--- title: Optimize Simple IS-IS Deployments « IS-IS Labs | daily.dev description: Running IS-IS routers as level-1-2 (the default in most implementations) in a single-area network wastes resources: it doubles the LSP database size and... canonical: https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Optimize Simple IS-IS Deployments « IS-IS Labs | daily.dev og:description: Running IS-IS routers as level-1-2 (the default in most implementations) in a single-area network wastes resources: it doubles the LSP database size and... og:url: https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz og:image: https://api.daily.dev/og/posts/iEetqcXBZ.png og:image:alt: Optimize Simple 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. # Optimize Simple IS-IS Deployments « IS-IS Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 11 min read · 0 upvotes · 0 comments ## Summary Running IS-IS routers as level-1-2 (the default in most implementations) in a single-area network wastes resources: it doubles the LSP database size and triggers two SPF runs per topology change. In RFC 1195-compliant implementations like Arista cEOS, level-2 LSPs also bloat by including all intra-area prefixes, amplifying flooding overhead. The recommendation is to configure all routers as level-2-only using `is-type level-2`, which keeps routing functional and forward-compatible if the network is later split into multiple areas. FRRouting's incomplete IS-IS implementation avoids the worst-case bloat but still duplicates topology databases. Practical lab commands and output from FRRouting and Arista cEOS illustrate the problem and the fix. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What happens if I leave IS-IS routers at the default level-1-2 type in a single-area network? Running level-1-2 IS-IS in a single-area network doubles the topology database — every router maintains separate level-1 and level-2 LSP databases with identical information. Every topology change triggers two SPF runs with identical results, one of which is discarded. On RFC 1195-compliant implementations like Arista cEOS, level-2 LSPs also include all reachable intra-area prefixes, causing further bloat and increased flooding. _Network engineers tuning IS-IS deployments track protocol behavior and vendor quirks like these on daily.dev._ ### Should I configure IS-IS as level-1 or level-2 in a single-area network? Level-2-only is the better choice for a single-area IS-IS network. It avoids the overhead of running parallel level-1 and level-2 topologies, and it remains forward-compatible if you later decide to split the network into multiple areas. Configure it with a command like `is-type level-2` on each router. _Engineers planning IS-IS network design decisions find relevant discussions and lab write-ups on daily.dev._ ### Does FRRouting support IS-IS level-1-2 redistribution between levels? FRRouting's IS-IS implementation (as of October 2024) cannot distribute intra-area level-1 information into level-2 LSPs or vice versa, which is required by IS-IS standards. This means running level-1-2 on FRRouting in a single-area network causes duplicate databases but avoids the worst-case LSP bloat seen in fully compliant implementations like Arista cEOS. _Developers and network engineers following FRRouting limitations and IS-IS protocol support find updates on daily.dev._ ## Similar posts on daily.dev - [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 - [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/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz) ```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":"Optimize Simple IS-IS Deployments « IS-IS Labs","url":"https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz"},"datePublished":"2026-08-11T08:51:47.974Z","dateModified":"2026-08-11T08:53:29.268Z","description":"Running IS-IS routers as level-1-2 (the default in most implementations) in a single-area network wastes resources: it doubles the LSP database size and...","image":"https://media.daily.dev/image/upload/s--58gMhC4P--/f_auto/v1722860399/public/Placeholder%2012","thumbnailUrl":"https://media.daily.dev/image/upload/s--58gMhC4P--/f_auto/v1722860399/public/Placeholder%2012","isAccessibleForFree":true,"articleSection":"ipSpace.net Blog","inLanguage":"en","publisher":{"@type":"Organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180}},"author":{"@type":"Organization","name":"ipSpace.net Blog","logo":"https://media.daily.dev/image/upload/s--xS7E82MT--/f_auto,q_auto/v1786438303/logos/ipspace?_a=BAMAMicg0","url":"https://daily.dev/sources/ipspace"},"commentCount":0,"discussionUrl":"https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz","interactionStatistic":[{"@type":"InteractionCounter","interactionType":{"@type":"LikeAction"},"userInteractionCount":0},{"@type":"InteractionCounter","interactionType":{"@type":"CommentAction"},"userInteractionCount":0}],"keywords":"","timeRequired":"PT11M"} {"@context":"https://schema.org","@type":"BreadcrumbList","itemListElement":[{"@type":"ListItem","position":1,"name":"Home","item":"https://daily.dev"},{"@type":"ListItem","position":2,"name":"ipSpace.net Blog","item":"https://daily.dev/sources/ipspace"},{"@type":"ListItem","position":3,"name":"Optimize Simple IS-IS Deployments « IS-IS Labs"}]} {"@context":"https://schema.org","@type":"FAQPage","@id":"https://daily.dev/posts/optimize-simple-is-is-deployments-is-is-labs-ieetqcxbz#faq","mainEntity":[{"@type":"Question","name":"What happens if I leave IS-IS routers at the default level-1-2 type in a single-area network?","acceptedAnswer":{"@type":"Answer","text":"Running level-1-2 IS-IS in a single-area network doubles the topology database — every router maintains separate level-1 and level-2 LSP databases with identical information. 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