--- title: Explore IS-IS Data Structures « IS-IS Labs | daily.dev description: A hands-on lab guide for exploring IS-IS routing protocol data structures, covering adjacencies and the LSP database (LSPDB). It walks through key show... canonical: https://daily.dev/posts/explore-is-is-data-structures-is-is-labs-9bwpsulaa twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Explore IS-IS Data Structures « IS-IS Labs | daily.dev og:description: A hands-on lab guide for exploring IS-IS routing protocol data structures, covering adjacencies and the LSP database (LSPDB). It walks through key show... og:url: https://daily.dev/posts/explore-is-is-data-structures-is-is-labs-9bwpsulaa og:image: https://api.daily.dev/og/posts/9BWPsUlAA.png og:image:alt: Explore IS-IS Data Structures « 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. # Explore IS-IS Data Structures « IS-IS Labs **[ipSpace.net Blog](https://daily.dev/sources/ipspace)** · 13 min read · 0 upvotes · 0 comments ## Summary A hands-on lab guide for exploring IS-IS routing protocol data structures, covering adjacencies and the LSP database (LSPDB). It walks through key show commands on FRRouting and Arista cEOS to inspect IS-IS summary info, neighbor adjacencies, and LSP contents. The guide explains IS-IS concepts including NET/System ID, router levels (L1/L2), TLV types used in LSPs, the SPF algorithm's use of LSPDB, and how LSP fragmentation works when the maximum LSP size is reduced. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### What is the format of an IS-IS LSP ID? An IS-IS LSP ID always follows the format systemid.nodeid-fragmentid. The systemid is the 6-octet System ID from the NET address (displayed as a hostname in most show commands). The nodeid is zero for a router's own LSP and non-zero for LAN pseudonodes. The fragmentid is the sequence number of the LSP fragment when a router's information spans multiple LSPs. _Network engineers working with IS-IS deployments track protocol details like these on daily.dev._ ### What TLVs are commonly found in IS-IS LSPs in a modern IPv4 network? Common TLVs in a modern IPv4 IS-IS LSP include: TLV 1 (Area address), TLV 22 (Extended IS Reachability — adjacency cost to neighbors), TLV 129 (Supported protocols), TLV 132 (IPv4 Interface Address), TLV 135 (IPv4 Extended Reachability — prefix costs), TLV 137 (Hostname), and TLV 242 (Router capabilities). The full registry is maintained by IANA. _Engineers tuning IS-IS networks keep up with protocol specifics like TLV structures on daily.dev._ ### How does IS-IS handle LSP fragmentation when a router has too much information for a single LSP? When a router's information exceeds the maximum LSP size, IS-IS generates multiple LSP fragments identified by an incrementing fragment ID (e.g., rtr.00-00, rtr.00-01). The same information is split across fragments. IS-IS cannot fragment LSPs in transit, so the maximum LSP size must not exceed the lowest MTU on any link in the network and must be configured consistently across all routers using a command like lsp-mtu. _Keeping IS-IS MTU and fragmentation issues from breaking production networks is easier when you follow routing protocol 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 - [BGP-Free Core with SR-MPLS « ipSpace.net blog](https://daily.dev/posts/bgp-free-core-with-sr-mpls-ipspace-net-blog-h7kronzy3) · ipSpace.net Blog · 0 upvotes · 0 comments --- [View this post on daily.dev](https://daily.dev/posts/explore-is-is-data-structures-is-is-labs-9bwpsulaa) ```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":"Explore IS-IS Data Structures « IS-IS Labs","url":"https://daily.dev/posts/explore-is-is-data-structures-is-is-labs-9bwpsulaa","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/explore-is-is-data-structures-is-is-labs-9bwpsulaa"},"datePublished":"2026-08-11T08:51:58.742Z","dateModified":"2026-08-11T08:54:38.749Z","description":"A hands-on lab guide for exploring IS-IS routing protocol data structures, covering adjacencies and the LSP database (LSPDB). 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