--- title: Multilevel IS-IS Deployments « IS-IS Labs | daily.dev description: A hands-on lab guide exploring multilevel IS-IS deployments using a 5-router topology. Covers configuring L1, L2, and L1/L2 router types, IS-IS adjacency... canonical: https://daily.dev/posts/multilevel-is-is-deployments-is-is-labs-13tqa0dfq twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Multilevel IS-IS Deployments « IS-IS Labs | daily.dev og:description: A hands-on lab guide exploring multilevel IS-IS deployments using a 5-router topology. Covers configuring L1, L2, and L1/L2 router types, IS-IS adjacency... og:url: https://daily.dev/posts/multilevel-is-is-deployments-is-is-labs-13tqa0dfq og:image: https://api.daily.dev/og/posts/13tQa0DFQ.png og:image:alt: Multilevel IS-IS Deployments « IS-IS 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. # Multilevel IS-IS Deployments « IS-IS Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 22 min read · 0 upvotes · 0 comments ## Summary A hands-on lab guide exploring multilevel IS-IS deployments using a 5-router topology. Covers configuring L1, L2, and L1/L2 router types, IS-IS adjacency rules, circuit types, the ATT (Attached) flag and default route generation, automatic redistribution of L1 routes into L2 LSPs, suboptimal inter-area routing trade-offs, and the consequences of partitioning the L2 backbone. FRRouting is noted as incompatible with multilevel deployments as of August 2025 due to incomplete L1-to-L2 prefix propagation. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### Why does a level-1 IS-IS router lose all its adjacencies when I change its type from level-2? A level-1 IS-IS router can only form adjacencies with level-1 or level-1-2 routers in the same area. If its neighbors are configured as level-2-only routers, no adjacency can form because the IS-IS hello (IIH) types are incompatible. The fix is to change the neighboring routers to level-1-2, which allows them to maintain both L1 adjacencies within the area and L2 adjacencies toward the backbone. _Engineers reconfiguring IS-IS router types in production track these adjacency rules on daily.dev before touching live devices._ ### Why does FRRouting not work in multilevel IS-IS deployments? FRRouting's IS-IS implementation does not propagate level-1 IP prefixes into the level-2 LSP database as required by RFC 1195. This means L1 routes from an area are never redistributed into the L2 backbone, breaking inter-area reachability. FRR works correctly in single-level IS-IS deployments, but multilevel topologies require a different platform. _Networking teams choosing between IS-IS implementations for multilevel designs compare platform support on daily.dev._ ### How does an IS-IS level-1 router get a default route to reach other areas? Level-1-2 routers set the ATT (Attached) bit in their L1 LSPs when they can reach at least one other area via the L2 backbone. Level-1 routers receiving an LSP with the ATT bit set automatically install a default route pointing to that router. When multiple L1/L2 routers advertise the ATT bit, ECMP default routes are installed toward all of them. _Network engineers designing IS-IS exit-point redundancy for L1 areas find related coverage on daily.dev._ ## Similar posts on daily.dev - [Multivendor SR-MPLS « ipSpace.net blog](https://daily.dev/posts/multivendor-sr-mpls-ipspace-net-blog-qekokwduk) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/multilevel-is-is-deployments-is-is-labs-13tqa0dfq) ```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":"Multilevel IS-IS Deployments « IS-IS Labs","url":"https://daily.dev/posts/multilevel-is-is-deployments-is-is-labs-13tqa0dfq","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/multilevel-is-is-deployments-is-is-labs-13tqa0dfq"},"datePublished":"2026-08-11T08:54:45.446Z","dateModified":"2026-08-11T09:05:43.810Z","description":"A hands-on lab guide exploring multilevel IS-IS deployments using a 5-router topology. 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