policies track BGP techniques like this on daily.dev."}},{"@type":"Question","name":"Why doesn't AS-path prepending work to shift inbound traffic away from a link when the ISP uses local preference?","acceptedAnswer":{"@type":"Answer","text":"AS-path prepending loses to local preference in BGP's decision process. If an ISP assigns a higher local preference to directly connected customer routes (e.g., localpref 200 vs. the default 100), the local preference attribute is evaluated before AS-path length, so prepending extra AS hops has no effect on that ISP's path selection. 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